Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 Oct 2015

Insecticidal Control of Megacopta cribraria (Hemiptera: Plataspidae) in Soybean

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Page Range: 263 – 283
DOI: 10.18474/JES15-15.1
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Abstract

The invasive kudzu bug, Megacopta cribraria (F.) (Hemiptera: Plataspidae), has become an economic pest of soybean, Glycine max (L.) Merrill, in the southeastern United States since its initial discovery in Georgia. Information on management practices, including insecticides, is limited due to its uncertain pest status in its native range and recent introduction to the United States. We evaluated the efficacy in controlling M. cribraria and economic benefits of a variety of insecticides labeled for use in soybean from different chemical classes in field trials in South Carolina and Georgia from 2010 through 2012. Several pyrethroids were among the most effective insecticides for control of M. cribraria. The pyrethroid bifenthrin had an immediate (2–6 d after treatment application) percentage of control of 97.5 ± 0.2% (SEM), which was the highest of the active ingredients tested. Likewise, net marginal benefits were typically greatest for pyrethroids, either alone or tank-mixed with other materials. Our results confirm that chemical control of M. cribraria in commercial soybean production is economically viable, but the number of effective chemical classes is limited.

The kudzu bug, Megacopta cribraria (F.) (Hemiptera: Plataspidae), poses a significant concern to soybean, Glycine max (L.) Merrill, production in the southeastern United States. A native of eastern Asia, M. cribraria was discovered in Georgia in 2009 (Eger et al. 2010), and by 2015 had spread to 13 states plus the District of Columbia (Gardner et al. 2013b; W. Gardner, University of Georgia, pers. comm.; N.J.S. unpubl. data). Although the invasive weed kudzu, Puereria montana (Loureiro) Merrill variety lobata (Willdenow), is a major host in its invasive range (Zhang et al. 2012), populations of M. cribraria have been found in soybean in the United States since 2010 (Suiter et al. 2010). Megacopta cribraria undergoes two generations per year in the southern United States (Zhang et al. 2012). Development from egg to adult can occur on soybean plants beginning with the early vegetative stages, and takes 45–50 d at 28°C (Del Pozo-Valdivia and Reisig 2013).

Reports from its native range have indicated that M. cribraria can be a damaging pest of soybean (Hosokawa et al. 2007, Takagi and Murakami 1997, Thippeswamy and Rajagopal 2005, Zhixing et al. 1996,). Yield losses of up to 50% have been reported in China (Zhixing et al. 1996), and soybean growth can be reduced due to feeding by M. cribraria (Kikuchi and Kobayashi 2010). In the southeastern United States, yield losses of up to 60% were observed when adults were confined to caged soybean plots and their offspring were allowed to feed and develop (Seiter et al. 2013b). Due to its potential to reduce yields, management tactics for M. cribraria in soybean need to be evaluated. Effective control of M. cribraria in soybean with deltamethrin has been reported in China (Zhixing et al. 1996). Several insecticides, including pyrethroids, neonicotinoids, and an oxadiazine, provided residual control of adults when applied to building-material surfaces (Seiter et al. 2013a), as M. cribraria is also considered a nuisance pest in its invasive range. However, there is currently no published information on the field efficacy of labeled insecticides to control populations of M. cribraria in soybean in the United States. Our objective was to evaluate the field efficacy of insecticides from a variety of chemical classes against natural infestations of M. cribraria. In addition, we conducted an economic assessment of the tradeoff between the cost of these materials and their economic benefits in terms of preserved soybean yield.

Materials and Methods

Field trials were established in Georgia and South Carolina in 2010 (Georgia only), 2011, and 2012 to study the efficacy of insecticides labeled for use in soybean to control M. cribraria. Trials were located at the University of Georgia Plant Sciences farm (Oconee Co.), the Southeast Georgia Research and Education Center (Burke Co.), the University of Georgia Tifton Campus (Tift Co.), and the Clemson University Edisto Research and Education Center (Barnwell Co., SC) (Table 1). Each trial was a randomized complete block design with four replicate blocks and 4 to 13 insecticide treatments applied at rates labeled for use in soybean (Table 2), as well as an untreated control. The insecticides represented a variety of chemical classes and modes of action (Insecticide Resistance Action Committee 2014). Soybean varieties were planted using a 91.4-cm (Georgia trials) or a 96.5-cm (South Carolina trials) row spacing during a planting window typical of the southeastern United States (late May or early June). Seeding rates were ≈255,000 seeds/ha in the South Carolina trials and ≈287,000 seeds/ha in the Georgia trials. Insecticides at all sites were applied at a spray volume of 93.5 L/ha using a high-clearance self-propelled sprayer, with hollow cone tip nozzles at 345 kPa (South Carolina trials; ConeJet TXVS-6, TeeJet Technologies, Wheaton, IL) or standard flat fan nozzles at 276 kPa (Georgia trials; TeeJet 8002, TeeJet Technologies). Populations of M. cribraria were assessed using sweep-net (10 trials) or beat-cloth (two trials) sampling. Sweep nets (38 cm diameter) were swung at 180° across one (Georgia trials) or two (South Carolina trials) soybean rows through the upper plant canopy 10, 20, or 25 times in each plot. In the South Carolina trials, samples were alternately taken from rows two and three or rows six and seven (eight-row plots) or rows 10 and 11 (12-row plots) to avoid sampling the same rows on consecutive evaluations. Insects were counted in the field or collected in plastic bags and frozen at −20°C until they were counted in a laboratory. In the Georgia trials, only adults and nymphs of M. cribraria were counted. In the South Carolina trials, adults and nymphs of M. cribraria were counted, along with other pests of soybean. White beat cloths (91.4 cm long × 71.1 cm wide) were placed between two soybean rows; the sections of row (91.4 cm long) parallel to the cloth on either side were vigorously shaken over the cloth surface, and dislodged adults and nymphs of M. cribraria and other pests of soybean were counted. This was repeated twice in each plot for each sample, for a total of 3.7 m of row sampled per plot. Densities of M. cribraria were assessed one to three times for each trial from 2 to 21 d after insecticide treatments were applied. In cases where treatments were applied multiple times, only population density assessments made between the first and second applications were included.

Table 1. Plot information for trials conducted in Georgia and South Carolina in 2010, 2011, and 2012.

          Table 1.
Table 2. Chemical and rate information for insecticide treatments. Insecticide Resistance Action Committee mode of action classification is given in parentheses under Insecticide Class.

          Table 2.

In the South Carolina trials, the center four rows were harvested using a two-row plot combine (model 8-XP, Kincaid Equipment Manufacturing, Haven, KS). Soybean yields and moisture content were measured, and yields were converted to kg/ha at 13% moisture prior to data analysis. Net economic benefit was calculated for each plot. The cost per hectare of each insecticide rate was computed based on average prices obtained from up to six local agricultural input dealers. This was added to a base application cost (representing fixed and variable costs of operating a high-clearance self-propelled sprayer) of $13.84/ha (Clemson University Cooperative Extension 2014) to give the total cost per application in U.S. dollars per hectare. The marginal benefit of a treatment was the within-replicate difference in yield (kg/ha) from the untreated control, multiplied by a hypothetical soybean price of $0.4042/kg ($11.00/bushel at 13% moisture), which was close to the low end of the range of average prices received by U.S. soybean growers from 2010 to 2012 (U.S. Department of Agriculture Economic Research Service 2013). Net marginal benefit was calculated by subtracting the total insecticide cost (cost per application × number of applications) from the marginal benefit.

Data analysis

Data for densities of M. cribraria (expressed as number of insects per sweep or number of insects per 3.7 m of row) measured by both sweep-net and beat-cloth sampling were generally nonnormal and positively skewed, approximating a Poisson distribution. Therefore, all population data were transformed prior to analysis using the function. Each trial was analyzed separately using analysis of variance, with insecticide treatment as a fixed effect and replicate as a random effect (PROC MIXED [SAS Institute 2010]). Transformed population data (all trials), yield (when available), and net marginal benefit (when available) were analyzed as dependent variables using this model. When multiple population density assessments were taken from a given trial, a repeated measures model was used that included time (in days postapplication), insecticide treatment, and their interaction as fixed effects. A repeated measures statement was included in these models with the combination of replicate and treatment (i.e., the plot) as the subject and time as a repeated effect with a first-order ante-dependence covariance structure. A first-order ante-dependence covariance structure was used because it resulted in comparatively low values of the Akaike information criterion (data not shown) while allowing for unequal time intervals between density assessments (Wolfinger 1996). Mean separations were performed using Fisher's method of least significant difference (α = 0.05). Percentage of control (based on the sum [not corrected for number of sweeps] of adults and nymphs of M. cribraria for each treatment compared with the sum in the corresponding untreated plots) was calculated across trials for chemicals that included a single active ingredient and were included in two or more trials.

Results

Insecticide treatment significantly affected densities of M. cribraria in 11 out of 12 trials (Table 3). Days after treatment application significantly affected densities in three of the nine trials in which it was included as a fixed effect, and the interaction between insecticide and days after treatment application was significant in five out of nine trials (Table 3). Insecticide treatment affected yield and net marginal benefit in two of the five trials in which these dependent variables were assessed (Table 3).

Table 3. Tests of fixed effects for field efficacy trials, 2010–2012.

          Table 3.

2010 trials

Only adults were present in the trials conducted in Georgia in 2010 when populations were monitored after the first insecticide application (Fig. 1). In trial GA-1, all insecticides significantly reduced densities of M. cribraria compared with the untreated control after 2 d, but by 9 d after treatments were applied, only plots treated with methyl parathion remained different from the untreated control. In trial GA-2, all insecticides reduced densities compared with the untreated control; however, peak populations in the untreated control were low compared with the other trials.

Fig. 1. Populations of Megacopta cribraria in two insecticide efficacy trials conducted in Oconee Co., GA, in 2010. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 sweeps per plot) on the indicated dates (DAT = days after treatment application). Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-1) or treatment (GA-2).Fig. 1. Populations of Megacopta cribraria in two insecticide efficacy trials conducted in Oconee Co., GA, in 2010. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 sweeps per plot) on the indicated dates (DAT = days after treatment application). Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-1) or treatment (GA-2).Fig. 1. Populations of Megacopta cribraria in two insecticide efficacy trials conducted in Oconee Co., GA, in 2010. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 sweeps per plot) on the indicated dates (DAT = days after treatment application). Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-1) or treatment (GA-2).
Fig. 1. Populations of Megacopta cribraria in two insecticide efficacy trials conducted in Oconee Co., GA, in 2010. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 sweeps per plot) on the indicated dates (DAT = days after treatment application). Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction (GA-1) or treatment (GA-2).

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

2011 trials

Only adults were present in trial GA-3 conducted in 2011, whereas both adults and nymphs were present in trial GA-4 (Fig. 2). Densities of M. cribraria in the untreated control of GA-3 peaked at less than two per sweep, and none of the insecticide treatments had populations that were different from the untreated control at 9 d after treatments were applied. Overall populations were much higher in trial GA-4, and all insecticides except for indoxacarb reduced densities compared with the untreated control. Adults and nymphs were present in both trials conducted in South Carolina in 2011 (Fig. 3). Densities of M. cribraria were reduced by 5 of the 10 insecticides 3 d after treatments were applied in trial SC-1, whereas all insecticides significantly reduced densities compared with the untreated control in trial SC-2 at 3 and 11 d after treatments were applied. Additional pests were present at low densities in SC-1 and SC-2 (Table 4). Yields in the untreated controls were statistically lower than most treatments in both trials (SC-1 and SC-2) (Fig. 3). In SC-1, yields in the esfenvalerate, acephate, and indoxacarb treatments were statistically not different from the untreated control. The highest-yielding treatment (methoxyfenozide) did not reduce densities of M. cribraria 3 d postapplication. In SC-2, two of the highest-yielding treatments (λ-cyhalothrin + thiamethoxam and chlorpyrifos + λ-cyhalothrin) had the lowest densities of M. cribraria; however, yields were reduced in the ζ-cypermethrin + bifenthrin treatment even though densities of M. cribraria were as low as in the λ-cyhalothrin + thiamethoxam and chlorpyrifos + λ-cyhalothrin treatments. Net marginal benefit in SC-1 and SC-2 was generally highest with insecticides that contained a pyrethroid ingredient (Fig. 3). Exceptions in SC-1 were the molting hormone agonist methoxyfenozide, which had the highest net marginal benefit, and the pyrethroid esfenvalerate, which had a reduced net marginal benefit.

Fig. 2. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Oconee Co. (GA-3) and Burke Co. (GA-4), GA, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (30 [GA-3] or 10 [GA-4] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-3) or treatment (GA-4).Fig. 2. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Oconee Co. (GA-3) and Burke Co. (GA-4), GA, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (30 [GA-3] or 10 [GA-4] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-3) or treatment (GA-4).Fig. 2. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Oconee Co. (GA-3) and Burke Co. (GA-4), GA, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (30 [GA-3] or 10 [GA-4] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction (GA-3) or treatment (GA-4).
Fig. 2. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Oconee Co. (GA-3) and Burke Co. (GA-4), GA, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (30 [GA-3] or 10 [GA-4] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction (GA-3) or treatment (GA-4).

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

Fig. 3. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (25 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 3. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (25 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 3. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (25 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.
Fig. 3. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (25 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

Table 4. Peak densities of pest species in addition to Megacopta cribraria in trials conducted in South Carolina from 2011 to 2012. Trials SC-1, SC-2, and SC-5 are expressed as mean ± SEM insects per sweep, while trials SC-3 and SC-4 are expressed as mean number of insects per 3.7 m of soybean row. The earliest soybean growth stage where each peak was observed is given in parentheses.

            Table 4.

2012 trials

Adults and nymphs were present in two trials conducted in Georgia in 2012 (Fig. 4). All insecticides resulted in reduced densities of M. cribraria compared with the untreated control at 3 and 10 d after treatments were applied in trial GA-5. In trial GA-6, all but three treatments resulted in reduced densities of M. cribraria at 3 d after treatments were applied, and all treatments resulted in reduced densities at 10 d after treatments were applied. Adults and nymphs were present in two trials conducted in South Carolina in 2012 (Fig. 5). All treatments resulted in reduced densities of M. cribraria 6 d after treatments were applied in trial SC-3. Only imidacloprid did not result in reduced densities 6 d after treatments were applied in trial SC-4. Soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), was the only other pest that exceeded a density of one per 3.7 m of soybean row in both trials (Table 4). Yield and net marginal benefit did not differ among treatments (Fig. 5). Adults and nymphs of M. cribraria were present in two trials evaluating insect growth regulator insecticides in 2012 (Fig. 6). In trial SC-5, only one treatment resulted in reduced densities of M. cribraria, and all insect growth regulator insecticides were statistically not different from the untreated control. Other insect pests were present, but at low densities (Table 4). Yield and net marginal benefit were not affected by treatment. In trial GA-7, the effect of insecticide treatment was not significant, but densities of M. cribraria were relatively low. Across trials, the active ingredients that provided the highest percentage of control of M. cribraria belonged to the pyrethroid and carbamate insecticide classes (Table 5).

Fig. 4. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Burke Co., GA, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (10 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction.Fig. 4. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Burke Co., GA, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (10 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction.Fig. 4. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Burke Co., GA, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (10 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for the date × treatment interaction.
Fig. 4. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Burke Co., GA, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (10 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction.

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

Fig. 5. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using beat cloth sampling (3.7-m row per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 5. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using beat cloth sampling (3.7-m row per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 5. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using beat cloth sampling (3.7-m row per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.
Fig. 5. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using beat cloth sampling (3.7-m row per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

Fig. 6. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC (SC-5), and Tift Co., GA (GA-7), in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 [SC-5] or 10 [GA-7] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 6. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC (SC-5), and Tift Co., GA (GA-7), in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 [SC-5] or 10 [GA-7] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.Fig. 6. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC (SC-5), and Tift Co., GA (GA-7), in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 [SC-5] or 10 [GA-7] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference (α = 0.05) for treatment.
Fig. 6. Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC (SC-5), and Tift Co., GA (GA-7), in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 [SC-5] or 10 [GA-7] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.

Citation: Journal of Entomological Science 50, 4; 10.18474/JES15-15.1

Table 5. Single active ingredients across all trials, sorted by immediate (2–6 DAT*) percentage of control (mean ± SEM) of Megacopta cribraria . Only treatments that were included in two or more trials are displayed.

            Table 5.

Discussion

Several insecticide active ingredients in the pyrethroid class, as well as carbaryl (a carbamate), showed the highest immediate (2–6 d after the initial application) efficacy across trials based on reduction of densities of M. cribraria. Of these, the pyrethroid bifenthrin was the most consistently effective; bifenthrin reduced densities compared with the untreated control in each treatment that it appeared either alone or mixed with another chemical. Extended (7–12 d after the initial application) efficacy was more variable and might have been influenced by adult migration patterns in some cases (for example, trials GA-1 and GA-3, where densities in the untreated control declined from the first evaluation to the second). In the limited examples where yield and net marginal benefit varied among treatments, several pyrethroids were among the best performers. Methoxyfenozide was a notable exception in trial SC-1, although its yield impacts were not accompanied by a reduction in densities of M. cribraria. Because these trials were conducted in open plots of soybean and relied on natural infestations, other insect pests would have been differentially affected by the treatments as well. The densities of additional pests that we observed in the South Carolina trials (from which yields were analyzed) were generally low, and were below thresholds for treatment that are recommended in the state (Greene 2013). Based on these comparatively low densities, any contributions to yield reductions by these additional pests were likely minor.

Several of the pyrethroid insecticides that showed the greatest efficacy for control of M. cribraria are already recommended for use in soybean to control pests such as stink bugs (Hemiptera: Pentatomidae), corn earworm (Helicoverpa zea [Boddie]; Lepidoptera: Noctuidae), green cloverworm (Hypena scabra [F.]; Lepidoptera: Noctuidae), and velvetbean caterpillar (Anticarsia gemmatalis [Hübner]; Lepidoptera: Noctuidae) (Greene 2013). The stink bug complex infesting soybean in the southeastern United States consists primarily of the southern green stink bug, Nezara viridula (L.), the brown stink bug, Euschistus servus (Say), and the green stink bug, Acrosternum hilare (Say) (Bundy and McPherson 2000). The brown stink bug is less susceptible to pyrethroid and organophosphate active ingredients than the southern green stink bug or green stink bug (Snodgrass et al. 2005). In addition, field populations of the soybean looper, a major soybean pest in the region, have exhibited resistance to pyrethroids (Felland et al. 1990), which are consequently not recommended for soybean looper management in South Carolina (Greene 2013).

Population densities of M. cribraria differed among trials, as represented by the highly variable levels observed in the untreated controls. Greater interpretive weight should be given to trials that had higher overall population densities and contained both adults and nymphs (i.e., trials GA-4, SC-3, etc.). The presence of nymphs appears to be a key contributor to soybean yield reductions (Seiter et al. 2013b). Therefore, successful control of nymphs in particular will be a key determinant of a particular insecticide's usefulness for management of M. cribraria. Based on our results, nymphs of M. cribraria were at least as susceptible as adults to the insecticides we tested. Timing of insecticide applications based on density and life stage of M. cribraria and/or plant phenology could also impact the initial efficacy, duration of control, and soybean yield. Likewise, the number of applications needed to effectively manage M. cribraria was not evaluated here. Other experiments have indicated that a single application is typically sufficient to prevent yield losses in soybean caused by this insect (N.J.S., unpubl. data). The economic data presented here should be interpreted in the context that multiple applications were used in most trials, likely inflating actual management costs for M. cribraria.

The limited number of insecticide classes demonstrating efficacy for control of M. cribraria presents some concerns for ongoing management. Repeated use of a limited number of insecticides could favor resistance development, particularly for an insect with a narrow host range. Development of reduced susceptibility or resistance to pyrethroids has been reported in many hemipteran pests, including the common bed bug (Cimex lectularius L.; Romero et al. 2007), the tarnished plant bug (Lygus lineolaris [Palisot de Beauvois]; Snodgrass 1996), Triatoma infestans Klug (Picollo et al. 2005), and the southern chinch bug (Blissus insularis Barber; Cherry and Nagata 2005). In addition, the insecticides that were most effective for control of M. cribraria have a broad spectrum of affected insect taxa, and these materials can negatively impact beneficial insects in addition to the target pests (Tillman and Mulrooney 2000, Wilkinson et al. 1979).

Several insecticides, in particular from the pyrethroid class, appear to be effective and economically viable for control of M. cribraria in soybean. Future research should include the development of economic injury levels and thresholds. In addition, the impact of natural enemies on populations of M. cribraria should be investigated in more detail. Although an early report indicated that this impact might be limited (Ruberson et al. 2013), parasitoid species have recently been discovered in the United States that target the adults (Golec et al. 2013) and eggs (Gardner et al. 2013a) of M. cribraria. In addition, the fungal pathogen Beauveria bassiana (Balsamo) Vuillemin (Seiter et al. 2014) and a nematode from the family Mermithidae (Stubbins et al. 2015) attack M. cribraria. Laboratory assays should be used to determine median lethal concentration (LC50) values and potential sublethal effects of insecticides on M. cribraria, as well as to monitor for the development of insecticide resistance in field populations. These efforts will place insecticidal control of M. cribraria within an integrated pest management framework, allowing soybean producers to more effectively manage this emerging invasive pest.

Acknowledgments

The authors thank Dan Robinson, James Smoak (Clemson University), and Dean Kemp (University of Georgia) for technical assistance. This is technical contribution no. 6255 of the Clemson University Experiment Station. This material is based upon work supported by USDA National Institute of Food and Agriculture, under project numbers SC-1700441, SC-1700470, and SC-1700455. Funding for this work was provided by the South Carolina Soybean Board, the Georgia Agricultural Commodity Commission for Soybeans, and the United Soybean Board. Funding sources had no role in study design, data collection/interpretation, report preparation, or the decision to submit this manuscript for publication.

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<bold>Fig. 1</bold>
.
Fig. 1 .

Populations of Megacopta cribraria in two insecticide efficacy trials conducted in Oconee Co., GA, in 2010. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 sweeps per plot) on the indicated dates (DAT = days after treatment application). Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction (GA-1) or treatment (GA-2).


<bold>Fig. 2</bold>
.
Fig. 2 .

Populations of Megacopta cribraria in insecticide efficacy trials conducted in Oconee Co. (GA-3) and Burke Co. (GA-4), GA, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (30 [GA-3] or 10 [GA-4] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction (GA-3) or treatment (GA-4).


<bold>Fig. 3</bold>
.
Fig. 3 .

Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2011. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (25 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.


<bold>Fig. 4</bold>
.
Fig. 4 .

Populations of Megacopta cribraria in insecticide efficacy trials conducted in Burke Co., GA, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (10 sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for the date × treatment interaction.


<bold>Fig. 5</bold>
.
Fig. 5 .

Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC, in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using beat cloth sampling (3.7-m row per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.


<bold>Fig. 6</bold>
.
Fig. 6 .

Populations of Megacopta cribraria in insecticide efficacy trials conducted in Barnwell Co., SC (SC-5), and Tift Co., GA (GA-7), in 2012. The rate of active ingredient in kg/ha is given in parentheses for each insecticide treatment. Populations were assessed using sweep nets (20 [SC-5] or 10 [GA-7] sweeps per plot) on the indicated dates. Columns with a letter in common are not different based on the Fisher method of least significant difference ( α = 0.05) for treatment.


Contributor Notes

Corresponding author (email: nseiter@uaex.edu). Current address: University of Arkansas Division of Agriculture, Southeast Research and Extension Center, PO Box 3508, Monticello, Arkansas 71656.
Received: 09 Mar 2015
Accepted: 31 May 2015
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