Hessian Fly (Diptera: Cecidomyiidae) Management Using Seed Treatments in Winter Wheat1
The Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), is a potentially severe pest of wheat, Triticum aestivum L. em Thell, in the southeastern United States. Plant resistance is an effective method to control Hessian fly, but when adapted high-yielding varieties with effective resistance are not available, neonicotinoid insecticide seed treatments may provide an alternative method of control of Hessian fly on susceptible varieties of winter wheat. A series of experiments were conducted to examine the efficacy of neonicotinoid seed treatments for control of Hessian fly in winter wheat. Infestations and immature numbers per tiller were assessed during the vegetative stages in autumn and winter and the wheat reproductive stage during the spring. Both imidacloprid 480FS at 0.31 g active ingredient (a.i.)/kg of seed and clothianidin at 0.39 g a.i./kg or higher rates provided consistent reductions in Hessian fly infestations during autumn and early winter. Lower rates were less effective, and they did not provide consistent reductions in autumn infestations. Thiamethoxam was evaluated at one rate in two experiments and was similar in efficacy to imidacloprid and clothianidin at the same rate. None of the seed treatments provided effective control of spring infestations during the wheat reproductive stage. Imidacloprid and clothianidin at rates of 0.31 g a.i./kg of seed or higher rates had a positive yield response in eight of nine comparisons, with an average increase of 285.9 ± 92.7 kg/ha. Neonicotinoid insecticide seed treatments at higher rates provide a useful method for managing Hessian fly on susceptible varieties of winter wheat.Abstract
The Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), is a potentially severe pest of wheat, Triticum aestivum L. em Thell, in the southeastern United States. This insect species was one of the first exotic, invasive pests in North America. It was first reported near Brooklyn, NY, in the 1770s and is speculated to have been introduced in infested straw (Schmid et al. 2018). The Hessian fly was documented to have spread from New York and reached Georgia in the 1850s (Webster 1899), although multiple sources of introduction are possible (Schermerhorn et al. 2015). Increased winter wheat production in the Southeast in the 1980s lead to outbreaks that cause extensive damage and losses (Buntin et al. 1992). Winter wheat is still widely grown in a double-crop system in the Southeast, and the Hessian fly periodically causes considerable damage.
The Hessian fly is a gall-forming insect, although galls by this insect are indistinct. It is a cool-season insect and aestivates in wheat stubble as a puparia during the summer (McColluch 1923). Infestations occur in the fall and winter in vegetative stages of wheat and during the spring in wheat reproductive stages. Injury is caused by larval feeding at the base of the vegetative-stage plants or on stems above the nodes of reproductive-stage plants. Larval feeding injects toxic substances into plants that may cause stunted growth, seedling or tiller death, and increased susceptibility to cold temperature injury (Buntin 1999). In reproductive-stage plants, larvae feed on the stem above the nodes, where they weaken and disfigure the stems, which reduces grain filling and may cause stem lodging. In the Midwest and northern areas, there usually is a single autumn and single spring generation (McColloch 1923, Walton and Packard 1930). But, in the southeastern United States, there usually are two autumn generations, a winter generation, and a spring generation (Buntin and Chapin 1990).
Hessian fly management has historically been with the use of resistant wheat varieties and delayed planting of wheat to avoid autumn infestations. Reduced or no-tillage practices and lack of control of volunteer wheat during the summer can enhance Hessian fly abundance (Buntin et al. 1991, Chapin et al. 1991). Delayed autumn planting until after a certain date is based on the historical date when adult activity normally ceases or becomes limited because of cold weather. These dates for a given location are referred to as the “fly-free” dates and were developed >100 yr ago for most of the eastern United States (Walton and Packard 1930). Fly-free dates are still effective in most areas, but delaying planting must be balanced with the reduced yield potential of late-planted winter wheat. The fly-free date approach is not effective in the coastal plain region for the southern United States because early plantings are at risk of infestation, but although late plantings may avoid the fall generation injury, they may be heavily attacked by the winter and spring infestations (Buntin et al. 1990). Plant resistance has been widely used to control Hessian fly for many decades (Painter 1951, Ratcliffe et al. 2000), with at least 37 resistance (R) genes being identified (Li et al. 2013, Zhao et al. 2020). However, only a small number of the known R genes may be effective in an area and only a small number have been deployed in commercial wheat varieties (Cambron et al. 2010, Shukle et al. 2016). Indeed, the four original R genes (H3, H5, H6 and H7H8) that were widely deployed in the past in wheat have become completely infective in the eastern United States (Shukle et al. 2016). When resistance fails in an area, it may occur rapidly without time to develop and deploy high-yielding agronomically acceptable varieties with new sources of resistance.
Insecticides are another option for Hessian fly management. In the past, Hessian fly infestations could be controlled using systemic insecticides, such as disulfoton, phorate, and carbofuran (Bigger et al. 1965, Brown 1960, Buntin 1990, Chapin et al. 1991, Morrill and Nelson 1976). These insecticides were applied as granules in-furrow at planting for best results. Seed applications of these type of insecticides caused phytotoxicity to germinating seed, and broadcast applications were not effective except at unacceptable high rates (Bigger et al. 1965, Brown 1960). Furthermore, these organophosphate and carbamate insecticides are no longer available or registered for use on cereal grains in North America. Neonicotinoid seed treatments are now available on a wide variety of crops, including cereal grains. Few studies have examined the efficacy of neonicotinoid seed treatments for management of Hessian fly. Wilde et al. (2001) showed that imidacloprid and thiamethoxam reduced autumn Hessian fly infestations, but did not control spring infestations, in Kansas. In North Carolina, imidacloprid partly reduced autumn infestations in winter wheat, but control was variable and effect on yield was not measured (Howell et al. 2017). Winn et al. (2023) also found that imidacloprid at twice the maximum labeled rate (3.13 ml/kg of seed [4.8 fl oz/100 lb of seed]) partly reduced Hessian fly infestations, with a significant increase in yield in two of three susceptible wheat varieties. Control of Hessian fly using foliar applied insecticides is inconsistent and difficult to achieve good results because the spray must be timed to coincide with adult peak egg laying (Buntin and Hudson 1991, Howell et al. 2017).
The objective of this study was to examine the efficacy of neonicotinoid seed treatments for control of Hessian fly in the Coastal Plain region of Georgia where the insect has multiple generations per season. Studies were conducted when these insecticides were being developed for this purpose for winter wheat in the 1990s and more recently when insecticide seed treatments were widely available to growers.
Materials and Methods
Experiments were conducted at the University of Georgia Southwest Branch Experiment Station near Plains (Sumter Co.) in all years (1993, 1994, 1995) and in a farmer’s field located 11 km (7 mi) south of Sandersville (Washington Co.) in 1993 and 1994. Soil types were a Tifton sandy loam at Plains and a Marlboro sandy loam at Sandersville. Tillage was conventional with plowing or disk-harrowing before planting. At Plains, a 5-m-wide strip of a Hessian fly–susceptible cultivar was sown around the plot area in late August to encourage the buildup of Hessian fly populations. A complete fertilizer (5–10–15, N-P-K) was applied at the rate of 560 kg/ha and incorporated before planting. Experiments were planted with a small-plot grain drill in 17.8-cm rows at a soil depth of 1.3 cm at the rate of 66 seeds per m of row or 101 kg of seed per ha. Plots measured 14 rows (3.1 m) by 6.1 or 7.8 m. Experiments were planted on 12 and 19 November 1993 at Plains and Sandersville, respectively, and on 9 and 10 November at Plains in 1994 and 1995, respectively. Plots at Plains were irrigated with 1.3 cm of water at 7 and 21 d after planting. Plots were not irrigated at Sandersville. Ammonium nitrate fertilizer was top-dress applied in February at 247 kg N per ha. Diclofop-methyl (Hoelon) 3EC at 3.16 L/ha (Bayer CropScience, St. Louis, MO) and Harmony Extra (thifensulfuron-methyl + tribenuron-methyl) SG at 31.5 g/ha (FMC Corp., Philadelphia, PA) were applied to plots to control annual ryegrass (Lolium multiflorum Lam.) and broadleaf weeds, respectively.
Seed of the Hessian fly–susceptible wheat ‘Savannah’ was treated with imidacloprid (Gaucho 480FS) insecticide by Gustafson Inc. (now Bayer CropScience) in late September in 1993, 1994, and 1995. Treatments are listed in Tables 1 and 2. An untreated check and a standard of disulfoton (DiSyston) 15G at 1.12 kg active ingredient (a.i.)/ha applied in-furrow at-planting were included in each experiment. In 1993 and 1994, a treatment of dimethoate (Dimethoate 4E, FMC Corp.) at 0.56 kg a.i./ha applied as a foliar spray at 30 d after planting was included. In 1996, dimethoate was replaced with lambda cyhalothrin (Karate 2.02SC, Bayer CropScience) at 0.028 kg a.i./ha. Imidacloprid seed treatments in all years also were treated with triadimenol (Baytan) 30FL at 81.25 ml/100 kg of seed and captan (Captan) 400 at 130 ml/100 kg of seed (Bayer CropScience).
In Experiment 1 in 2012, seed of the Hessian fly–susceptible ‘USG 3295’ winter wheat was treated with various rates listed in Table 3 of clothianidin (NipsIt Inside) combined with metconazole (0.44%) and metalaxyl at (0.99%) fungicides by Valent BioSciences (Libertyville, IL). In Experiment 2 in 2012, USG 3295 wheat seed was treated with selected rates of imidacloprid (Attendant 480) combined with Rancona fungicide (ipconazole at 2.29% + metalaxyl at 1.94%) by Chemtura Corp. (Philadelphia, PA). Treatment plots in all experiments were arranged in a randomized compete block design with four or five replications.
Plant stand was measured at 25–40 d after planting by counting all plants in two 1-m sections of row per plot. Hessian fly populations were assessed in March at grain head emergence stage in 1993–1994 and in early February at fill tiller stage and late April at milk-early dough stage in 1995 and 1996. Infestations in 2013 were assessed on 12 December, 9 January, 4 February, and 24 April when plants were in the three-leaf, three- to four-tiller, full tiller, and milk stage of development (Feekes 10.5), respectively. Hessian fly populations were sampled by collecting a 30-cm section of row from each plot and dissecting all tillers or stems. The number of Hessian fly immatures (larvae and pupae) per plant and the proportion of infested tillers per stem were recorded. Insect samples were collected in one half of each plot. The other half was harvested with a Hege small plot combine in late May each year to measure seed weight, test weight, and moisture content. Grain yields were calculated and adjusted to 13.5% moisture content.
Data in each experiment were analyzed with a one-way analysis of variance for a randomized complete block design with treatment modeled as a fixed effect and replication modeled as a random effect by using PROC MIXED (SAS Institute 2013). Degrees of freedom were calculated using the Kenwood–Rogers method. To normalize data, percentage values were transformed with an angular transformation before analysis and insect counts and yield were transformed with a log(x + 1) before analysis. Means and SEs from PROC MEANS are presented. When significance was indicated, LS means were separated using pairwise t test of the PROC PLM procedure at α = 0.05 (SAS Institute 2013).
Results
Plant stands at 25–40 d after planting were not affected by treatments at Sandersville in 1993 (P = 0.4568) and Plains in 1994 (P = 0.2336), but were significantly lower in the disulfoton treatment than the untreated check and seed treatments at Plains in 1993 (P = 0.03571) and 1995 (P = 0.0167; data not shown). In the 2012 and 2013 experiments, the plant stand was less in the untreated check than all of the fungicide and insecticide plus fungicide seed treatments at 22 d after planting (F = 5.24; df = 7, 21; P = 0.0014 and F = 4.38; df = 7, 21; P = 0.0039, respectively). At 35 d after planting, plant stands were lower in the untreated check than the other treatments, but these differences were not significant (F = 1.85; df = 7, 21; P = 0.1308 and F = 1.95; df = 7, 21; P = 0.1118; data not shown).
Hessian fly infestations and immature numbers per stem were significantly lower in all seed treatments and the disulfoton treatment than the untreated control at both locations in 1993 and 1994 (Table 1). In 1994 and 1995, only the disulfoton and the 0.62-g rate of imidacloprid (Gaucho 480FS) had reduced Hessian fly infestation and immatures per stem compared with the untreated check in the winter sample in January at 105 d after planting (Table 2). The 0.155- and 0.31-g rates of imidacloprid did not reduce infestation and immature numbers per stem. Dimethoate had lower infestation at Plains in 1993 and 1994, but did not reduce infestations at Sandersville in 1993 and 1994 and at Plains in 1994 and 1995. Spring infestations in April 1995 at 185 d after planting were similar to those of the untreated check in all treatments. In the 1995–1996 experiment, all treatments significantly reduced the winter infestations in late January at 77 d after planting, but disulfoton was the most effective treatment with the imidacloprid seed treatments being partly effective in reducing infestations (Table 2). The disulfoton and 0.31-g rate of imidacloprid (Gaucho 480FS) also reduced the spring infestation, but the 0.155- and 0.233-g rates did not as compared to the untreated check.
For Experiment 1 in 2012 and 2013, all clothianidin seed treatments significantly reduced Hessian fly infestations and immature numbers per stem on 12 December and 9 January, except for the lowest rate of 0.098 g a.i./kg of seed (Table 3). On 9 January, the highest rate of clothianidin at 0.70 g a.i./kg (1.79 fl oz) was more effective than the lower rates. Hessian fly infestations and immature numbers were not significantly different among treatments in the 4 February and 24 April samples at full tiller and anthesis stage, respectively. For Experiment 2 in 2012 and 2013, all the Attendant imidacloprid (Attendant 480) seed treatments had significantly lower infestations and immature numbers than the two fungicide seed treatments and the untreated check on 12 December, 9 January, and 4 February (Table 4). The spring infestations on 24 April were not significantly different among treatments. The thiamethoxam (Cruiser 5FS + Dividend Extreme) standard also reduced Hessian fly infestations and immature numbers, but was less effective than the imidacloprid treatments on 9 January.
Grain yield was not significantly affected by treatments at either location in 1993 and 1994 (Table 1). Treatments also did not significantly affect grain yield at Plains in 1994 and 1995, although the 0.313- and 0.626-g rates of imidacloprid (Gaucho 480FS) yielded 239 and 413 kg/ha more than the untreated check (Table 2). Grain yield in 1995 and 1996 was significantly lower in the disulfoton and untreated check than the 0.157- and 0.313-g rates of imidacloprid (Table 2). These treatments yielded 504 and 719 kg/ha more, respectively, than the untreated check. Grain yields in Experiment 1 in 2012 and 2013 with clothianidin seed treatments were not significantly different among treatments, even though the 0.70 g a.i./kg rate of clothianidin yielded 572 kg/ha more than the untreated check (Table 3). In Experiment 2 in 2012 and 2013, the thiamethoxam at 0.32 g a.i./kg and imidacloprid (Attendant + Foothold fungicide) at 0.50 g a.i./kg of seed yielded significantly more than the untreated check and fungicide-only treatments (Table 4). Grain test weights were not significantly affected by treatments in any experiment (P > 0.10; data not shown).
Discussion
The most effective insecticide treatment in the first three experiments was an in-furrow application of disulfoton at 1.12 kg a.i./ha, but disulfoton is no longer available for use in the United States (U.S. EPA 2016). None of the foliar applied insecticide treatments were effective in controlling Hessian fly infestations. All the neonicotinoid seed treatments tested provided control of Hessian fly infestations in the autumn after planting of winter wheat. Both imidacloprid (Guacho 480FS and Attendant 480) at 0.31 g a.i./kg of seed (1.0 fl oz/100 lb of seed) and clothianidin 600FS at 0.39 g a.i./kg (1.0 fl oz/100 lb of seed) or higher rates provided consistent reductions in Hessian fly infestations during autumn and early winter. The lower rates of these products were less effective and did not provide consistent reductions in autumn infestations. Thiamethoxam was only evaluated at the rates of 0.295 and 0.32 g a.i./kg of seed (0.75–0.81 fl oz/100 lb of seed) in two experiments and was less effective at this rate than the highest rates of imidacloprid and clothianidin, but was similar in efficacy to imidacloprid and clothianidin at the same rates. None of the seed treatments persisted to control spring infestations during the wheat reproductive stage, which is consistent with results of previous studies (Howell et al. 2017, Wilde et al. 2001, Winn et al. 2023).
Yield responses were variable, with significant yield differences occurring on two of the five experiments. Nevertheless, considering the numerical yield differences in yield between the untreated check and imidacloprid and clothianidin at 0.31–0.62 g a.i./kg of seed (1.0–2.0 fl oz per 100 lb of seed) in all experiments, the average yield response was positive in eight of nine comparisons and equaled 285.9 ± 92.7 kg/ha or 4.76 bushels/acre at 60 lb per bushel. The price of soft red winter wheat in November 2023 (U.S. Wheat Associates 2023) was approximately $US0.22/kg (US$6.00/bushel), which indicates the seed imidacloprid treatments at 0.31 g a.i./kg of seed and clothianidin treatments at 0.39 g a.i./kg of seed or more provided an average benefit of US$70.57/ha. The current cost for an insecticide-fungicide seed treatment at a standard seeding rate is approximately US$37.00/ha, based on price quotations from seed dealers in Georgia. Therefore, the use of neonicotinoid seed treatments at high rates may be justified when the risk of Hessian fly damage is present for susceptible varieties. Although the use of plant resistance is the most effective method to control Hessian fly (Buntin et al. 1992), when adapted high-yielding varieties with effective resistance are not available and the risk of Hessian fly damage is present, neonicotinoid insecticide seed treatments provide an alternative method of control of Hessian fly on susceptible varieties of winter wheat.
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