Fall armyworms, Spodoptera frugiperda (J. E. Smith), can be severely damaging to turfgrass, especially in the southeastern United States. Problems often occur in late summer or fall after migrating populations have increased during the season (Reinert and Cobb 2012). Seashore paspalum, Paspalum vaginatum Swartz, is a warm-season perennial grass valued for its salt tolerance (Duncan and Carrow 2000). Additionally, it is tolerant to drought, poor soils and low mowing levels, resulting in its popularity for use in golf courses and other turfgrass settings (Duncan and Carrow 2000). Previous experiments have identified a gradient in resistance among warm-season turf grasses to the fall armyworm (Braman et al. 2000, 2002). Paspalum vaginatum taxa, in general, were more susceptible to fall armyworm than other warm-season grasses such as centipedegrass [Eremochloa ophiuroides (Munro) Hack.], zoysiagrass (Zoysia Willd.), and some bermudagrasses (Cynodon Rich.). Identification and utilization of turfgrasses with resistance to the major turfgrass pests is an important component of integrated pest management strategies in turf production.

Various evaluations of pasture and turf grasses have revealed antibiosis as a mechanism of resistance against the fall armyworm (Wiseman et al. 1982, Chang et al. 1986). This was indicated by smaller size and delayed development of larvae feeding on the resistant grasses. A study using diet-incorporated plant tissue, identified antibiosis among some Paspalum species (Wiseman and Duncan 1996) when 81 Paspalum taxa were evaluated for potential resistance to fall armyworm. The objective of the experiments described herein was to compare the response of fall armyworm on fresh plant tissue among turf-type Paspalum taxa with that on other Paspalum species. The zoysiagrass taxon ‘Cavalier’ (Braman et al. 2000, Reinert and Engelke 2010) was used as a resistant control.

Materials and Methods

Paspalum vaginatum taxa were obtained from the UGA Paspalum Breeding Program whereas Paspalum spp. taxa were grown from seed obtained from the USDA National Plant Germplasm System (Table 1). Seeds were germinated under mist conditions and then transplanted to 15cm standard plastic pots containing Metro Mix 300 (Sun Gro Horticulture Canada Ltd.). Plants were fertilized at transplanting with a controlled-release fertilizer (Osmocote 18 - 6-12; The Scotts Company, Marysville, OH) and regularly thereafter with a water-soluble general purpose fertilizer (Peter's 20 - 20 - 20; The Scotts Company, Marysville, OH).

Table 1. Developmental parameters of fall armyworm when fed grass clippings of 22 turfgrass taxa in a growth chamber study (Trial 1).

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Table 1. Continued.

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For growth chamber studies, clippings from the Paspalum spp. and Zoysia sp. grasses grown and maintained in greenhouses at the UGA Griffin campus were provided daily to the fall armyworm larvae. Fall armyworm eggs from a commercial supplier (Benzon Research, Carlisle, PA) were held at 27°C until eclosion. Neonate larvae were confined individually in individual 32-ml clear plastic diet cups with snap-on paper lids (Bio-Serv, Frenchtown, NJ) containing fresh clippings of the grasses of interest. These cups were arranged in trays. Three different growth chamber trials (no-choice) were conducted, using randomized complete block design. In the first trial, 22 turfgrass taxa (Trial 1, Table 1) were included with 60 larvae per taxon (12 replications of 5 larvae each). In the second and third trials (Trials 2 and 3, Table 2), 10 turfgrass taxa were selected to represent a range in response to the fall armyworm. Thirty larvae were tested per taxon in 6 replications of 5 larvae each. Each cup within a tray was labeled with a number that corresponded to one of the turfgrass taxa selected as a treatment. The trays were randomly stacked within an environmental chamber maintained at constant temperature (24°C) and photoperiod (15:9 Light hours: Dark hours) for the duration of the study.

Table 2. Developmental parameters of fall armyworm when fed grass clippings of 10 turfgrass selections in growth chamber studies (Trials 2 and 3).

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Daily feeding of grass clippings continued until all larvae had either successfully pupated or died. Larval survival per treatment was recorded at 8 d and at pupation. In addition to larval survival, larval weight at 15 d, pupal weight, days to develop to pupal stage and adult emergence were recorded. Larval and pupal weights, days to develop to pupal stage and survival were used to determine potential antibiosis among Paspalum taxa in comparison with resistant ‘Cavalier’ zoysiagrass.

Two greenhouse studies on whole plants (Trial 4 and 5), were performed using 10 turfgrass taxa selected to represent a range in response to the fall armyworm (Table 3). Both studies used randomized complete block design. Plants in 15cm diam plastic pots were maintained at 31°C and photoperiod (12:12 L:D) for the duration of the study. The plants were watered daily during the study. Ten larvae were placed on a plant, and 2 such plants were enclosed in a cage (BugDorm, Bioquip, Inc.) in Trial 4, whereas 3 plants were placed in a cage in Trial 5, and this was replicated twice. Thus, there were a total of 40 larvae (8 replications of 5 larvae each) per treatment in Trial 4, and 60 larvae (12 replications of 5 larvae each) in Trial 5. Larval survival per treatment, larval weight at 15 d, pupal weight, days to develop to pupal stage and adult emergence were recorded as in the growth chamber studies.

Table 3. Developmental parameters of fall armyworm feeding directly on 10 turfgrass selections in greenhouse studies (Trials 4 and 5).

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Statistical analyses. Data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) procedure in SAS (SAS Institute 2003). Mean separation was by least significant difference (LSD).

Results and Discussion

Growth chamber trials. Six of the 22 turfgrass taxa used in Trial 1 did not support development to at least 8 d and these included ‘Cavalier’ zoysiagrass and 5 of the 13 Paspalum spp. taxa: P. macrophyllum Kunth. (PI 282,807), P. notatum Flüeggé (PI 310.146), Paspalum notatum Flüeggé var. saurae Parodi (PI 422,024), P. plicatulum Michx. (PI 462,290) and P. quadrifarium Lam. (PI 462,298) (Table 1). Sixteen of the 22 taxa supported growth and development of the fall armyworm, to at least 8 d.

Larval and pupal weights and days to develop to the pupal stage were significantly influenced by turfgrass selection. Survival of the initial 60 neonate larvae to 8 d ranged from 3.3% on P. minus (PI 404,465) to 76.7% on P. vaginatum ‘Sea Isle Supreme’. Larvae on the Paspalum selection P. thunbergii Kunth ex Steud. (PI 286,486) had the lowest numerical larval weights, but these were statistically similar to those on P. dilatatum Poir. (PI 419,929), P. minus Fourn. (PI 404,465), P. ionanthum Chase (PI 404.449), P. vaginatum ‘Aloha’, P. nicorae Parodi (PI 209,983), P. conjugatum P.J. Bergius (PI 404,643), P. vaginatum ‘Sea Spray’, P. vaginatum ‘Sea Isle 2000’ and P. dilatatum Poir. subsp. flavescens Roseng. et al. (PI 477,100).

Larvae did not survive to the pupal stage on the Paspalum spp. taxa P. ionanthum (PI 404,449), P. minus (PI 404,465) and P. thunbergii (PI 286,486). Among the taxa supporting larval development beyond 8d, survival to pupation ranged from 5% on P. vaginatum ‘Sea Spray’ to 71.7% on P. vaginatum ‘Sea Isle Supreme’ and P vaginatum ‘Durban’. The lowest numerical pupal weight was observed on P. pubiflorum Rupr. ex Fourn. (PI 304,147), which was statistically similar to those on P. conjugatum (PI 404,643), P. nicorae (PI 209,983) and P. dilatatum subsp. flavescens (PI 477,100). Larvae on P. vaginatum ‘Sea Spray’ took the greatest number of days to reach pupal stage, but this duration was statistically similar for larvae on P. nicorae (PI 209,983), P. conjugatum (PI 404,643), P. pubiflorum (PI 304,147), P. vaginatum ‘Aloha’ and P. dilatatum (PI 419,929). The lowest percentage of adult emergence was observed on P. vaginatum ‘Sea Spray’, which was statistically similar to those on P. pubiflorum (PI 304.147) and PI 404,643 (P. conjugatum), P. vaginatum ‘Salam’, P. nicorae (PI 209,983) and P. dilatatum subsp. flavescens (PI 477,100).

The taxa P. vaginatum ‘Sea Isle Supreme’, P. vaginatum ‘Sea Dwarf’, P. vaginatum ‘Sea Isle 1’ and P. vaginatum ‘Durban’ were highly susceptible to the fall armyworm, as indicated by high larval and pupal weights, shorter time to pupation and high percentage of adult emergence (Table 1).

In growth chamber trials with the 10 selected taxa (Table 2 shows averages over Trials 2 and 3), lowest numerical larval weights at 8 d were observed on P. ionanthum (PI 404,449), and these were statistically similar to those on P. notatum var. saurae (PI 422,024), P. quadrifarium (PI 462,298), P. notatum (PI 310,146) and P. minus (PI 404,465). Percentage of larval survival ranged from 3.33% on P. ionanthum (PI 404.449) to 66.67% on P. vaginatum UGA-22. Although larval weights were low on P. minus (PI 404,465), the survival of these larvae was high, and similar to that on the highly susceptible P. vaginatum UGA-22. Larvae did not survive to pupation on P. ionanthum (PI 404,449). Pupal weights were significantly low on P. notatum (PI 310,146) and P. thunbergii (PI 286,486) than the other taxa, which were statistically similar with respect to pupal weights. Highest percentage of adult emergence was on P. minus (PI 404,465) and this was similar to that on the highly susceptible P. vaginatum UGA-22 and P. vaginatum ‘Seastar’, indicating the suitability of these taxa to the fall armyworm. Larvae did not survive and develop on ‘Cavalier’ zoysiagrass in these trials.

Greenhouse trials. In the greenhouse trials on whole plants (Table 3 shows averages over Trials 4 and 5), developmental parameters were not always consistent with those obtained on grass-clipping studies in the growth chamber. Although 2% of the larvae survived to 8 d in the second greenhouse trial on ‘Cavalier’ zoysiagrass, none of the larvae survived to pupation. The larval weights on this taxon were similar to those on all the others except the P. vaginatum taxa UGA-22 and ‘Seastar’, and P. quadrifarium (PI 462,298). Larvae on P. ionanthum (PI 404,449) did not survive to pupation. Lowest pupal weights were observed on P. thunbergii (PI 286,486), which was significantly lower than those on P. vaginatum ‘Seastar’, but similar to those on all the other taxa. Larvae on P. macrophyllum (PI 282,807) took the longest time to reach pupation, whereas those on the P. vaginatum taxa UGA-22 and ‘Seastar’ and P. quadrifarium (PI 462,298) took the shortest. Paspalum vaginatum ‘Seastar’ and P. vaginatum UGA-22 appeared to be suitable (susceptible) hosts, as indicated by high larval and pupal weights, fewer days and higher survival to pupation, and to adult emergence, although differences with other taxa were not always significant. The least suitable (resistant) hosts among Paspalum spp. taxa was PI 404,449 (P. ionanthum), because larvae did not survive to pupation on this taxon in both trials.

Our studies compare the performance of fall armyworm on turf -type Paspalum taxa with that on other Paspalum species measured through studies on excised grass clippings in a growth chamber and on whole plants in a greenhouse (Table 4). Whereas many of the tested taxa supported growth and development of fall armyworm larvae in at least one experiment, PI 404,449 (P. ionanthum) showed consistent low susceptibility in growth chamber, excised-clipping and greenhouse, whole-plant studies. The taxa PI 310,146 (P. notatum), PI 422,024 (P. notatum var. saurae), PI-282807 (P. macrophyllum) and PI 286,486 (P. thunbergii) showed low susceptibility in growth chamber, excised-clipping studies, but low susceptibility was not as marked in greenhouse trials. Feeding on these taxa resulted in low larval and pupal weights, and longer time to reach the pupal stage, indicating antibiosis to some degree. Paspalum vaginatum ‘Seastar’ and P. vaginatum UGA-22 appeared to be the most suitable (susceptible) hosts, as shown by larval and pupal weights and days to develop to pupal stage. The zoysiagrass selection ‘Cavalier’ was resistant to fall armyworm, consistent with earlier reports (Braman et al. 2000, Reinert and Engelke 2010). Low input requirement of P. vaginatum has resulted in increased interest in the development of various taxa and selections that require assessment of their pest resistance. Our results are consistent with those from earlier studies (Braman et al. 2000) and also demonstrate resistance in additional species.

Table 4. Response of fall armyworm on Paspalum taxa.

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Significant morphological variability exists within P. vaginatum (Duncan and Carrow 2000) and, likewise, the degree of fall armyworm resistance may also vary among individuals within species. Therefore, information on pest susceptibility of various taxa within the species would help breeding efforts to develop of pest-resistant taxa and simplify management. Managed turf demands high levels of aesthetic perfection, and breeding objectives are often driven by this demand, whereas pest management is usually achieved by the use of effective chemical pesticides. However, concern for environmental stewardship and development of insecticide resistance provide impetus for incorporating pest resistance into breeding objectives, along with other desirable horticultural and agronomic traits.