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1 June 2016 Parasitism of Megacopta cribraria (Hemiptera: Plataspidae) by Paratelenomus saccharalis (Hymenoptera: Platygastridae) in Organic Soybean Plots in Georgia, USA
Glynn Tillman, Julia Gaskin, Dinku Endale, Carroll Johnson, Harry Schomberg
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Abstract

Megacopta cribraria (F.) (Hemiptera: Plataspidae) is a newly invasive, exotic pest of soybean (Glycine max [L.] Merr.; Fabales: Fabaceae) in the southeastern United States. In 2013, the exotic egg parasitoid Paratelenomus saccharalis (Dodd) (Hymenoptera: Platygastridae) was discovered parasitizing eggs of this pest in kudzu (Pueraria montana var. lobata [Willd.] Maesen & S. Almeida; Fabales: Fabaceae) and soybean in 3 states in this region of the United States. We evaluated parasitism of M. cribraria egg masses by P. saccharalis in conventional tillage and no-till organic soybean experimental plots in 2013. Density of M. cribraria egg masses was significantly higher in conventional tillage soybean than in no-till soybean in weeks 2 through 5 for the 7 wk period M. cribraria egg masses were detected on soybean. Percentage of parasitism of M. cribraria egg masses by P. saccharalis was significantly higher in conventional tillage soybean (58.4%) than in no-till soybean (44.9%). In general, parasitism rates of egg masses were higher in conventional tillage soybean, where M. cribraria egg mass density was higher, than in no-till soybean.

The kudzu bug, Megacopta cribraria (F.) (Hemiptera: Plataspidae), is a newly invasive, exotic pest of soybean (Glycine max [L.] Merr.; Fabales: Fabaceae) in the southeastern United States. This insect is native to Asia, where it tends to be an occasional pest of legumes, but it can be a serious pest at times (Wang et al. 1996; Thippeswamy & Rajagopal 1998). It was first reported in Georgia, where large numbers were discovered feeding on kudzu, Pueraria montana var. lobata (Willd.) Maesen & S. Almeida (Fabales: Fabaceae), in 9 northeastern counties in the fall of 2009 (Suiter et al. 2010). It spread rapidly from the original 9 Georgia counties to 11 states in 2013 (Megacopta Working Group 2015). The primary reproductive hosts of M. cribraria in Georgia are kudzu and soybean (Zhang et al. 2012). In soybean, excessive feeding by M. cribraria on stems, petioles, and leaves appears to weaken and stress plants, resulting in fewer pods per plant, fewer seeds per pod, and smaller seed size for infested plants compared with uninfested plants (Greene et al. 2012). In 11 field trials in Georgia in 2010 and 2011, M. cribraria populations reduced soybean yield by an average of 20% (Roberts & Whitaker 2012). Parasitism of eggs by native egg parasitoids was not observed in surveys conducted in Georgia in 2010 and 2011 (Ruberson et al. 2012).

In 2013, an exotic egg parasitoid identified as Paratelenomus saccharalis (Dodd) (Hymenoptera: Platygastridae) was found in M. cribraria eggs in kudzu and soybean in 3 states in the southeastern United States (Gardner et al. 2013). This egg parasitoid is widely distributed throughout the Eastern Hemisphere, and known hosts of the parasitoid are restricted to the family Plataspidae (Ruberson et al. 2012). It has been recorded attacking eggs of Megacopta species in various locations throughout Asia, including Japan where overall parasitism rates of eggs in the spring are often high (43–100%) (Takasu & Hirose 1986). In Alabama in 2013, parasitism of egg masses in conventional soybean ranged from 52 to 85% (Gardner et al. 2013). We evaluated parasitism of M. cribraria egg masses by P. saccharalis in conventional tillage and no-till organic soybean experimental plots in Georgia in 2013.

The study site was located within the University of Georgia Ponder Farm (31.5113889°N, 83.6444444°W) in Tift County, Georgia. The 2 treatments were conventional tillage soybean and no-till soybean. Each experimental plot was approximately 0.1 ha. Each treatment was randomly assigned to a plot within a replicate for each of 4 replicates in a randomized complete block design. Blind cultivation with a tine weeder was used for weed management in conventional tillage soybean. In the no-till treatment, cool-season wheat (Poales: Poaceae) was grown to suppress weeds. Wheat (‘Georgia Gore’) was planted on 7 Nov 2012. Group VII soybean (‘Woodruff’ soybean) was planted on 20 Jun 2013, 7 d after wheat had been harvested.

Beginning on 23 Jul 2013, soybean was sampled weekly for a 7 wk period. For each sample, all plants within a 1.83 m length of row were examined for M. cribraria egg masses. Twelve random samples were obtained per plot. Egg masses detected during sampling were collected and held in the laboratory for parasitoid emergence. Voucher specimens of all collected M. cribraria egg masses and emerged parasitoids were placed in ethanol and deposited at the United States Department of Agriculture, Agricultural Research Service, Crop Protection & Management Research Unit in Tifton, Georgia.

All data were analyzed using SAS statistical software (SAS 9.3; SAS Institute 2010). Kudzu bug egg mass count data were modeled by a Poisson distribution. The analyses were done using PROC GLIMMIX. Model fit was evaluated by use of the chi-squared and df statistic provided by PROC GLIMMIX (Littell et al. 2006). Fixed effects were treatment, week, and the treatment by week interaction. Random effects were replicate and residual error. Subsamples (12 per plot) were pooled. Means were back transformed using the ILINK option in the LSMEANS statement and compared using Tukey's honestly significant difference (HSD) test. Data for parasitism rates of kudzu bug egg masses were analyzed using PROC MIXED. Fixed effects were treatment, week, and the treatment by week interaction. Random effects were replicate and residual error. Subsamples were averaged. Arcsine square-root transformation was used to normalize percentage parasitism data. Means were separated using Tukey's HSD test when appropriate.

Megacopta cribraria egg masses were detected on conventional tillage and no-till organic soybean for 7 wk. The treatment by week interaction was significant for density of M. cribraria egg masses (F = 14.55; df = 6,39; P = 0.001). The mean number of M. cribraria egg masses per sample was significantly higher in conventional tillage soybean than in no-till soybean in weeks 2 through 5 (Fig. 1A). In conventional tillage soybean, density of M. cribraria egg masses was significantly higher in weeks 3 and 4 than in the remaining weeks. In no-till soybean, density of M. cribraria egg masses was significantly higher in weeks 4 and 6 than in the remaining weeks. Megacopta cribraria may have been able to colonize conventional tillage soybean more easily than no-till soybean with wheat stubble.

Fig. 1.

A) Density of Megacopta cribraria egg masses (mean ± SE) in organic soybean in 2013. Means with the same lowercase letter are not significantly different between treatments per week, and means with the same uppercase letter are not significantly different among weeks per treatment (Tukey's HSD test, P > 0.05). B) Percentage of parasitism (mean ± SE) of M. cribraria egg masses by Paratelenomus saccharalis over time in organic soybean in 2013. Means with the same uppercase letter are not significantly different (Tukey's HSD test, P > 0.05).

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Paratelenomus saccharalis parasitized M. cribraria egg masses during the time eggs were present on soybean. Percentage of parasitism of M. cribraria egg masses by P. saccharalis was influenced by treatment (F = 8.44; df = 1,39; P = 0.006) and week (F = 19.38; df = 6,39; P = 0.0001), but the treatment by week interaction was not significant (F = 2.26; df = 6,39; P = 0.0574). Percentage of parasitism of M. cribraria egg masses by P. saccharalis was significantly higher in conventional tillage soybean (58.4%) than in no-till soybean (44.9%). Parasitism rates of egg masses were high (82–95%) the last 3 wk of the study and were significantly higher in these last 3 wk than in the previous weeks (Fig. 1B). In general, parasitism rates of egg masses were higher in conventional tillage soybean, where M. cribraria egg mass density was higher, than in no-till soybean.

Acknowledgments

The authors thank Kristie Graham, Jacob Barrett, and Chance Myers (United States Department of Agriculture, Agricultural Research Service, Crop Protection & Management Research Unit, Tifton, Georgia) for their technical assistance.

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Glynn Tillman, Julia Gaskin, Dinku Endale, Carroll Johnson, and Harry Schomberg "Parasitism of Megacopta cribraria (Hemiptera: Plataspidae) by Paratelenomus saccharalis (Hymenoptera: Platygastridae) in Organic Soybean Plots in Georgia, USA," Florida Entomologist 99(2), 300-302, (1 June 2016). https://doi.org/10.1653/024.099.0223
Published: 1 June 2016
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