Weekly Survivorship Curves of Soybean Aphid Biotypes 1 and 4 on Insecticidal Seed-Treated Soybean

Abstract

Thiamethoxam, an insecticide used in soybean seed treatments, effectively suppresses soybean aphids (Aphis glycines) Matsumura (Hemiptera: Aphididae) for a short time after planting. However, exactly when and how quickly soybean aphid populations could increase is unknown. Likewise, we lack data on virulent soybean aphid biotypes (that can overcome soybean resistance) when fed on seed-treated soybean. Determining the survival of soybean aphids over time on insecticidal seed-treated soybean is critical for improving soybean aphid management and may provide insights to manage aphid virulence to aphid resistant-soybean. In greenhouse and field experiments, aphid-susceptible soybean plants (with and without an insecticidal seed treatment) were infested at 7, 14, 21, 28, 35, and 42 days after planting (DAP). We compared aphid survival among biotypes 1 (avirulent) and 4 (virulent) and insecticide treatment 72 h after infestation. We also measured thiamethoxam concentrations in plant tissue using liquid chromatography-tandem mass spectrometry. As expected, soybean aphid survival was significantly lower on seed-treated soybean up to 35 DAP for both biotypes, which correlates with the decrease of thiamethoxam in the plant over time. Moreover, we found no significant difference between avirulent and virulent biotype survivorship on insecticidal seed-treated soybean plants, although we did find significantly greater survival for the virulent biotype compared with the avirulent biotype on untreated soybean in the field. In conclusion, our study further characterized the relative short duration of seed treatment effectiveness on soybean aphid and showed that survivorship of virulent aphids on seed-treated soybean is similar to avirulent aphids.

Thiamethoxam Differentially Impacts the Survival of the Generalist Predators, Orius insidiosus (Hemiptera: Anthocoridae) and Hippodamia convergens (Coleoptera: Coccinellidae), When Exposed via the Food Chain

“…The effects of thiamethoxam on natural enemies via the food chain are likely stronger at early time points after plant treatment ( Grafton-Cardwell and Gu 2003 , Douglas et al 2015 ), when presumably the insecticide concentration within plants is still high ( Castle et al 2005 , Krupke et al 2017 , Esquivel et al 2019 ). However, in some systems thiamethoxam has several weeks of insecticidal bioactivity after treatment ( Castle et al 2005 , Diez-Rodrĩguez et al 2006 , McCornack and Ragsdale 2006 , Seagraves and Lundgren 2012 , Krupke et al 2017 , Esquivel et al 2019 ) which might facilitate toxicity to predatory natural enemies via the food chain at later time points. Yet, evidence of thiamethoxam toxicity via the food chain is limited to few predatory natural enemy species, and available mostly for early time points after thiamethoxam treatment.…”

“…seeds were Mycogen (a subsidiary of Corteva Agrisciences Indianapolis, IN) variety 5N248R2, treated with Cruiser Maxx (Syngenta, Greensboro, NC) containing thiamethoxam (56.3 g of active ingredient), and the fungicides mefenoxam (3.75 g of active ingredient) and fludioxonil (2.5 g of active ingredient), per 100 kg of seed. To obtain untreated seeds (without insecticide or fungicides), we removed the treatment following the serial washing protocol utilized by Esquivel et al (2019) . Three soybean seeds (treated or untreated) were planted into a plastic pot (Kord Regal, Toronto, Canada; 10.1 cm upper diameter, 7.6 cm lower diameter, 8.9 cm height) filled with soilless media Pro-Mix BX (Québec, Canada).…”

“…Immediately after collecting leaves from plants, we transferred a total of 10, 7-d-old adult soybean aphids, green peach aphids or melon aphids to soybean, zinnia or green pepper plants, respectively, to estimate the toxicity of thiamethoxam present in plants. Aphids on plants were confined with clip cages as described in Esquivel et al 2019 . Mortality of soybean aphids, green peach aphids, and melon aphids was recorded 72 h after plant infestation (note: soybean aphid mortality values in the experiment with insidiosus flower bug, also include the mortality caused by the predator and it was not performed using clip cages).…”

“…For mean separation we used Tukey HSD post-hoc test with 95% family-wise confidence for mean separation of main effects, and LSMEANS for interactions (SAS version 9.4). For details about thiamethoxam residues in soybean and zinnias after planting/treatment under similar experimental conditions, please refer to Esquivel et al (2019) and Derksen et al (2015) .…”

“…These studies imply that the prey functioned as a biological bridge for thiamethoxam to move from treated plants to predators up to 10 wk after application ( Grafton-Cardwell and Gu 2003 ). Empirical evidence shows that thiamethoxam can remain bioactive against target pests for 5–25 wk ( Castle et al 2005 , Diez-Rodrĩguez et al 2006 , Qureshi et al 2014 , Krupke et al 2017 , Esquivel et al 2019 ), suggesting that the time period of thiamethoxam trophic toxicity could also last for several weeks. Nonetheless, the severity and length of trophic thiamethoxam toxicity on natural enemies is expected to change under different conditions (e.g., crop and its phenological stage, thiamethoxam dose, insect pest feeding on plant, susceptibility of the natural enemy to thiamethoxam, among others).…”