The effects of insecticide dose and herbivore density on tri-trophic effects of thiamethoxam in a system involving wheat, aphids, and lady beetles

Authors: BREDESON, MICHAEL - South Dakota State University; REESE, R - South Dakota State University; Lundgren, Jonathan

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2014
Publication Date: 12/27/2014
Citation: Bredeson, M.M., Reese, R.N., Lundgren, J.G. 2014. The effects of insecticide dose and herbivore density on tri-trophic effects of thiamethoxam in a system involving wheat, aphids, and lady beetles. Crop Protection. 69:70-76.

Interpretive Summary: Neonicotinoid insecticides are commonly used in modern cropping systems, including in wheat production. As the crop ages, these systemic insecticides begin to dissipate to sublethal concentrations. There is concern whether these sublethal doses may affect beneficial insects like predators (i.e., lady beetles) when their herbivore prey (aphids) consumes the treated plants. We tested whether thiamethoxam-treated plants can harm the native lady beetle, Coleomegilla maculata when it eats bird-cherry oat aphids (Rhopalosiphum padi). We found that the dose of the insecticide and the number of aphids on the wheat plants both affected the quantity of toxin detected in the plants and aphids, as well as how these aphids affected young lady beetle larvae. Specifically, aphids that were less crowded on the plants were of poorer quality for the beetle larvae. These results may be important for scientists interested in designing toxicity assays that can inform risk assessments of these insecticides.

Technical Abstract: We assess how herbivore density and insecticide dose affects the tri-trophic effects between thiamethoxam-treated wheat (Triticum sp.), Rhophalosiphum padi and the predatory Coleomegilla maculata DeGeer. In the first experiment 2nd and 4th instar C. maculata were fed aphids reared for 24 h on wheat plants treated with sub-lethal thiamethoxam soil drenches to assess the effect of insecticidal dose on the predator. In the second experiment a constant, sub-lethal thiamethoxam soil drench was used on wheat plants, but the plants were infested with different densities of aphids. Aphids from each density treatment were then fed to C. maculata 2nd instars. Insecticide levels within wheat and aphid tissues were quantified using a competitive ELISA. In experiment one, 4th instars of C. maculata displayed no negative effects after consuming aphids off of treated plants. However, 2nd instars of C. maculata ate more treated aphids than untreated aphids and larval development times varied between treatments. In experiment two, the 2nd instar C. maculata displayed slower walking speeds after consuming aphids from low aphid density plants. Thiamethoxam within wheat tissue increased with increasing dose and was not observed in untreated plants. Thiamethoxam was found at higher levels in wheat plants with the lowest aphid density. Clothianidin was found in aphids that had fed on thiamethoxam-treated wheat but no differences between treatments were observed. The neonicotinoid seed-treatment altered the quality of R. padi as prey for 2nd instar C. maculata, but this depended on the aphid infestation level on the plants. In addition to revealing a potential deleterious effect of thiamethoxam systemic insecticides in wheat, our assays provide insight into the design and interpretation of toxicity assays involving systemic neonicotinoids and herbivores.