Assessment of acute toxicity tests and rhizotron experiments to characterise lethal and sub-lethal control of soil-based pests

Authors: AGATZ, ANNIKA - University Of York; SCHUMANN, MARIO - Georg August University; French, Bryan; BROWN, COLIN - University Of York; VIDAL, STEFAN - Georg August University

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2018
Publication Date: 5/17/2018
Citation: Agatz, A., Schumann, M., French, B.W., Brown, C.D., Vidal, S. 2018. Assessment of acute toxicity tests and rhizotron experiments to characterise lethal and sub-lethal control of soil-based pests. Pest Management Science. https://doi.org/10.1002/ps.4922.
DOI: https://doi.org/10.1002/ps.4922

Interpretive Summary: As an underground pest of corn, it is difficult to assess the effectiveness of insecticides used to control the larvae of western corn rootworm (WCR). In part, this is due to the complex interactions between plants, soil, and pests. There are three common insecticides producers use to control WCR populations; clothianidin (neonicotinoid), chlorpyrifos (organophosphate), and tefluthrin (pyrethroid). In acute toxicity tests using petri dishes we found that the concentrations needed to effectively prevent WCR larvae from feeding on corn roots were higher based on when the insecticides were introduced to the market; organophosphate > pyrethroid > neonicotinoid. However, when using simulated soil profiles to ascertain the effective concentrations in controlling WCR larval, the concentrations did not increase based on when the insecticides were introduced to the market. This may be due to less insecticide contact with the larvae because of interactions of the insecticides with the soil and larval ability to avoid contact with the insecticides in the soil. Our experiments have the potential to enhance efficacy testing and product development and might be useful tools for assessing WCR resistance in the future.

Technical Abstract: Assessing the efficacy of plant protection products is particularly challenging for products designed to combat soil based pests due to the complex and dynamic interplay of the system components. Here we present two types of efficacy studies, acute toxicity experiments (homogenous exposure of individuals in soil) and rhizotron experiments (heterogeneous exposure of individuals in soil) conducted with larvae of the western corn rootworm Diabrotica virgifera virgifera LeConte and three pesticide active ingredients (clothianidin (neonicotinoid), chlorpyrifos (organophosphate), and tefluthrin (pyrethroid)). The order of compound concentrations needed to invoke a specific effect intensity (EC50 values), within the acute toxicity tests of organophosphate > pyrethroid > neonicotinoid follows the chronological order of their introduction onto the market. This is not the case for the rhizotron experiments because application type, fate and transport of the compounds in the soil profile and sub-lethal effects on larvae (i.e. avoidance behaviour and emergency pupation) also affect their effectiveness in controlling larval feeding on corn roots. Beyond the pure measurement of efficacy through observing relative changes in plant damage to control plants, the tests allow to gather mechanistic understanding for drivers of efficacy apart from acute toxicity. The experiments have the potential to enhance efficacy testing and product development and might be useful tools for assessing resistance development in the future.