Investigating the combined effect of drought stress and neonicotinoid use on two-spotted spider mite, Tetranychus urticae, Koch, outbreaks in corn

Recently, much of the Intermountain West has been under the influence of drought and this is predicted to increase due to climate change. Neonicotinoids have also become the most extensively used class of insecticides in pest management. The consequences of these two factors for agricultural systems and plant-arthropod interactions are not well understood. Drought and neonicotinoid insecticides are two factors implicated in mite outbreaks. Elevated temperatures and prolonged dry conditions are the environmental factors that mostly promote spider mite proliferation. Neonicotinoids have been shown to modify plant physiological pathways that enhance spider mite reproductive rate and make plants more susceptible to arthropod herbivory.

What is not clear is how these factors interact on spider mites feeding on corn. The objective of our study is to identify corn plant inducible defenses that are significantly affected by the combination of the two factors in order to verify their consequences on spider mite ecology. We setup a field study where we provided high and low levels of water to different neonicotinoid seed treatments of corn (in this case we used Clothianidin). Mites were placed on plants at the beginning of the corn biological cycle and counted by the end of the summer season. Lab bioassays have been done to test plant defense protein production (trypsin, chitinase, peroxidase, polyphenoloxidase) in leaves.

The results showed that Clothianidin stimulated the proliferation of spider mites, especially on those plants subjected to low water irrigation. No significant differences have been found in the total protein concentration. However high water and absence of spide mites promotes the highest protein production. The lowest trypsin concentration was found in Clothianidin treated plants not fed by spider mites. As for chitinase, peroxidase and polyphenoloxidase there is no significant difference among the treatments.