Oviposition response of monarch butterfly (Lepidoptera: Nymphalidae) to imidacloprid-treated milkweed

Authors: MULLINS, ALEXANDER - Iowa State University; BRADBURY, STEVEN - Iowa State University; Sappington, Thomas; ADELMAN, JAMES - University Of Memphis

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2021
Publication Date: 5/19/2021
Citation: Mullins, A., Bradbury, S., Sappington, T.W., Adelman, J. 2021. Oviposition response of monarch butterfly (Lepidoptera: Nymphalidae) to imidacloprid-treated milkweed. Environmental Entomology. 50(3):541-549. https://doi.org/10.1093/ee/nvab024.
DOI: https://doi.org/10.1093/ee/nvab024

Interpretive Summary: Monarch butterflies migrate from northern states to Mexico to spend the winter, then migrate back northward in the spring. Maintenance and growth of monarch populations depends largely on successful summer reproduction in the Midwestern U.S., but this is also a region of great agricultural land use. Monarch caterpillars can feed only on milkweed plants, and milkweed populations have declined as herbicidal control of weeds in row crops has greatly improved over the last 25 years. The reduction in milkweed is responsible in part for a steady decline in monarch population size to an alarmingly low level. Monarch conservation efforts are calling for restoration of 1.3 billion milkweed stems into the Midwestern landscape to support summer reproduction. To reach this goal, establishment of habitat containing milkweed in the agricultural landscape is being attempted, but the nearness of such habitat to crops can result in exposure to insecticides. A class of insecticide called neonicotinoids is widely used as a treatment on seeds of many different crops, and is taken up into the plant through the roots. Such "systemic" treatment protects the seedling crop plant from early season insect pests for several weeks. A common neonicotinoid is imidacloprid, which is also sprayed on the leaves as a foliar treatment in some pest management situations. Because imidacloprid from seed treatments can move through the soil to field margins in rain runoff where it can be taken up by milkweed roots, or can drift through the air to settle outside the crop field when sprayed as a foliar treatment, there is potential exposure of monarch larvae and adults to this insecticide. We conducted laboratory cage experiments to determine if monarch adult females avoid milkweed for laying eggs when the plants are treated with imidacloprid systemically through uptake of insecticide-laced water, or by foliar treatment of imidacloprid directly onto leaf surfaces. In both cases, monarch females laid their eggs equally on treated and untreated control plants. The results imply that imidacloprid contaminated milkweed in the field will be as attractive to egg-laying females as normal uncontaminated milkweed, thus potentially exposing the future larvae to any toxic effects of this common insecticide. This information will be used by scientists, regulatory agencies, and conservation managers to help weigh the relative advantages and disadvantages of placing restored milkweed habitat patches at different distances from cropland.

Technical Abstract: Monarch butterfly (Danaus plexippus) populations have declined over the last two decades, attributable in part to declines in its larval host plant, milkweed (Asclepias), across its breeding range. Conservation efforts call for restoration of 1.3 billion milkweed stems into the Midwestern landscape. To reach this goal, habitat establishment in marginal croplands is required and the proximity of such habitat to crops presents potential exposure to agrochemicals. Corn and soybean are frequently treated with neonicotinoid insecticides that provide systemic protection against insect pests. Foliar treatments with neonicotinoids are also used in some cropping systems. Here, we investigate whether ovipositing monarchs discriminate against milkweed plants exposed systemically or through topical leaf application, to the neonicotinoid insecticide imidacloprid. In our first experiment, we placed gravid females in enclosures containing a choice of two cut stems for oviposition: one in 15mL, 0.5mg/mL aqueous solution of imidacloprid and one in 15mL water. In a second experiment, females were given a choice of milkweed plants surface treated with applications of 30µL of 0.825mg/mL imidacloprid-surfactant solution or control plants. To evaluate oviposition preference, eggs were counted and removed daily from each plant over three days. Video data of a subset of butterflies was used to evaluate landing behavior. Results indicate imidacloprid did not influence oviposition in female monarchs in either systemic or foliar treatments. This suggests that female oviposition preference is not affected by imidacloprid. The implications of these findings for monarch conservation practices will be informed by the results of ongoing egg and larval toxicity studies.