Submitted to: Journal of Stored Products Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 11, 2014
Publication Date: March 1, 2014
Citation: Tucker, A.M., Campbell, J.F., Arthur, F.H., Zhu, K. 2014. Mechanisms for horizontal transfer of methoprene from treated to untreated Tribolium castaneum (Herbst). Journal of Stored Products Research. 57:36-42. DOI: http://dx.doi.org/10.1016/j.jspr.2014.02.004. Interpretive Summary: Our earlier research has shown that methoprene (an insecticide that mimics insect growth regulators and disrupts the development of immature insects) can be transferred from a red flour beetle sprayed with the insecticide to another untreated individual and negatively affect the development of that second individual. This process, called horizontal transfer, can increase the effectiveness of insecticide applications in food facilities, but how transfer actually occurred was not determined. In a series of experiments to assess different potential mechanisms of transfer, we showed prolonged contact between treated and untreated insects caused negative effects on development and survival, but that brief periods of contact were not sufficient to cause negative effects. Feeding on the treated insects did occur, but increased feeding did not increase the negative impacts of the methoprene, suggesting that cannibalization is not an important mechanism in horizontal transfer of methoprene. Transfer of methoprene from treated individuals to flour and then to another untreated individual did cause negative effects on development. These results suggest that horizontal transfer of methoprene is probably due to prolonged contact with methoprene treated individuals or more likely with flour that had been in contact with methoprene treated individuals. That flour can absorb methoprene and transfer it to other insects suggests another route of methoprene exposure in food facilities. Flour residues on surfaces within a food facility can be contaminated with methoprene during insecticide applications or by coming in contact with treated surfaces afterward, and the flour can then be moved during sanitation activities into untreated hidden areas containing insect populations, potentially increasing insect exposure to methoprene.
Technical Abstract: Experiments were performed to determine the relative impact of different mechanisms of horizontal transfer of methoprene by Tribolium castaneum (Herbst), the red flour beetle. Insects exposed to 5 methoprene treated developmental stages (late-stage larvae, pupae, or adults) resulted in 100% mortality, no adult emergence, and a shorter survival time compared to the combined control group (flour only; no food; or water treated larvae, pupae, or adults). This suggests that cuticle contact is involved, but since some cannibalization also occurred, this alternative mechanism could not be ruled out. The average level of cannibalization was not different between methoprene and control treatments, but the distribution in the level of feeding did appear to be shifted to greater consumption for larvae exposed to methoprene treated individuals relative to the controls. The role of cannibalism on horizontal transfer was further evaluated by scoring the level of feeding by untreated larvae on either water or methoprene treated pupa and then evaluating the larva’s development and survival among different levels of cannibalization. In this experiment, the level of feeding on methoprene treated individuals did not affect the percentage of normal surviving adults (insects which emerged as adults, exhibited no external morphological malformations, and were alive at the end of the observational period), which suggests that consumption of treated individuals was not contributing significantly to horizontal transfer. Confining a methoprene or water treated adult with an untreated late-stage larva for 24 h did not effect larval development and survival compared to the controls, which indicates that brief contact between individuals was not sufficient for the expression of horizontal transfer. Transfer of methoprene from treated individuals to a flour substrate and then to an untreated individual through contact with and/or feeding on the flour was evaluated. Untreated larvae in methoprene contaminated flour had significantly greater deformities and mortality, lower adult emergence, and a shorter survival time as compared to the controls. These results suggest that horizontal transfer of methoprene is probably due to prolonged contact with methoprene treated individuals or methoprene contaminated flour.