Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2007
Publication Date: 9/5/2008
Citation: Chanbang, Y., Arthur, F.H., Wilde, G.E., Throne, J.E., Subramanyam, B. 2008. Susceptibility of eggs and adult fecundity of the lesser grain borer, Rhyzopertha dominica, exposed to methoprene. Journal of Insect Science 8:48. 6 pgs. Interpretive Summary: One of the insecticides used to control the lesser grain borer on stored rice is the insect growth regulator methoprene. While this insecticide limits development of immature insects, we have limited information regarding direct toxicity to eggs of the lesser grain borer. We exposed their eggs on filter paper and on rice treated with methoprene, and also exposed adults on treated rice. Mortality of eggs increased as the concentration of methoprene on filter paper increased, and eggs exposed directly on rice either failed to hatch or larvae died before they could penetrate the hull, or the immature insect died inside the kernel and did not reach the adult stage. When adult females were exposed on rice treated with methoprene, egg-laying was reduced. Results show that eggs of the lesser grain borer are extremely sensitive to methoprene, and it could be used effectively in management programs that are targeted toward this insect.
Technical Abstract: A series of tests was conducted to determine the susceptibility of eggs of Rhyzopertha dominica (F.), the lesser grain borer, exposed to the insect growth regulator (IGR) methoprene on filter paper and on rough rice. In the first test, the hatch rate of eggs exposed on filter paper treated with methoprene at the rate of 0.0003 mg [AI] /cm2 was 50.0 ± 7.3% compared to 93.0± 3.3% on untreated controls. In the second test, eggs were exposed to a dose-response series of 0.00003 to 0.03 mg[AI]/cm2. Neonate emergence was directly proportional to concentration and ranged from 85 ±2.0% on untreated controls to 26.7 ± 8.3% at the highest concentration tested. In the third test, 1 ppm of methoprene was sprayed on long grain rough rice (Cocodrie variety), and then individual kernels were cracked and paired with an egg. About 70% of untreated eggs hatched and larvae were able to bore inside and eventually emerge as adults, while only 40% of the eggs exposed to treated rough rice were able to develop to the larval stage. Eggs either failed to hatch on the treated rice or the larvae died before they could penetrate the rice hull. In the final test, newly-emerged adults were exposed on rough rice treated with 1 ppm methoprene. Number of eggs per female from adults on clean grain was 27.7 ± 4.3 eggs per female, and on treated grain was 4.8 ± 1.1 eggs per female during 24 days after emergence. Methoprene as either a protectant or residual spray severely affects development of egg and neonate R. dominica, and also reduces fecundity of exposed parent adults.