Submitted to: Florida Entomologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/8/2011
Publication Date: 6/1/2012
Citation: White, W.H., Erwin, T.L., Viator, B.J. 2012. Leptotrachelus dorsalis (F.) (Coleoptera: Carabidae): A candidate biological control agent of the sugarcane borer in Louisiana. Florida Entomologist. 95(2):261-267. Interpretive Summary: Advances in insecticide chemistry has resulted the development of highly selective insecticides; those that only kill a specific pest such as the sugarcane borer. A moth whose caterpillars tunnel into cane stalks causing extensive damage. The beneficial-insects that control this moth are left unharmed. One insect that has benefited from the use of these insecticides is the ground beetle, Leptotrachelus dorsalis. Immature stages of this beetle forage behind leaf sheaths of the sugarcane stalk where they are voracious predators of sugarcane borer caterpillars. We discovered that L. dorsalis requires approximately 37 days to develop from an egg, through three mobile immature stages, a cocoon, and finally an adult beetle. While developing through the three mobile immature stages up to 800 sugarcane borer caterpillars can be devoured. Although classified as a ground beetle, L. dorsalis appears to be exclusively canopy dwelling and all three mobile immature stages are found in sugarcane fields throughout the growing season. We found that this ground beetle is capable of preventing economic damage to sugarcane by the sugarcane borer if the adult beetles migrate into sugarcane fields early enough in the growing season and then develop high numbers. Preliminary studies indicate that it is possible to hold adult beetles over the winter in cold storage for releasing into sugarcane fields the following spring. These individuals would help jumpstart beetle numbers in cane fields and where they would increase in numbers to a level that insecticide applications to control the sugarcane borer can be reduced or eliminated.
Technical Abstract: With the registration and wide-spread use of insect growth regulators (e.g. tebufenozide and novaluron) for control of sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae) in Louisiana, larvae of the ground beetle, Leptotrachelus dorsalis (F.) (Coleoptera: Carabidae) have become apparent in sugarcane fields. In a survey of the 18 parishes growing sugarcane in Louisiana, L. dorsalis was found in 8 (44%) of those parishes. The highest number occurred in a field where 10% of the stalks sampled were harboring L. dorsalis larvae. Laboratory studies revealed a developmental period of 37 days from egg deposition to adult emergence. Three larval instars were identified with the first two lasting five days, but the third instar required 17 days. In a voracity study, L. dorsalis larvae were found to consume on average 798 first instar sugarcane borer larvae per ground beetle larvae. Seventy-five percent of those sugarcane borer larvae were consumed by third instar larvae. Field surveys suggest that adults migrate into sugarcane fields when above-ground internodes form on the sugarcane stalk (June) and increase in numbers thereafter. All three instars of the beetle can be found in sugarcane fields during the critical period of June – September when sugarcane is subject to economic injury by sugarcane borer. If L. dorsalis are abundant in fields during establishment of second generation sugarcane borer, our data suggests they are capable of holding the average season-long damage at or below 10% bored internodes. This level of damage is the recognized economic injury level for sugarcane borer in Louisiana. Our study indicates that L. dorsalis is a good candidate for continued research in the areas of augmentative releases as a possible strategy to increase beetle numbers in sugarcane fields early in the growing season and thus enhance their efficacy as predator of sugarcane borer larvae.