Leptotrachelus dorsalis (Coleoptera: Carabidae): A Candidate Biological Control Agent of the Sugarcane Borer in Louisiana
Following registration and the 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 more abundant 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 of beetles occurred in a field where 10% of the stalks sampled were harboring larvae. Laboratory studies revealed a developmental period of 37 d from egg deposition to adult emergence. Three larval instars were identified with the first 2 lasting 5 d, and the third instar requiring 17 d. In a voracity study, larvae were found to consume on average 798 first instar sugarcane borer larvae per ground beetle larva. Seventy-five percent of those sugarcane borer larvae were consumed by third instar beetle 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 3 instars of the beetle can be found in sugarcane fields during the critical period of Jun to Sep 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 on augmentative releases as a strategy to increase beetle numbers in sugarcane fields early in the growing season and thus enhance their efficacy as predators of sugarcane borer larvae.