Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 5, 2003
Publication Date: August 5, 2003
Citation: Martin, P.A., Blackburn, M.B., Shropshire, A.D. 2003. Two new bacterial pathogens of colorado potato beetle. Journal of Invertebrate Pathology.
Interpretive Summary: Colorado potato beetles are major leaf-eating pest insects on potatoes, eggplant, and tomatoes. These beetles are normally controlled with chemical insecticides. Problems with chemicals persisting in the environment and toxic effects on applicators, as well as consumers, make it crucial to reduce chemical insecticide use. Insect pathogenic bacteria, which kill plant-damaging beetles without harming plants, people, or beneficial insects, are a more environmentally friendly alternative to chemical pest control. Two unusual, naturally occurring bacteria have been discovered that may be prospective alternatives. One glows in the dark (Photorhabdus luminescens) and the other is purple, (Chromobacterium violaceum). Under laboratory conditions, Colorado potato beetle larvae consume these bacteria, and die within 2-3 days. However, the bacteria do not kill the adults, but only stops them from feeding on plant leaves. The ebacteria do not have to be alive in order to kill. They produce compounds which are themselves toxic to beetle larvae and easily isolated from the bacterial cells. Scientists can use these two bacteria and the toxins they produce to develop alternative and safer ways to protect crops against pest insects.
Other than Bacillus thuringiensis, few bacteria are lethal to the Colorado potato beetle, a major pest of potatoes and eggplant. Expanded use of biologicals for the control of Colorado potato beetle will improve resistance management, reduce pesticide use, and produce novel compounds for potential use in transgenic plants. While using a freeze dried and re-hydrated diet pellet to screen bacteria lethal to other insects, we found strains of Photorhabdus luminescens which killed Colorado potato beetle larvae. The LC50 for second instar larvae was 6.4 +/- 1.87 x 10**7 cells per diet pellet. In an attempt to find additional naturally occurring P. luminescens strains toxic to Colorado potato beetle larvae, we recovered bacteria which produced a purple pigment. These bacteria, identified as Chromobacterium violaceum, were also toxic to Colorado potato beetle larvae within three days. The LC50 for second instar larvae for these bacteria was 2.0 +/- 0.79 x 10**8 cells per diet pellet. The LC50 for C. violaceum was approximately a log lower for third instar larvae. P. luminescens appeared to kill, by means of a protein toxin, which may be similar to the described lepidopteran protein toxins. The toxin or toxins which C. violaceum produces, while not fully characterized, do not appear to be proteins. In both cases these toxins are made after exponential growth ceases.