Soybean looper larva. (Photo by Matthew E. Baur)

Matthew E. Baur and David J. Boethel

Two insect pest species that would be targets for insect-resistant, transgenic soybean varieties—velvetbean caterpillar and soybean looper—originate south of the Tropic of Cancer and migrate into Louisiana.

The soybean looper is difficult to control with insecticides because it has developed resistance to several products used for insect control on soybeans. Researchers are puzzled at how the soybean looper has developed and maintained resistance to organophosphate, carbamate and pyrethroid insecticides when exposure within Louisiana soybeans to these insecticides has been low.

In the region where the soybean looper originates, it completes development on vegetable and ornamental crops that receive many insecticidal applications. This continued exposure to insecticidal pressure may have selected individuals with higher levels of tolerance to these products and may have contributed significantly to control problems in Louisiana soybeans.

LSU AgCenter scientists have focused efforts on identifying soybean looper populations in areas south of the Tropic of Cancer (south Texas, south Florida and the Caribbean) to determine the susceptibility of these populations to insecticides. If we find populations in these geographic areas that provide migrants to Louisiana with high levels of tolerance or resistance, this would explain why populations within Louisiana are difficult to control.

We used DNA fingerprinting to determine the genetic similarity among populations, and we assessed susceptibility to four insecticides: Condor (Bacillus thuringiensis or Bt), Larvin (thiodicarb), Tracer (spinosad) and Denim (emamectin benzoate). Three of the four insecticides (Bt, thiodicarb and spinosad) were chosen because they are recommended for soybean looper control in soybeans in Louisiana, and emamectin benzoate was included because it is not registered for use on soybeans in Louisiana. We did not include pyrethroid or organophosphorous insecticides in our screening panel because soybean looper populations are not susceptible to these products. Therefore, these are not recommended for control.

Populations in Puerto Rico were the most resistant to the insecticides; populations from Texas were intermediate, and those from Florida were the most susceptible. In three of the four years studied, populations from Texas and Florida appeared to provide most of the migrants to Louisiana, but evidence from the last year of the study (2000) indicated some migrated from Puerto Rico.

These studies demonstrate that soybean looper populations, separated by large geographic distances, manage to exchange genetic information. There is widespread interbreeding among populations. The results from these studies also point to the possibility of rapid resistance development in the soybean looper populations. This information could affect the way in which the insect-resistant, transgenic soybean varieties may be used.

To assess the threat of resistance in soybean looper populations adequately, scientists still need to discover the mode of inheritance of resistance traits, and, if possible, the mechanisms of resistance. As more sophisticated molecular biological techniques become available, researchers will be better able to address many of these questions about insecticide resistance development. Although only DNA fingerprinting was used here, these molecular techniques have been important in determining insecticide resistance mechanisms.


(This article appeared in the fall 2003 issue of Louisiana Agriculture.)