Environmental factors impacting leg coloration in Chrysodeixis includens

Authors: Allen, Kerry; Elkins, Blake; Little, Nathan

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2025
Publication Date: 3/27/2025
Citation: Allen, K.C., Elkins, B.H., Little, N. 2025. Environmental factors impacting leg coloration in Chrysodeixis includens. Environmental Entomology. 54. https://doi.org/10.1093/ee/nvaf031.
DOI: https://doi.org/10.1093/ee/nvaf031

Interpretive Summary: The soybean looper is a caterpillar pest that is widely distributed throughout North and South America. It appears on cotton and soybean, along with a similar caterpillar, the cabbage looper. Although these two insects are identical in appearance and damage caused by host plants, soybean loopers have developed insecticide resistance to insecticides that provide adequate control of cabbage loopers. It had been noted that the black legs on soybean loopers could be used to distinguish them from cabbage loopers, but this characteristic is variable and not under simple genetic control. We examined three different environmental factors (temperature, hours of light, and rearing density) to determine if these had an impact on the development of dark pigmentation on soybean looper legs. We found that all three factors impacted the development of dark pigmentation in the legs. Caterpillars reared at the coldest temperature, and most light expressed darker pigmented legs as as young caterpillars. As the rearing density of caterpillars increased, dark leg pigmentation also increased, but this wasn’t apparent until the later stages of caterpillar development. There was a general trend for the weight of pupae to decrease and days to pupation to increase as the percentage of dark pigmentation on soybean loopers’ legs increased.

Technical Abstract: The soybean looper, Chrysodeixis includens (F.), is widely distributed throughout North and South America. It appears on important crop hosts, including cotton, Gossypium hirsutum L. and soybean, Glycine max (L.), with mixed populations of other “looper” species, especially the cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae). Although these two caterpillars are similar in appearance and damage caused on host plants, soybean loopers have developed insecticide resistance to insecticides that provide adequate control of cabbage looper. One of the earliest attempts to distinguish between the two species was to examine the actual legs of caterpillars, and those that possessed “black” legs were considered soybean loopers. However, it was demonstrated that this characteristic was variable and not under simple genetic expression. We examined three different environmental factors (temperature, light intensity, and rearing density) to determine if these had an impact on the development of dark pigmentation on soybean looper legs. Larvae were exposed to five distinct levels of each environmental variable. We found that all three factors impacted the development of dark pigmentation in the legs. Larvae reared at the coldest temperature, and most light expressed darker pigmented legs as early as the 3rd instar of larval development. As larval rearing density increased, dark leg pigmentation also increased, but this wasn’t apparent until the 5th instar of larval development. There was a general trend for pupal weight to decrease and days to pupation to increase as the percentage of dark pigmentation on soybean loopers’ legs increased within the various treatments of the study.