Environmental factors impacting leg coloration in Chrysodeixis includens (Lepidoptera: Noctuidae)

The soybean looper, Chrysodeixis includens (F.) (Lepidoptera: Noctuidae), is widely distributed throughout North and South America. It appears on important crop hosts with mixed populations of other looper species, especially the cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae). One of the earliest attempts to distinguish between the 2 species was to examine the 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 3 different environmental factors (temperature, light duration, and rearing density) to determine if these had an impact on the development of dark coloration on soybean looper legs. Larvae were exposed to 5 distinct levels of each environmental variable. We found that all 3 factors impacted the amount of dark coloration in the legs. Larvae reared at the coldest temperature and most light displayed darker-colored legs as early as the third instar of larval development. As larval rearing density increased, dark coloration on legs also increased, but this was not apparent until the fifth instar of larval development. There was a general trend for pupal weight to decrease and days to pupation to increase as the percentage of darkness on soybean loopers' legs increased within the various treatments of the study. Overall, dark leg coloration was largely dependent on environment and an apparent fitness cost was associated with larvae exhibiting darker legs within the various rearing conditions examined.