Glandless Cotton in New Mexico: Beet Armyworm, Spodoptera Exigua and Bollworm, Helicoverpa Zea Development and Field Damage

Cotton glands produce gossypol, a natural defense against insect pests. Glandless cotton varieties are available, but losses from pests have prevented commercial development.  Some areas of New Mexico have somewhat lower insect pressure, with high predation and desiccation, suppressing pest populations.  With appropriate management and monitoring of insect pests, growers could potentially produce glandless varieties as a niche crop with greatly added seed value. In 2011-2013, field trials were conducted on a New Mexico State University farm to determine susceptibility to lepidopterous pests and to investigate potential control measures.  

Bollworm and beet armyworm survival was 5-10 times higher at pupation on glandless cotton. Among survivors beet armyworm fourth instar larval and pupal weights were not significantly different on glandless or glanded cotton in 2012. However, those on glandless leaves required 4 less days to reach pupation.  Bollworm fourth instar larvae were 69% heavier, pupae were 16% heavier on glandless cotton squares, and pupated in 9 fewer days compared to those on glanded cotton. Survival was not affected by nitrogen although there were some impacts on development.  

Glandless cotton had twice as many damaged squares with 6% compared to 3% damaged squares. The number of damaged squares were not affected by nitrogen levels.  Predation of sentinel bollworm eggs was similar in glanded and glandless plots. Results in 2013 were similar to 2012. However, in 2013 monitoring of each instar indicated that beet armyworm was significantly more likely to molt into a sixth instar when feeding on conventional rather than glandless cotton. This delay in development allowed beet armyworm to compensate for slower weight gain and pupate at weights similar to those fed on glanded cotton despite being significantly smaller at 14 days.  Glandless cotton is more susceptible to insect injury, but will likely be manageable as a valuable niche crop.