Glandless Cotton in New Mexico: Impact of Nitrogen On Gossypol Associated Resistance to Cotton Bollworm and Beet Armyworm

Eradication of boll weevil and pink bollworm have produced a renewed interest in glandless (gossypol free) cotton as a specialty crop for both livestock and human consumption. Glandless cotton was not considered a viable crop in some areas since the lack of gossypol resulted in a plant that was highly susceptible to yield loss associated with injury from insects and other pests. New Mexico has lower insect pressure than most other areas of the cotton belt and might be an area where gossypol free cotton is a viable product. In 2012, field trials were conducted on an NMSU University farm to evaluate the influence of two nitrogen rates (low-N and high-N) on cotton bollworm and beet armyworm in field and lab trials using the glandless Acala GLS and a local standard Acala 1517-99. Petiole nitrate levels were 1573-1765 ppm in low-N treatment plots and 5211-6243 in high-N treatment plots when a field to lab bioassay was initiated. There was no significant effect of nitrogen on survival of first instar beet armyworm or cotton bollworm after 48 hours. There was significantly higher mortality for both insects on glanded vs. glandless cotton. Beet armyworm fed glanded cotton had 70-83% mortality vs 27-43% mortality in glandless cotton in high-N and low-N treatments. Cotton bollworm fed glanded cotton had 73-87% mortality in glanded cotton vs 17-20% mortality in high-N and low-N treatments. Glandless, high-N field plots had significantly more square damage than glanded plots. High-N glandless plots averaged 8.2% damaged squares compared to 6.1% damaged squares in glandless low-N plots and 3.2-3.6% damaged squares in glanded cotton. Petiole nitrate levels during August were also significantly higher in high-N plots which averaged 4313ppm compared to 853ppm in low-N plots.