Evaluation of potential relay strip crops for predator enhancement in Texas cotton
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A two-year study was conducted at the Texas Agricultural Experiment Station farm at Munday, Texas to evaluate different relay strip crops for predator enhancement in cotton. The study evaluated eight relay strip crop treatments, including three fall crops (fall planted canola, Brassica rapa L., hairy vetch, Vicia villosa Roth, and wheat, Triticum aestivum L.), three spring crops (spring planted canola, forage sorghum, Sorghum bicolor L., and grain sorghum, Sorghum bicolor L.), a relay crop system comprised of canola planted in the fall and grain sorghum in the spring, and cotton planted adjacent to cotton as control. Each treatment consisted of four rows of the relay crop planted on both sides of an 8-row 23-m cotton plot, replicated three times. Predator abundance was monitored weekly in strip crops and cotton throughout the crop season by taking a 15 s D-vac sample from each plot. Cotton aphids, Aphis gossypii Glover, and bollworm- budworms, Helicoverpa zea (Boddie) and Heliothis virescens (F.) combined, were monitored in cotton plots only. Aphid abundance was estimated during the peak aphid period by counting the number of aphids on 10 upper and 10 lower leaves of cotton plants within each plot. Bollworm- budworm abundance was estimated by counting the number of larvae per 4 row-m of cotton in each plot. Predator species composition varied significantly among strip crops, but it did not significantly vary among cotton plots. Overall, strip crops significantly enhanced predator numbers in adjacent cotton plots during the first half of the growing season, but this effect dissipated after mid-July. Average aphid abundance in cotton was significantly affected by the adjacent strip crop, with the highest average number of aphids per leaf in control plots compared with numbers in plots adjacent to strip crops. A significant effect of strip crops on bollworm- budworm abundance was not detected, but the average bollworm- budworm larval abundance in our experimental plots remained below the economic threshold, whereas the rest of the 83 ha of cotton in the same experimental farm which employed the standard cropping system suffered from a severe bollworm- budworm infestation in both years. Average lint yields did not significantly vary among strip crop treatments, but average yields in the strip crop study plots were similar (1996) or significantly higher (1997) than in the plots where strip crop strategy was not employed. All the cover crops evaluated in this study enhanced the predator numbers and suppressed the aphid abundance in cotton in both years. However, wheat and spring canola were the two candidate cover crops that significantly suppressed aphid numbers in cotton both years, indicating a higher relay strip crop potential than the other cover crops evaluated. Although strip cropping of wheat or spring canola significantly suppressed aphid populations in cotton, a relay system consisting of canola planted in the spring adjacent to wheat planted in the fall might be more efficient than using a single strip crop in a pesticide-free cotton production system. 1999 Taylor & Francis Group, LLC.
