ABSTRACT

Low relative humidity and high temperatures in cotton fields in the New Mexico Chihuahuan desert dramatically reduced immature boll weevil Anthonomus grandis grandis Boheman survival. The degree of survival was also affected by cultural practices that affected crop microclimate. Boll weevils in infested squares placed in cotton furrows for 10 days in early season had 100% mortality. After canopy closure when temperatures were lower and relative humidity higher, some adults emerged from squares, but emergence varied with management practices. Rows oriented north/south had 38% emergence compared to 9% emergence in rows oriented east/west. Narrow row cotton with 17-cm row widths had higher survival of boll weevils, lower temperatures and higher relative humidity than standard 96-cm row widths. Mean daily high temperature in-furrow was 22^oC lower in 17-cm rows compared to 96-cm rows with mean highs of 32^oC and 54^oC respectively. Relative humidity in-furrow was also significantly lower in 96-cm rows with lows of 17% RH compared to 24-25% RH in 34-cm and 17-cm rows. Squares under the plant canopy had 34% emergence of adults compared to 6% emergence of squares in the center of the furrow. In a controlled chamber test at 26^oC, low RH reduced adult emergence, and resulted in an extended emergence time and a shorter adult lifespan. Adult emergence was 91% at 50% RH vs. 41% at 17% RH. Adult lifespan was much shorter at 17% RH with mean lifespan of 20 days compared to 42 days for those reared and maintained at 50% RH. Adult emergence was also delayed 2 days at 17% RH compared to those at 50% RH. Results for cotton bollworm, were more variable but hatch rates were lower in east-west oriented rows and 96 cm row spacing. These results indicate that natural mortality to insects due to high temperatures and low RH can be very high in the desert valleys of the Chihuahuan desert. Also, higher relative humidity, lower temperatures and higher survival of boll weevil in narrow row cotton suggest that the increasing use of narrow row cotton in arid/semi-arid areas will result in higher boll weevil populations and damage.