Desiccation resistance and water balance in populations of the maize weevil Sitophilus zeamais

Water balance is a challenge for insects due to their small size and body with high surface area-to-volume ratios, and it is particularly important for insects that inhabit dry environments, such as stored grains, and warm climates. This subject has been curiously neglected in stored product insects, including the maize weevil Sitophilus zeamais Motschulsky, were broad population differences have been reported with management consequences. Here, we assessed variation in desiccation resistance and water balance among three strains of the maize weevil exhibiting differences in body mass. The survival of male and female weevils from three strains was therefore assessed at relative humidities ranging from 0 to 100%, in which their water content and water loss were also assessed. The effect of insect aggregation (1–25 insects per group) on water balance was also assessed, as was the water gain by the drinking of free water and drinking preference when exposed to insecticide-contaminated water. The median survival time (LT50) of individual insects increased with relative humidity and varied significantly among the weevil strains. The larger strain exhibited 60–80% higher LT50 at 50% relative humidity (r.h.) and above, with little difference between strains at 0% r.h. Heavier weevils exhibited higher water content and lower overall rates of water loss; these qualities made heavier weevils more resistant to dehydration. Larger groups of weevils facilitate water retention in individual insects. Weevils actively drink water to achieve favorable water balance when dehydrated, which is a major source of water intake. Under dehydration, weevils in general were not able to discriminate between insecticide-contaminated and uncontaminated water. However, females of the insecticide-susceptible strain avoided contaminated water.