Uscana lariophaga, egg parasitoid of bruchid beetle storage pests of cowpea in West Africa: the effect of temperature and humidity.

The stored-product bruchid pests, Callosobruchus maculatus (Fabricius) and Bruchidius atrolineatus (Pic) cause considerable production losses in cowpea in West Africa. Uscana lariophaga Steffan parasitizes the eggs of the bruchids both in the field and in storage. As chemical control of bruchids in traditional granaries is not appropriate for poor farmers, enhancement of the efficacy of the parasitoid by environmental manipulation has been investigated. The effect of temperature on the capacity of U. lariophaga to parasitize eggs has been studied at eleven constant and three fluctuating temperatures within the range 10 to 45 °C. Longevity of the female wasp decreased with increasing temperature. The rate of development increased linearly at temperatures from 17.5 to 35 °C, but decreased from 35 to 40 °C. Mortality of the developing wasp remained below 20% from 20 to 37.5 °C, but outside this range, mortality reached 100% at 15 and at 42.5 °C. Most parasitization occurred at temperatures of 25 and 30 °C. Sex ratio (percentage females) increased with temperature in the high temperature range. The intrinsic rate of increase (rm) for U. lariophaga was highest in the temperature range from 30 to 37.5 °C and was higher than that of C. maculatus at all temperatures. While the rm value of C. maculatus did not vary much at temperatures from 25 to 35 °C, the rm value of the wasp doubled. Relative humidity did not effect longevity, egg-laying capacity, mortality, development time and sex ratio of the wasps with C. maculatus as host. However, with B. atrolineatus as the host, development time and mortality increased at lower relative humidity levels. The results indicate that temperature is the major regulating factor on the parasitoid. As the type of storage structure and its location (sun or shade) affects the temperature inside the store, ways are being investigated of manipulating the storage environment through temperature regulation to increase the impact of the parasitoid.