Distribution of psocids (Psocoptera) in temperature gradients in stored wheat

Psocids have become important pests of stored products during the last two decades, but little is known about their behavior or ecology. We examined distribution of Liposcelis bostrychophila Badonnel (Psocoptera: Liposcelididae), Liposcelis entomophila (Enderlein), and Liposcelis paeta (Pearman), three of the main psocid pests of stored grain throughout the grain growing regions of the world, in temperature gradients of 20°-24 °C, 20°-30 °C, and 20°-42 °C in small bulks of wheat to determine their ecological preferences. Psocids consistently preferred the warmest regions of the gradients, except in the 20-42 °C gradients. Over 80 and 78% of psocids moved to the warmest region of the grain in the 20°-24° and 20°-30° gradients, respectively. Liposcelis bostrychophila females and both sexes of L. paeta preferred the warmer region of the grain in the 20°-42° gradient, while densities of L. entomophila males were higher in more moderate temperature regions of the grain and densities of L. entomophila females were evenly split between the warm and hot regions of the grain. Temperatures can be below 20 °C during much of the storage season for grains, so the current results help to explain why psocids move to warmer regions of the grain which occur toward the center of the grain mass as grain temperatures cool in the fall. This may allow psocid populations to continue to grow during the colder months.