Diatomaceous earth impairment of water balance in the maize weevil, Sitophilus zeamais

Abstract

As water balance is a fundamental component for insect homeostasis, inert dusts such as diatomaceous earth (DE) are physical insecticides that allegedly impair water balance causing insect death by desiccation. Death by desiccation and water loss were indeed reported when using DE against a few stored product insects at discreet relative humidities, which may vary with species, strain, and DE deposit. Here we assessed the activity of two DE formulations from distinct deposits, Insecto® and KeepDry®, against the maize weevil (Sitophilus zeamais), a key pest species of stored cereals in warmer climates. We tracked their action on the insect integument, and impairing water balance in a range of relative humidity, assessing their effect on insect water content and rate of water loss. Both DE formulations compromised insect survival at the full range of relative humidities tested (i.e., from 0 to 100 %) reducing in a third the median survival time. Both DE formulations damaged the insect tegument surface, including sensilla and pores, and compromised water balance reducing body water content and increasing the rate of water loss in over twofold. However, such activity and efficacy differed between DE formulations with KeepDry® leading to greater insecticidal activity and impairment of water balance in the maize weevil than the Insecto® formulation.