Chapter 11 - Insect Quarantine

Abstract

Insect mortality during low-pressure storage (LP) might stem from at least three factors: dehydration, a lack of O2, and a low pressure per se. There are two distinct phases in the transport of gases and vapors into and from insects: (i) diffusion in the circulating blood and (ii) diffusion through air in the tracheal system. The diffusive resistance of an insect’s liquid-filled pathway to O2 entry exceeds that of the air-filled pathway because O2 diffuses 100,000 times faster through air than through the insect’s blood. Hypoxia and anoxia cause toxic levels of lactate and pyruvate to accumulate in insects, but in plant matter the respiratory quotient (RQ) remains close to unity and ethanol and acetaldehyde do not accumulate when as little as 0.08% [O2] is available during hypobaric storage. When decreasing the relative humidity kills surface insects by desiccation, the lethal vacuum time is directly related to insect weight loss, but when insects oviposit their eggs inside stored plant matter, the life stages which develop are protected from desiccation by the saturated atmosphere present inside the host’s intercellular system. Internal insects are not reliably killed during LP storage even though the stored plant matter’s respiration causes the O2 partial pressure to be lower inside the commodity than it is in the storage atmosphere. The surface to volume ratio of plant matter typically is much smaller than that in an insect life stage, and therefore insects evaporate a larger percentage of their water much faster than plant matter when both are exposed to the same water vapor partial pressure gradient and to metabolic heat produced by the commodity. Lagunas-Solar tested a Metabolic System for Disinfection and Disinfestation (MSDD) that combines ethanol vapor with rapid cycles of low ambient pressure and high [CO2] to kill pathogens and arthropod pests. They speculated that the insect’s response is due in part to anatomical changes caused by rapid pressure cycling. MSDD lowers the humidity in a vacuum chamber and kills various insect life stages present on the commodity’s surface, but does not reliably kill eggs and larvae present inside plant matter where they are protected by the high intercellular humidity. Insects which are adapted to grow on decaying fruits metabolize ethanol and are not killed by it. Therefore, MSDD has limited potential as a disinfestation treatment for quarantine pests. Ionizing radiation from high-energy electron beams and gamma rays from cobalt 60 disrupt an insect’s normal cellular function by breaking chemical bonds within DNA and other molecules, making this an effective way to provide quarantine security by preventing insect reproduction, rather than by killing the insect pest immediately.