Evaluation of freezing to control wood-destroying insects (Isoptera, Coleoptera)

Structural insect pests may be killed by exposure to freezing temperatures. The critical thermal minimum (CT sub(min)) is the threshold low temperature at which an insect succumbs from momentary exposure to temperatures decreasing at 1 degree C/min. The CT sub(min) was -21.3 degree C for the western drywood termite, Incisitermes minor (Hagen); -17.2 degree C for workers of Incisitermes snyderi (Light); and -13.9 degree C for workers of the Formosan subterranean termite, Coptotermes formosanus Shiraki. The CT sub(min) for the southern lyctus beetle, Lyctus planicollis LeConte, was -35.6 degree C for eggs, -26.2 degree C for small larvae, -30.0 degree C for medium larvae, and -24.3 degree C for large larvae. The effectiveness of liquid nitrogen (N sub(2)) to chill wall voids below the CT sub(min) values for these structural pests was tested in a mock-up wall. The time to the CT sub(min) was affected by the rate at which liquid N sub(2) was released into the void and the vertical location of the release point. Fiberglass insulation in the wall affected the pattern of cooling by deflecting the liquid N sub(2) around the insulation. Plumbing and electrical fixtures in the wall were not damaged by liquid N sub(2), even though they were exposed to temperatures as low as -50 degree C. However, liquid N sub(2) cracked vinyl floor tiles adjacent to the wall. The oxygen level in confined spaces where liquid N sub(2) was released rapidly dropped below 19.5%, the minimum acceptable level for worker safety. Consequently, fans probably should be used to circulate fresh air into the areas where liquid N sub(2) is used, and electronic monitoring should be done to ensure safe oxygen level. Because insects survive if they are not exposed to at least their CT sub(min), it is important to monitor accurately the internal temperature of target wood with thermocouples to ensure that the CT sub(min) has been reached where the insect actually occurs. To achieve an effective low temperature throughout a structure, virtually every void would need to be treated. Otherwise, it is unlikely that isolated pockets of termites could survive by escaping exposure to the CT sub(min). Proper placement of thermocouples could be extremely important.