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 (CTmin) is the threshold low temperature at which an insect succumbs from momentary exposure to temperatures decreasing at 1°C/min. The CTmin was -21.3°C for Incisitermes minor; -17.2°C for workers of Incisitermes snyderi; and -13.9°C for workers of Coptotermes formosanus. The CTmin for the beetle Lyctus planicollis was -35.6°C for eggs, -26.2°C for small larvae, -30.0°C for medium larvae, and -24.3°C for large larvae. The effectiveness of liquid nitrogen (N2) to chill wall voids below the CTmin values for these structural pests was tested in a mock-up wall. The time to the CTmin was affected by the rate at which liquid N2 was released into the void and the vertical location of the release point. Fibreglass insulation in the wall affected the pattern of cooling by deflecting the liquid N2 around the insulation. Plumbing and electrical fixtures in the wall were not damaged by liquid N2, even though they were exposed to temperatures as low as -50°C. However, liquid N2 cracked vinyl floor tiles adjacent to the wall. The oxygen level in confined spaces where liquid N2 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 N2 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 CTmin, it is important to monitor accurately the internal temperature of target wood with thermocouples to ensure that the CTmin 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 CTmin. Proper placement of thermocouples could be extremely important.