Spider mites are tiny arachnids that damage plants by feeding on individual cells. Since they are not insects, they are often resistant to common insecticides, making environmental factors like temperature a primary focus for control. Understanding the thermal limits of these pests is essential for managing an infestation, as temperature dictates their life cycle speed and survival.
The Lethal Upper Limit
Spider mites are highly susceptible to high temperatures, and heat is a rapid way to achieve complete mortality. The exact temperature required for immediate death depends on the duration of exposure and the mite’s life stage. For effective eradication, temperatures must be sustained above 104°F (40°C) for a prolonged period, or much higher for a short burst.
A temperature of 135°F (57°C) will kill all life stages, including eggs and adults, with an exposure time of approximately six hours. Eggs tend to be the most vulnerable stage to heat, while adult females are often the most tolerant, requiring the highest temperatures and longest exposures for a complete kill.
Survival Range and Reproduction Rate
The survival and proliferation of spider mites are accelerated by warm, dry conditions. The optimal temperature range for the two-spotted spider mite, the most common species, is between 86°F and 89°F (30°C to 32°C). Within this range, their metabolic rate peaks, allowing them to complete their entire life cycle in as little as seven days.
This rapid development allows populations to explode quickly. Temperatures slightly below the lethal threshold, such as 90°F to 100°F, continue to increase their reproduction speed. Low humidity, typically below 50%, further enhances these conditions, creating an environment conducive to widespread outbreaks.
Control Through Cold
Cold temperatures do not typically provide a quick kill for spider mites but instead serve to slow their development or induce a state of dormancy. Below 50°F (10°C), the mite’s activity decreases significantly, and reproduction slows to a near halt. This is why infestations often subside during the colder months outdoors.
For actual mortality, freezing temperatures are required. The exact lethal point depends heavily on the mite’s stage and its ability to enter diapause, a protective overwintering state. Non-diapausing mites have a 50% mortality rate at approximately 6°F (-14.3°C). The most cold-tolerant diapausing females can survive down to about -9.8°F (-23.2°C). This cold hardiness allows eggs and overwintering adults to survive typical outdoor winters, only to re-emerge when temperatures rise.
Practical Application of Thermal Treatments
Growers can use these thermal limits to their advantage, most commonly through targeted hot water dips. A plant-safe method involves fully immersing the infested plant or cuttings in water heated to between 109°F and 111°F (43°C to 44°C) for a period of 30 minutes. This temperature range is often sufficient to kill the mites and eggs without causing thermal shock or scalding to the plant foliage.
Hot Water Treatment
Using a thermometer is necessary to maintain the water within this narrow margin, as temperatures a few degrees higher or for a longer duration can damage the host plant.
Soil and Cold Treatments
For treating soil, steam pasteurization is a non-chemical option, which requires heating the growing medium to 150°F (66°C) for about 10 minutes to kill pests and pathogens. Cold treatments are primarily used to manage dormant plant material, where a plant can be placed in cold storage, such as a refrigerator, to halt the mites’ life cycle and prevent spread.