Two-spotted spider mites (Tetranychus urticae) are a widespread pest known for their ability to rapidly colonize and damage a vast array of garden plants and houseplants. Their minute size and habit of feeding on the undersides of leaves make them difficult to detect until an infestation is severe, often marked by fine webbing and stippling damage on the foliage. The challenge of managing these pests is compounded by their explosive reproductive rate and their capacity for prolonged survival when environmental conditions become unfavorable. Understanding the survival window of their eggs, and how that is extended through a process of true dormancy, is necessary for effective control.
The Standard Egg Cycle and Non-Dormant Survival
Under ideal growing conditions, the two-spotted spider mite life cycle proceeds rapidly, driven primarily by warm temperatures. The non-dormant egg is minute, spherical, and nearly transparent when first laid, becoming a pale, pearly white as the embryo develops. These eggs are typically laid on the underside of leaves, often secured within the protective silk webbing produced by the adult mites.
When temperatures are optimal, around 85 to 90 degrees Fahrenheit, the egg incubation period is short, often lasting only three to five days. The entire life cycle, from egg to adult, can be completed in as little as seven to twelve days, leading to a quick turnover of generations. This rapid development is characteristic of the summer-active mite population, explaining how populations can explode so quickly during hot, dry weather.
Diapause: The Mechanism of True Dormancy
The extended survival of spider mites is not typically due to their non-dormant eggs but rather a physiological state of true dormancy called diapause. Diapause is a protective metabolic shutdown that allows the species to survive periods when food is scarce or temperatures are hostile. For the two-spotted spider mite, this overwintering stage is almost always entered by the mated adult female, not the egg itself.
The environmental cues that trigger this shift are primarily a shortening of the day length, or photoperiod, and a drop in ambient temperatures, signaling the approach of winter. The adult females stop feeding and reproducing, their color changes from a pale green or yellow to a distinct orange-red, and they seek sheltered locations like leaf litter, soil crevices, or under bark. This state is a pre-programmed arrestment of development, which is metabolically distinct and highly resistant to environmental stressors.
Environmental Factors Governing Dormancy Duration
The duration of this protective diapause is directly influenced by the severity and consistency of the cold temperatures the mites experience. The diapausing adult females can survive for several months under consistent cold. In environments with harsh winters, overwintering females can remain dormant and survive for three to nine months.
This prolonged survival relies on the mites maintaining a state of metabolic depression and often accumulating cryoprotectant chemicals like polyols, which increases their cold hardiness. Termination of diapause requires a specific period of chilling followed by rising temperatures in the spring. Fluctuating winter temperatures or warmer indoor environments can shorten the overall survival window, as the mites may deplete their energy reserves attempting to break dormancy prematurely.
Management Strategies for Dormant Eggs
Since overwintering occurs primarily in the form of diapausing adult females hiding in sheltered areas, control measures must target these specific sites and life stages. Standard contact miticides are often ineffective against these dormant forms due to their non-feeding state and protected locations.
A key strategy is thorough sanitation, which involves removing all crop residue and weeds from growing areas, as these provide the most common overwintering sites. In greenhouses and indoor settings, a complete clean-up between crops is important, including cleaning cracks and crevices in benches and structures where diapausing mites hide. Applying specialized dormant oils can be an effective tactic, as these products work by physically smothering the mites and eggs in their protected locations. Temperature manipulation can also be a tool, as consistently low temperatures can prevent the initiation of the reproductive cycle once mites emerge from diapause in the spring.