Spider mite populations often increase during the hot, dry summer months, and these high numbers can lead to greater pressure in the spring. But there are strategies that can reduce the numbers of these overwintering pests.
As fall arrives, we see a rise in the environmental cues that lead to diapause, such as fewer hours of daylight, lower nighttime temperature, and decrease in food quality. Diapause is similar to hibernation, when changes in their physiology (body) and behaviour ensures greater chances of survival through the winter. Physically, they appear to have a more reddish-orange colour, rather than the typical yellowish-green of non-diapausing spider mites. The diapausing stage is more tolerant to pesticides, and feeds very little before leaving the crop to hibernate. As they start diapausing, the female spider mites mate but delay egg-laying. They also tend to move downwards towards the floor and away from light.
With this knowledge, we can act to reduce their numbers now and save ourselves from headaches in the future. One strategy is to aggressively treat for spider mites while they’re still on the plants. While populations that have started the switch to diapausing forms are less susceptible to chemical control, they are still susceptible to predation by beneficial mites. Predatory mites that feed on spider mites located on the foliage, without favouring diapause themselves, include Phytoseiulus persimilis, and Neoseiulus californicus.
Another strategy is to target the diapausing spider mites at their hibernation sites. This can be any openings in the house structure, even irrigation equipment and pipe fittings. Their habit of moving downward means that many will end up in the soil. Where plastic groundcover is used, they are especially found at the edges of the plastic, and at openings for equipment, posts, and pipes. To target these areas, the predatory mite, Stratiolaelaps scimitus is preferred, since they live and hunt in soil and other media.
Releasing Stratiolaelaps (formerly Hypoaspis miles) will be most effective when applied to the soil, or openings in the plastic, after any cleanup or sanitation activities have been completed - and before the new plants go in. With these predators in play, we can expect to see fewer spider mites emerging into the new crop.
For more information, please speak with your TerraLink sales rep, or contact Rachel Hagel, our beneficial insect specialist at 604-864-9044.