Thrips
Definition: Thrips are slender, fast-moving insects, about 1/25 inch long, that scrape open cannabis leaf surfaces and feed on the sap. Their damage shows as silvery or bronze streaks and patches dotted with tiny black specks of excrement, and heavy populations can slow growth by destroying large areas of leaf tissue.
How Thrips works
Thrips are easy to overlook because they’re tiny and hide in leaf folds and new growth. Their damage usually gets noticed first.
Identifying them
- Adults: thin, cigar-shaped, tan to dark brown, with fringed wings. They jump or fly when disturbed.
- Larvae: pale yellow or whitish, wingless, crawling on leaf surfaces.
- Damage: shiny silver patches where cells have been emptied, often with black dots of waste.
The silvery patches look different from the fine pinprick stippling of spider mites, which is a helpful way to tell the two apart.
The soil stage
Many thrips drop from the plant to pupate in the top layer of soil before emerging as adults. That’s why controls aimed only at leaves often miss part of the population.
Monitoring
Blue sticky traps attract thrips better than yellow ones. You can also tap a branch over a sheet of white paper and look for tiny, moving slivers.
Control options
- Predatory mites such as Neoseiulus cucumeris and Amblyseius swirskii eat young larvae.
- Minute pirate bugs (Orius) hunt adults and larvae.
- Beneficial nematodes or soil-dwelling predatory mites target the pupae in the medium.
- During veg, products labeled for thrips can help, used according to the label and your state’s allowed inputs.
As with aphids, a combined approach that covers both leaves and soil works best. Avoid spraying buds once flowers form; at that stage, beneficial insects and removing damaged fan leaves are safer tools.