Plant Health Initiative

Mites - Management

Natural suppression

The mite on the left has been parasitized by the fungus Neozygites floridana. Parasitized mites have a discolored, waxy or cloudy appearance and mite death occurs within 1 to 3 days of infection. Click on image to view a larger version. Photo credit: T. Klubertanz

Spider mite populations are generally suppressed through naturally- occurring biological control. Predatory mites colonize the same plants and feed on the two-spotted mites. Aphid predators such as lady beetles, minute pirate bugs, and lacewings also feed on mites.

The most important biological control agent is the widespread fungal pathogen, Neozygites floridana, that attacks all stages of mites and is host-specific to spider mites. Spores from the fungus attach to the mite’s body, germinate, and infect the body. During early infection stages, mites have a discolored, waxy or cloudy appearance and mite death occurs within 1 to 3 days.

The environmental conditions necessary for the fungus to be active are temperatures cooler than 85°F and high humidity. Rain itself does not trigger the fungal infections; the resulting high humidity is still necessary. When conditions are not favorable for the fungus to grow, it will produce resting spores in infected, dead mites until environmental conditions trigger the resumption of growth and spore release. When the fungal parasite is active, it can reduce populations of mites very rapidly.

Unfortunately, insecticide and fungicide applications will disrupt all these predator and parasite species.

Threshold levels

No specific threshold levels have been established for the two-spotted spider mite. Most Extension entomologists recommend treatment only if damage and mites are detected throughout the field. Use the following scale as a guide, and consider treatment when injury progresses to a rating of 3. Fields with ratings of 5 or worse may not be salvageable. Check fields every 4-5 days if drought persists since damaging infestations can develop quickly.

If mite presence is verified, it’s time to progress into the field. Move at least 100 feet into the field before making your first stop. Walk a “U” pattern checking at least 2 plants at each 20 locations. Assess mite damage using the following scale:

• 0 — No spider mites or injury observed.
• 1 — Minor stipling on lower leaves, no premature yellowing observed
• 2 — Stipling common on lower leaves, small areas or scattered plants with yellowing
• 3 — Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. (Spray Threshold)
• 4 — Lower leaf yellowing readily apparent. Leaf drop common. Stipling, webbing and mites common in middle canopy. Mites and minor stipling present in upper canopy. (Economic Loss)
• 5 — Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.

Insecticide selection

We now know that pyrethroids insecticides used to treat soybean aphids kill off the natural enemies that help control spider mites. They also stimulate movement of the mites, spreading the infestation, and there is some data from orchard systems to indicate they may increase the reproductive rate of existing female mites and increase the number of females being born into the population.

This worsening of mite infestations can occur in as little as 7-10 days after application. Organophosphate insecticides have provided a better kill rate, and are a better choice for mite management. However, there is still a risk of continued problems when environmental conditions remain hot and dry. Treatment may be difficult later in the season given the 21 and 28 day pre-harvest intervals of the organophosphates Dimethoate (a.i., dimethoate) and Lorsban (a.i., chlorpyrifos), respectively. Consider that yield expectations in mite-treated fields may require adjustment because of drought conditions.