Biology, scouting and control of spider mites
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Spider mite infestations are now being reported in southern Michigan. Be careful about spraying too early or too many times for mites, as they easily flare and become resistant.
Spider mites overwinter as adults. In spring, they move to new plant growth and lay eggs on the undersides of leaves. Mites can be blown by wind, so initial colonization of a field will often occur in the direction of prevailing winds, or along landscape features that disrupt air flow, such as tree lines, houses or even telephone poles. Infestations usually start on dusty edges of fields. It is thought that dust dries the leaf surface, protecting mites from disease, or perhaps that the dust provides a surface for the mites to anchor their webs. With a hand lens or microscope, you can see that webbing acts like an interstate highway, with mites moving back and forth above the leaf surface.
Eggs hatch in a few days, and small mites begin to feed. Immature mites resemble the adults, except that they are smaller in size. They grow by molting, and if you look closely at a mite-infested leaf, you sometimes see the shed skins of immature mites. A leaf heavily colonized by mites will have eggs, immatures, adults, shed skins and webbing.
Spider mites feed on numerous crops and under certain conditions will increase to the point where they affect yield. This is especially true under dry conditions or on sandy soils where water stress is an issue. Mites pierce individual plant cells and suck out the contents, initially causing tiny yellow spots (called stippling) on leaves. This feeding also increases water loss from the plant. As populations increase, symptoms include yellowing of leaves and in more severe cases browning, bronzing or death of foliage. As leaves become yellow and die from mite damage, the photosynthetic ability of the plant is reduced. With the additional water loss, leaf drop and reduced photosynthesis, yield and quality of crops is reduced by severe mite feeding.
If an infestation is identified early, spot treatments of the affected area plus a border strip may be enough to take care of mites. Otherwise, if mites spread across a field, estimate the percentage of leaf surface damaged by mites.
Thresholds for beans
Treatment thresholds for soybeans (based on percentage yellowing) vary with plant stage: pre-bloom = 40 percent; bloom R1 to pod fill R5 = 15 percent; R5 to early maturity R7 = 25 percent; after R7 = do not spray as damage at this point has little impact on yield. A handy evaluation scheme (originally from Ohio State) for mites in soybean is listed in this article.
Ohio State Evaluation for mites in soybean
Level 1 = Mites barely found on undersides of leaves in dry locations or on edges of fields. Damage: Barely detected.
Level 2 = Mites easily found on undersides of leaves in dry locations or on edges of fields, but mites are difficult to find on leaves on other locations in the field.
Level 3 = All plants infested when examined closely.
Level 4 = All plants heavily infested when examined closely.
Level 5 = Extremely high numbers.
Mite control is difficult, and more than one application may be needed. Unfortunately, multiple applications can lead to resistance. Later in the season, spraying obviously becomes more difficult due to closing of the canopy, but spraying is also less effective as plants mature (yield is no longer affected).
- Dry bean: dimethoate (0.5 1 pint/ acre). Will also control potato leafhopper.
- Sugarbeets: Lorsban (1-2 pints/ acre).
- Soybeans: Lorsban (0.5 - 1 pint/ acre) or dimethoate (1 pint/ acre).
- Field corn: Lorsban (1-2 pints/ acre; spider mite not on the label); dimethoate (0.5 - 1 pint/ acre).
Mites and aphids
Another consideration when both mites and aphids are present is choice of products for control. Aphid numbers have been low so far this year, so this may not be an issue.
- If aphids are your target, both organophosphate [OP] insecticides (Lorsban, dimethoate, Orthene) and pyrethroids (Asana, Baythroid, Mustang, Pounce, Warrior, etc) can be used.
- If mites are the target, then an OP is preferred. OPs generally are more effective against mites than pyrethroids, which tend to flare mite populations.
- If both pests are present, OPs again are the preferred option to reduce risk of flaring mites. However, Lorsban is the preferred option within the OPs. Although dimethoate is a low cost, effective mite control, it is less effective for aphid control compared to Lorsban.
What about natural controls? A heavy rain may reduce the population a little, but don't depend on rain alone. The real key is high humidity, since the fungal pathogens that kill mites require high, sustained humidity to grow and spread through the population. A brief rain, followed by quick drying, does not provide a long enough period for diseases to spread and kill a significant portion of the mite population. Instead, you need a sustained rainy pattern, or repeated dewy wet mornings.
Dr. DiFonzo's work is funded in part by MSU's AgBioResearch.