These tiny pests can give growers big problems. Here’s how to prevent, treat and control russet mite infestations using organic methods.
for Russet Mites
Hemp and russet mites are part of the eriophyid family of mites. Among the 100 or so plant-specific eriophyid species, including gall, rust, and blister mites, the tomato-attacking, microscopic russet mite is among the hardest to detect. Visible, if then, only in clusters, a single mite is too tiny to be seen by the human eye without magnification of 10x and higher. Their near invisibility makes these mites a particular threat to become established in your garden before you realize it.
Unlike spider mites, these voracious plant pests leave no webbing or other secretions when present. Visible damage to the plant is the first indication of its presence a damage often mistaken for mineral and other nutritional deficits.
Seen through a lens, russet-hemp mites are tapered, translucent, wedge-shaped cylinders that take on a yellow tint, especially in groups. Unlike most varieties of mites, eriophyid, including russet mites, have only two pairs of legs.
Like spider mites and others, they are increasing their range and are now common in places not previously seen. Because of their size, they’re effectively dispersed by wind.
Crop specific, they tend to multiply in areas of intensive growing, like the tomato raising regions of Florida, and also do quite well indoors where warm, moist environments facilitate rapid reproduction. In the past decade, they have spread from growers in California north into Oregon.
Females over winter just inside stems of the plants they infest or where twigs are joined to stems. Translucent eggs, nearly adult in size, are laid in spring and go through two nymph stages, both little different than the adult. The mites produce multiple, over-lapping generations through the course of a season, maturing in as little as eight days in warm, humid conditions.
Mites are sap suckers working at the cellular level. Damage typically appears first at the bottom of plants and moves upward as they feed. Lower leaves begin to yellow and curl, the leaf droops and the stem discolors. As the plant’s nourishment is sucked away, less vigorous green growth and flowering is observed. The tiny mites, in increasing numbers, spread to all parts of the plant. The mites seem particularly attracted to flower resins and will congregate in flowers and blossoms where they can effectively hide and do great damage. If left unchecked, the mites will eventually sap the entire plant.
Russet Mite Control
The best controls for both indoor and outdoor growers are preventive methods. Make sure you don’t introduce mites into your grow space by bringing in infected plants or contaminated potting soil that may carry female mites or eggs. Regular and close scrutiny of your plants, especially around the leaves nearest the soil line if outdoors, or anywhere a plant is flowering, is crucial to early detection. What might appear to be an iron or magnesium deficiency may well be an infestation of russet mites. If in doubt, treat for mites as well as adjusting nutrient solutions or amending soil. Finding and removing mites in their first generation, a period that can be short as a week under the right conditions, can short-circuit a cascade of generations once the mites begin laying eggs.
Many of the precautions and treatments used on spider mites also apply to this pest. Inspection of plants is crucial during treatment. Look to see if damage has stopped. Remove all damaged leaves, stems, and even entire plants and dispose in a way that won’t spread the tiny creatures or its eggs.
- Russet mites are most frequently found in outdoor container plantings. This suggests that they’re introduced both in potting soils and plants brought into the garden. Use only dependable, high-quality potting medium and plants from a nursery you trust. Don’t be afraid to ask if mites have been a problem. Even if they’ve been dealt with and the plant shows no signs of damage, eggs may still exist in the soil.
- Introducing beneficial nematodes just as soil temperatures begin to warm and ahead of planting can help destroy eggs that are in the soil and nymphs once they hatch. Apply a second round of vermiculite-carried nematodes if damage to lowest leaves is spotted.
- Avoid over-fertilizing plants. Mites are attracted to vigorous green growth that comes of too much nitrogen.
- Periodic releases of predatory mites can blunt infestations, giving you time to deal with them. (Yes, the same spray mentioned below will also kill beneficial mites.)
- Because of their near invisibility, it’s difficult to apply insecticidal soaps to suspected mite infestations. Home-made sprays using garlic, hot peppers, or citrus oils will not usually take down mites but may provide some deterrent. (Because they often arrive in the wind, russet-hemp mites aren’t easily deterred.)
- Neem oil will repel and kill mites. It should be applied at first signs of damage.
- Pyrethrum sprays have proven effective in killing mites but require complete coverage to ensure that none of the microscopic pests are overlooked. Tailor applications to the lifecycle of the mite. Spraying once a week — once every five days in warm conditions — for three to four weeks should cover adults, emerging nymphs, and eggs.
- No matter which method you’re using, close inspection of plants during treatment is crucial. Use a lens of 14X magnification or larger to see mites gathered at the center of curled leaves, at stem junctures, or in flowers. Don’t wait to see how effective one treatment is before continuing or using another. Once infestations spread up plants, it’s almost impossible to save the plant.
- Don’t hesitate to discard entire plants, even if they’re not entirely affected. While you’re treating affected plants, the mites are hurriedly spreading to others.
- Prevent spreading mites. Discard infested plants in plastic bags and dispose of them in sealed garbage containers.
- Do not bring uninspected plants or plants from any grower who’s had previous problems with mites into your grow space. This is especially important, whether taking clones or tomato starts from friends and professionals. Know your grower.
- Keep a clean grow space. Don’t introduce unnecessary tools used outside or suspect soil or growing mediums.
- Periodic releases of spider mite predators in a greenhouse can help keep pests at bay.
- At the first sign of mites, reduce their breeding environment by adjusting room temperatures and moisture content as much as possible. This will slow the breeding cycle, buying you time to inspect and treat plants before the mites next generation is spawned.
- Azamax will discourage mites from feeding and slow their breeding cycle, especially helpful if you’re moving towards harvest.
- Neem oil and pyrethrum/ canola oil sprays, used as above, will knock down mite infestations with repeated use.
- If your grow space has been infected, clean it from top to bottom. Scrub benches and other equipment with a mild bleach solution (1:10 bleach-to-water solution is said to be safe for cleaning according to the Centers For Disease Control), sterilize all hydroponic equipment, throw away any soil that can’t be sanitized, and leave nothing that was in the space during the infestation unscrubbed. The invisible eggs of this troublesome pest can survive in tiny, unseen places.
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