At this time in the growing season, you might start to see damage by broad mites. Broad mites can cause serious damage to a wide range of greenhouse crops such as New Guinea impatiens, begonia, garden impatiens, dahlias, gerbera, ivy, lantana, snapdragon, verbena, zinnia, and peppers. They use piercing mouthparts to feed on the epidermis of young leaves, causing leaf margins to curl and become shriveled. Their feeding causes twisted, hardened, and distorted growth in the terminal of the plant and bronzed (russeting) under the leaf surface, and on stems or buds. As infestations progress, terminal buds can be killed, flowers can be distorted, and plants will be stunted.
Broad mites are very small and are best seen with the help of magnification (15 – 20X hand lens or microscope). Broad mites are colorless-to-pale brown with a white stripe down the center of their backs. Eggs are elliptical but are covered by small whitish bumps that look like rows of diamonds. Eggs are usually found on the undersides of leaves.
Regularly inspect crops for symptoms, and rogue out infected plants as soon as possible. If detected early, it may be feasible to discard a small number of infested plants. Keep growing areas weed free and cleaned thoroughly between crops. Avoid working in the area with broad mites and then moving to a clean crop, as you can easily move broad mites on tools and clothing.
Broad mite damage may be mistaken for damage caused by Western flower thrips feeding on young buds or nutritional deficiencies or high salts. If you are unsure if broad mites are the cause of the damage, send a sample to the UMass Extension Plant Diagnostic Lab for assistance.
Predatory mites Neoseiulus (Amblyseius) cucumeris, Neoseiulus (Amblyseius) californicus and Amblyseius swirskii can be used to suppress broad mites on greenhouse-grown crops. It is important however to apply predatory mites early in the crop production cycle before populations become established.
Not all miticides are labeled for broad mites. Translaminar miticides such as abamectin (Avid 0.15EC, Minx 4), spiromesifen (Savate) and chlorfenapyr (Pylon) tend to be more effective than contact miticides. Do not use Savate on geraniums, New Guinea impatiens and fuchsia. Do not use Pylon on dianthus, poinsettia, roses, kalanchoe, salvia and zinnia. Broad mites often hide within tender buds and new growth, so miticides that only have contact activity will not be very effective. If leaf canopies are dense, coverage on the undersides of leaves is difficult. Miticides with translaminar activity will be translocated to the growing tip where mites are feeding, delivering greater control.
Always read pesticide labels for plant safety information and make sure to follow all label recommendations and restrictions.
For more information on the specific miticides to apply, refer to Table B6 of the latest edition of the
New England Greenhouse Floriculture Guide:
- Geoffrey Njue, Extension Specialist, UMass Extension Greenhouse Crops and Floriculture Program