Description
Phytoseiulus persimilis is a tropical predatory mite that was one of the first commercial greenhouse biological control agents available. Adults are bright reddish orange, with long legs and pear-shaped bodies, 1/50th inch (0.5mm) long. Immature predators are a pale salmon color. Eggs are oval and 0.3mm long.
Both adult and immature predators move rapidly over the leaves and both feed on two-spotted mite eggs, nymphs and adults. Unlike its prey, P. persimilis does not spin webbing.
Target Pest
Two-spotted spider mite (Tetranychus urticae).
Life Cycle
A complete life cycle takes from 5 days at 86°F to 25 days at 59°F. There are 4 times more females in the population than males (a sex ratio of 4:1). Females lay 2-3 eggs per day for an average of 60 eggs over their 35 day lifetime. The eggs hatch in 2-3 days. They are oval and twice the size of two-spotted mite eggs. Newly hatched predators do not eat, but later stages and adults feed on all stages of prey. Each predator consumes between 5-30 prey (eggs or mites) per day. Persimilis does not diapause, therefore remains active year-round in greenhouses.
Use in Biological Control
Persimilis is very effective against two-spotted spider mites in greenhouse vegetable and ornamental, interior plantscapes and conservatories. It is also used in warm climates on field crops such as strawberries.
Optimum conditions are 68-81°F and relative humidity over 60%. At these temperatures persimilis reproduces faster than spider mites. At temperatures outside this range however, spider mites reproduce faster.
Monitoring Tips
Phytoseiulus persimilis are usually easy to tell from their prey, which are slow moving pale tan mites with two darker spots on their sides. In winter, some two-spotted spider mites turn a reddish color, but they can still be distinguished from the predators by their slower movements.
Product Information
Persimilis is usually sold in a granular carrier like vermiculite and is available in high density mixtures for hotspots and low density formulations. Low density formulations are only for use before spider mites are detected in the crop. Once spider mites are detected it is essential to establish predators as soon as possible by introducing persimilis to the crops.
Persimilis should be applied the same day they are received as the quality of the product drops with storage. Gently rotate the bottle to mix the mites with the carrier and distribute the contents over the plants.
If persimilis must be stored, hold only at 50-59°F. Temperatures above or below that range cause mortality. When persimilis is refrigerated they tend to clump together. If the package is cold, lay the container on its side at room temperature (out of direct sunlight) for 15-20 minutes to allow the mites to warm up and move apart. Check the product by sprinkling some of the vermiculite onto a sheet of white paper. The active predators should be easy to see moving about.
Introduction Rates
Phytoseiulus persimilis is most effective when applied at th first sign of a two-spotted spider mite infestation. Because of its high reproduction rate, persimilis usually exhausts its food supply and eventually dies out, therefore repeated introductions are recommended until all sites with spider mite infestations have persimilis present.
Apply 0.5 persimilis/square ft. or 20 persimilis/infested leaf, weekly as needed. Apply predators to each infested plant. Apply before mites are detected and gently rotate the bottle prior to release.
For Greenhouse Cucumbers - 0.6 persimilis/sq. ft or 100 persimilis/infested plant, weekly or as needed. For larger areas use 24,000/acre (60,000/hectare).
For Greenhouse Tomatoes - Apply at a ratio of 1 persimilis to 10-20 spider mites. The glandular hairs on tomato leaves are toxic to persimilis so females will lay fewer eggs than on other crops. When food becomes scarce, persimilis disperses throughout the crop. The predator moves better within a crop when the leaves of adjacent plants are touching. It tends to move upward on plants, so it can be applied to the trunks of large plants in interior plantscapes.
When predators are found on each infested leaf, it usually means that biological control program will be successful. It may take another 2-6 weeks for new plant growth to show improvement, depending on growth rates.
For Best Results
If spider mite numbers are high (there is visible webbing and clusters of mites stringing down from leaves), use a compatible pesticide or insecticidal soap to reduce pest numbers before releasing predators.
Phytoseiulus persimilis needs a relative humidity greater than 60% to survive, particularly in the egg stage. In low humidity conditions, raise the humidity by lightly misting plants or wetting walkways. Where humidity above 60% rH can be maintained, the predator Feltiella acarisuga can also be used with persimilis. Where humidity is below 60% rH, the predatory beetle Stethorus punctillum can be used with persimilis. Stethorus feeds on all stages of spider mite and is effective at detecting individual mites colonies.
If average temperatures are often below or above the 68-81°F range for optimum use of persimilis, introduce Amblyseius fallacis along with persimilis.
Using Chemicals
For effects of pesticides on Phytoseiulus persimilis, contact a consultant at Sound Horticulture. The pesticide fenbutatin oxide can be used with persimilis for additional control of spider mites if hotspots develop. It does not harm persimilis, but avoid over spraying which reduces the persimilis food supply and their ability to reproduce. Spreader-stickers, supreme oils and soaps are harmful to the persimilis contacted by the spray, but have little residual activity.
