Many of us are all too familiar with plant pest mites such as the two-spotted spider mite and the damage they can cause on greenhouse crops; however, there are some mites that do not feed on plants. In contrast to their vegetarian cousins, predatory mites prey upon small insects and/or other mites. Several predatory mite species are currently available for biological control of various greenhouse pests. Some mites are generalist predators, feeding on many species of prey, while others are more selective in their diets. If used properly under the right conditions, predatory mites can be an important part of an effective biocontrol program.
Mites are not insects, but arachnids, like spiders and ticks. All mites have five life stages: egg, larva, two nymph stages, and adult. The larvae have six legs, while the nymph stages and adults have eight. The lifespans, reproductive rates, and performance of predatory mites used as biocontrol agents (BCAs) are affected by temperature, relative humidity, and food source. Some predatory mite species are also subject to diapause (a period of dormancy induced by decreased daylight hours in fall/winter), while others are not - this may be an important consideration for growers who are active in the winter months.
To increase biocontrol efficacy, two or more predatory mite species are often released at the same time, or they may be used in combination with other types of biocontrol agents. Some predatory mite species may be recommended for inundative biocontrol, which is the introduction of large numbers of BCAs in order to overcome and control an established pest population that has reached significant proportions. While inundative biocontrol may work in some instances, inoculative biocontrol is frequently the most effective tactic. Inoculative biocontrol involves the periodic release of lower numbers of BCAs in order to keep pest populations low, thereby preventing a problem before it starts. The frequency and method of application depends in part on the formulation of the product. Some products are designed to be broadcast, while others are distributed in small piles throughout a crop. Some predatory mite species are available in sachets, small packets that also contain bran mites, which serve as an initial food source for the predators, and something for the bran mites to eat too. If used properly these sachets will release predatory mites gradually over a number of weeks, thereby decreasing the need for frequent applications. Each product comes with specific instructions which must be followed. Remember also that predatory mites cannot fly, so their movement between plants will be limited if the leaves are not close together.
Because they need to hunt and catch their food, predatory mites move rather quickly in contrast to those that feed on plants. When prey is scarce, most predatory mite species will eat pollen; some also consume nectar and honeydew, the sweet excretion of phloem-feeding insects such as aphids. Some alternate food products are commercially available, and there is evidence that their use can enhance populations of some BCAs in greenhouses. Predators can survive on these food sources for some time, but populations will be more robust when the mites are able to feed on their preferred prey.
Part II of this article will discuss the more common species of predatory mites available for biological control.
Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab