Beneficials of the Month: Predatory Mites

We have been outlining information on some of the major (and minor) insect and mite pests of greenhouse crops in previous issues. However, in many cases pests are not the only insect and mites to be found. Beneficial insects and mites are also part of the picture - either because they were purposely introduced, or simply wandered into the crop from outside the greenhouse. We use the term beneficial because these insects and mites happen to be predators or parasitoids of insects and mites that we consider pests. They certainly are not beneficial to the pests! The following is a brief description of some of the most common and widely available predatory mites used for biological control in greenhouses. All of the predators are sold by biological control suppliers. For more detailed descriptions and suggested use rates consult the references listed at the end of this article.

1. Phytoseiulus persimilis (Figure 1.)

Probably the most widely used predatory mite, and perhaps the most widely used beneficial organism in greenhouse crop production, P. persimilis is used in biological control programs for two-spotted spider mites (Tetranychus urticae), and related Tetranychus species. P. persimilis has been in commercial use in greenhouses, and other sites such as interior plantscapes and outdoor foliage production nurseries, for more than 30 years. They are available from many commercial sources, and in some parts of the world will move into greenhouse and other crops from surrounding areas.

The mites, about 0.5 mm long as adults, are predators as nymphs and adults, mostly feeding on spider mite eggs and nymphs, but also consuming adults. Because they are meat eaters, and cannot survive on alternate food sources such as pollen, survival is poor if prey is in short supply. They will very nearly eliminate spider mites, but not completely. The predators will then starve, and may need to be re-introduced if spider mite populations rebound. Some pest management advisors advocate regular introductions (e.g. weekly) of low predator numbers to keep spider mites in check.

P. persimilis mites are very effective in the proper crop and environmental conditions. They do best at warm temperatures (70-85ºF) and higher relative humidity (over 60%). In these conditions, P. persimilis develops faster than its' prey. The mites generally will not perform well at temperature extremes (cold or hot), or in very low relative humidity, although there are some high temperature tolerant strains available. Predator movement among plants is much more efficient if the leaves are touching.

2. Neoseiulus (=Amblyseius) cucumeris (Figure 2.)

This predatory mite is used to control thrips, including the western flower thrips (Frankliniella occidentalis). First-instar thrips larvae are the main food source. Later stage larvae and adults can defend themselves against attack by this tiny mite (0.5 mm). N. cucumeris can also feed on other mites (e.g. cyclamen and broad mites) as well as pollen, so can survive in many different situations. N. cucumeris do best at temperatures between 66 and 80º F, and at a relative humidity of about 70%. Biological control may be more successful in double-poly covered greenhouses than in glass greenhouses because of higher humidity. For best thrips control, predators must be introduced before thrips appear in large numbers. Some strains of this predator enter diapause during short days in winter, but there are non-diapausing types available.

They are most effective if plant leaves are touching. One of the problems when using N. cucumeris on flowering plants is that the predators do not follow thrips into flowers very well. Very large numbers of predators are required, but the price/mite is low.

3. Iphiseius (=Amblyseius) degenerans (Figure 3.)

This predator, sometimes called the dark mite because it is dark brown in color, seems to be a bit more aggressive than N. cucumeris in attacking thrips. It will move into flowers in greater numbers as well. These predators are giants among mites, with adults a whopping 0.7 mm long, which may explain why they may be more aggressive. I. degenerans will also feed on spider mites and pollen. In fact, some have observed that pollen is preferred over thrips and mites. The best environmental conditions for the predators are temperatures between 65 and 85º F, and relative humidities between 60 and 85%. This predator does not enter diapause during short days. I. degenerans eggs are able to tolerate lower humidity better than other predators. Some growers rear their own I. degenerans on castor bean plants.

4. Hypoaspis miles (Figure 4.)

These are predators that live in soil and other growing media. They feed on thrips pre-pupae and pupae, as well as fungus gnat larvae. Springtails, which can be nuisances as well as pests in some cases, are also eaten. H. miles adults are 0.8 mm long, and range in color from tan to brown. These predators do not enter diapause, and remain active all year.

With the range of food eaten by these predators, they can become established and persist for long periods. In fact, they can survive for several weeks without any food. The mites are often introduced underneath greenhouse benches, over flats and pots, in a kind of preventive pest management program. Because the predators live beneath the growing media surface, they will escape contact with many pesticides used to control pests that feed on leaves and flowers. They do move around at night, so may come in contact with pesticides that are applied during the overnight hours.

References

Cherim, Michael S. 1998. The green methods manual, Edition IV. The Green Spot Ltd. Nottingham NH. e-mail: grnspt@internetMCI.com

Cloyd, Raymond A., Michael Brownbridge, and Clifford Sadof. 1998. Greenhouse biological control of western flower thrips. The IPM Practitioner XX(8): 1-9.

IPM Laboratories, Inc. Locke, NY. e-mail: ipmlabs@baldcom.net

Murphy, Graeme. 1997. Biological control experiences - an Ontario perspective. GrowerTalks. September 1997: 94-96.

Koppert Biological Systems. The Netherlands. Koppert Biological Systems