Greenhouse Grower Notes: Life stage timing for predators

April 30, 2008
Written by Gillian Ferguson
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Eggs and immature mobile stages of two-spotted spider mite.
PHOTO COURTESY GILLIAN FERGUSON/OMAFRA

Although it is wonderful to be able to choose from an array of predatory mites for biological control of the two-spotted spider mite, such choice has also made life more complicated. We need to gain as much information as possible about the various species when choosing the species of predator to release, and in determining the timing of release.

We know that predatory mites for spider mites can be specialized, less specialized, or general in their diet. This means that specialized predators (e.g., Phytoseiulus persimilis) will die in the absence of spider mites, whereas a less specialized one (e.g., Neoseiulus californicus or Amblyseius andersoni) will survive for a considerable period without spider mites, but with other food or moisture sources.

Another factor that may affect choice of predatory mite species is its preference for one life stage or another of the spider mite (i.e., eggs versus mobile immatures). Perhaps it may be desirable to combine the use of predators that have different food preferences, as this could not only minimize competition between them, but also lead to more efficient suppression of spider mite populations.

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Adult Phytoseiulus persimilis feeding on egg of two-spotted spider mite.
PHOTO COURTESY GILLIAN FERGUSON/OMAFRA
STUDYING LIFE STAGE PREFERENCES

Researchers associated with Oregon State University (J.S. Blackwood, P. Schausberger, and B.A. Croft), investigated life stage preferences in 13 predatory mite species and reported their findings in 2001. Among the predators investigated were Phytoseiulus persimilis, Neoseiulus (Amblyseius) fallacis, Neoseiulus (Amblyseius) californicus, Galendromus occidentalis, and Amblyseius andersoni. To determine the preference of the predators, the researchers offered eggs and larvae of the two-spotted spider mite to individuals of all species in controlled ratios. Then they manipulated the ratio of the prey stages to determine whether an abundance of one stage or another would influence feeding by the predators. Experimental conditions were held at about 26ºC, 70 per cent RH, and light exposure for 16 hours out of every 24-hour period. Numbers of each prey stage consumed were recorded after 12 hours and 24 hours.

RESULTS OF THE INVESTIGATION
Results indicated that P. persimilis and N. fallacis had significant preferences for eggs, that A. andersoni had a significant preference for larvae, and that N. californicus and G. occidentalis had no preference for either eggs or larvae. Table 1 summarizes the prey-stage preference of some of the predatory mites studied.

tableIMPLICATION OF RESULTS
The exact implication of these results in a practical sense is not immediately obvious. Such data certainly provide some more information about the various predator species, and point to potentially more efficient combinations of predators, if targeting different life stages is one’s objective. But the final outcome in biological control of spider mites will also be influenced by many other factors. Such factors include predation capacity under high or very low pest pressures, compatibility of the species used in combination, ability to withstand extremes in humidity and temperature conditions, and ability to disperse efficiently on different plant surfaces, etc. Only by experimenting with the various predators under commercial or commercial-like conditions can we really assess the practical implication of the various predators released in various combinations.

Gillian Ferguson is the greenhouse vegetable IPM specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs in Harrow. • 519-738-1258, or This e-mail address is being protected from spambots. You need JavaScript enabled to view it


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