Predators for Spider Mites on Greenhouse Veggies

Table of Contents

1. Introduction
2. Reasons for limited success of Phytoseiulus persimilis
3. Stethorus punctillum
4. Feltiella acarisuga
5. Amblyseius or Noseiulus californicus
6. Conclusion

Introduction

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Reasons for limited success of Phytoseiulus persimilis

Limited success in use of P. persimilis on tomatoes and cucumbers is partially attributed to the surface characteristics of these two crops, both of which are quite hairy. The hairs on tomato leaves have the additional virtue of producing a sticky exudate to which predators often become stuck and trapped. In contrast to tomatoes and cucumbers, peppers have quite smooth leaf surfaces that allow for free movement and predatory activity. Another limiting factor to use of P. persimilis, particularly on cucumbers, is the unfavourable climate encountered on the upper leaf canopy where temperatures often exceed 300C and humidities are often under 60% during late spring and summer. Such conditions adversely affect this predator. In fact, under such conditions, P. persimilis will migrate downward towards more moderate conditions in the lower leaves, leaving the spider mites to thrive in the hot dry conditions in the upper leaves along the support wires.

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Stethorus punctillum

Stethorus punctillum is a voracious small black lady beetle (Fig.2), developed in Canada as a biocontrol agent (workers include Dave Raworth, Agriculture & Agri-Food Canada, Agassiz, BC, and Jay Whistlecraft, Agriculture & Agri-Food Canada, London, ON). This predatory beetle feeds only on spider mites, disperses well, and is able to find small colonies of prey (spider mites). Development from egg to adult under greenhouse conditions is 2 to 3 weeks and the number of spider mite eggs eaten by this predator during its development can exceed 1,000. Raworth also observed that releases of this beetle in vegetable greenhouse as early as February, resulted in establishment of this predator on pepper and cucumber, but not on tomato. Laboratory studies on environmental effects on this predatory beetle in the UK indicate that humidities between 33% and 90% have relatively little effects on its movement and predatory activities, and that the amount of prey eaten was similar at 20, 25 and 300C. Because of their voracious appetite, these lady beetles are best released in 'hot spots' to facilitate their establishment.

Figure 2 - Adult Stethorus punctillum

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Feltiella acarisuga

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Amblyseius or Noseiulus californicus

Research in the UK indicates that A. californicus, a predatory mite, can be an efficient predator of spider mites under high temperature and low humidity conditions. Laboratory studies showed that temperatures as high as 300C had no effect on their activity or predatory behaviour, and their activity and number of spider mites eaten were highest at low humidity (30%). In comparing the effects of host plants, pepper was the best host, followed by tomato and cucumber. Eggplant was the least favourable host, as it often is for many other biocontrol agents. A. californicus and a relative, A. fallacis, are reported to be slower feeders but they are more tolerant of residues of some pesticides, and are able to persist in the absence of spider mites by feeding on other mites and pollen.

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Conclusion

Research suggests that release of multiple species of biological control agents for management of spider mites is more effective than use of just a single species. Competition among the different biocontrols can occur but research also suggests that this happens after there is a crash in the spider mite population, which is the goal of the grower anyway.

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