Biological Control of the Twospotted
Spider Mite in Greenhouses

University of Florida
Central Florida Research and Education Center
2807 Binion Road, Apopka, FL 32703

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Bulletin 853 (technical)12/85
(updated 7/12/99)
L.S. Osborne, L. E. Ehler, and J. R. Nechols *
 
 

Table of Contents:

INTRODUCTION

There are over 140 species of insects and mites that are known to be pests in greenhouses (Pritchard, 1949). In recent years, the major pest species involved--such as aphids, mealybugs, scales, and spider mites--have generally been controlled by insecticides and acaricides. However, it is becoming increasingly clear that the strategy of unilateral reliance on chemical control will not be the final solution to the problem. In this regard, there are four major problems attendant to chemical control: (1) Development of resistance to chemicals in target pest species; (2) the dwindling supply of useful, registered insecticides and acaricides; (3) the damaging (or detrimental) effect of these chemicals on nontarget species resulting in secondary pest outbreaks; and (4) phytotoxic reactions by treated plants. On the other hand, unilateral reliance on biological control should not be viewed as a sound strategy because biological control alone does not always give adequate protection.

A solution to this problem lies in the utilization of integrated control--or what is now called integrated pest management (IPM). This is a management system in which ecologically suitable and economically rewarding control tactics are employed to maintain pest populations at tolerable levels. With respect to the current situation in many commercial greenhouses, there is a critical need for integrating biological control agents with existing cultural and chemical control methods.

The purpose of this bulletin is to synthesize information relevant to biological control of one of the major plant pests found in greenhouses worldwide. This pest is the twospotted spider mite, Tetranychus urticae Koch. Its most effective control agent (at present) is the predatory mite, Phytoseiulus persimilis Athias-Henriot. The twospotted spider mite is a suitable subject for an IPM program in which biological control plays a major role, and may even be employed by itself for extended periods of time. Biological control programs are currently available for the control of greenhouse whitefly, but many of the remaining pest species--particularly aphids, leafminers, mealybugs, scales, and various species of Lepidoptera--will require additional research. With adequate support, the necessary information for these species and their biological control agents should be forthcoming.

The emphasis in this bulletin will be on information--not implementation. There are two reasons why we have chosen this approach. First, during 12 years of research on biological control of pests in greenhouses in Florida, California, and New York, we have detected a strong interest, on the part of growers and others, in the use of biological control. However, information about the available biological control agents was either lacking or too difficult for those, interested in the topic to obtain and eventually utilize. Second, because of the great diversity of plant species (and cultivars) grown in greenhouses, the wide variety of cultural practices used, and the differing environmental conditions encountered, it is virtually impossible to develop a set of guidelines for implementing biological control which would cover every conceivable situation. Instead, growers who are interested in using biological control agents should be able to draw upon the information contained herein and develop their own systems. In fact, this approach is already being practiced.

Biological control can be defined as the action of natural enemies which maintains a host's (or pest) population density at a level lower than would occur in the absence of these enemies. There are three classes of natural enemies of insects and mites: predators, parasites (or parasitoids), and pathogens. A predator is an insect (sometimes mite, spider, etc.) whose immature form (larva or nymph) develops at the expense of more than one host individual. The adult form can be either predacious or free-living. Common examples of predators include lady beetles, big-eyed bugs, lacewing larvae, and ground beetles. Parasites, on the other hand, are insects whose immature form (larva) develops in or on only one host individual. The adult of the parasite is usually free-living, feeding, for example, on nectar or insect body fluids. Parasites are usually either certain flies (Diptera) or small wasps (Hymenoptera) and can be considered "specialized predators." Finally, pathogens are those micro-organisms whose interactions with the host induce a disease (often lethal) in the insect. Such pathogens include fungi, bacteria, protozoa and viruses. Nematodes are often included in this category.

The fundamental premise for biological control of a plant-feeding (phytophagous) insect can be summarized as follows: in the native home (or region of origin) of a given phytophagous insect, there should exist a natural enemy or complex of natural enemies which maintains (or is capable of maintaining) the population density of the insect at comparatively low levels. Essentially, the same applies for plant-feeding arthropods such as spider mites. This action is called natural biological control and is the reason why so many of our phytophagous insects, including several potential pest species, are relatively rare and/or innocuous. Plant-feeding insects which are introduced into a new country without their effective natural enemies often reach outbreak proportions and are thus subjects for classical biological control, that is, importation of the appropriate natural enemies from the native home of the pest. Also, natural enemies from areas other than the native home of the pest species, but which are preadapted to exploit the pest, can be of considerable value in classical biological control.

The effectiveness of a given natural enemy under greenhouse conditions may not always coincide with its performance outside of the greenhouse. Natural enemies which are generally ineffective under field conditions may be relatively effective in the greenhouse and vice versa. For example, the parasite Encarsia formosa Gahan is effective against greenhouse whitefly in the greenhouse; however, the same species is generally ineffective in many field situations especially where temperature extremes either favor the pest or cause direct mortality of the parasites. The underlying reasons for the differential performances of other natural enemies are not always known. However, it is clear that certain greenhouse conditions may be quite favorable for one species of natural enemy, but not for another.

It is our view that, for biological control to be employed successfully in the greenhouse, a proper attitude on the part of grower is necessary. Indeed, colleagues in New York have also noted the importance of this aspect (see Tauber, 1977; Tauber and Helgesen, 1978). Some philosophical commitment to biological control, including commitment to the integration of biological and chemical control, is also necessary. A certain amount of patience and a firm resolve for making biological control work is generally needed. In our experience, one of the foremost obstacles to the implementation of biological control in the greenhouse is a negative attitude.

* Authors:
L.S. Osborne Associate Professor of Entomology, Central Florida Research and Education Center, University of Florida, IFAS, 2807 Binion Rd., Apopka, FL 32703-8504;

L. E. Ehler Professor of Entomology, Department of Entomology, University of California, Davis, CA 95616

J. R. Nechols Associate Professor of Entomology, Department of Entomology, Kansas State University, Manhattan, KS 66506.