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The use of biological control is one means of dealing with insect and mite pests of greenhouse-grown horticultural crops by making releases of parasitoids or predators. The characteristics of an efficient biological control agent include: 1) high female reproductive capacity, 2) short generation time, 3) sufficient dispersal capabilities, and 4) life cycle that is synchronized with generations of the prey or pest. Biological control agents have to compete with pest population development and reproduction in order to successfully regulate populations. In fact, the intrinsic rate of increase (rate of increase of populations that reproduce within discrete time intervals, and have generations that fail to overlap) of biological control agents must be similar to pests in order to provide sufficient regulation.

Furthermore, in terms of application, it is important to understand the procedures that will ensure success when using biological control agents, including: a) ordering biological control agents early, b) checking to make the biological control agents are alive, c) making releases immediately upon arrival, and d) applying before pest populations reach outbreak proportions.

However, another critical factor that may affect the effectiveness of biological control agents in regulating pest populations is the environmental conditions associated with greenhouses, such as temperature, relative humidity, day length (photoperiod) and light intensity. These environmental parameters, if not appropriate for the specific biological control agent, can influence reproduction, foraging behavior and survival. Therefore, the greenhouse environment must be taken into consideration when implementing a biological control program.

Temperature

Temperature can influence the rate of development and reproduction of both biological control agents and pests. For instance, the predatory mite, Phytoseiulus persimilis is most effective at temperatures between 20 and 30°C (68 and 86°F) because at this temperature range, the development time of Phytoseiulus persimilis is shorter than its prey, the twospotted spider mite (Tetranychus urticae). The optimal temperature for foraging, development and reproduction is 25 to 30°C (77 to 86°F). However, searching activity decreases when temperatures are greater than 30°C (86°F). Furthermore, at temperatures greater than 30°C (86°F), the development time of the twospotted spider mite is shorter than Phytoseiulus persimilis.

Relative humidity

A relative humidity less than 40 percent may affect egg survival, adult longevity and female reproduction of the predatory mite. The effects of temperature and relative humidity on the development of the larval and nymphal life stages of Phytoseiulus persimilis are presented in Table 1. Whiteflies, at temperatures between 17 and 30°C (63 and 86°F), disperse more quickly and across longer distances, which may impact the success of the greenhouse whitefly (Trialeurodes vaporariorum) parasitoid, Encarsia formosa.

Relative humidity may also impact the ability of parasitoids to attack prey. For instance, the parasitoid Encarsia formosa has a higher parasitism rate at a relative humidity between 50 and 70 percent. A relative humidity, below or above this range may result in decreased reproduction and longevity.

Day length and light intensity

Moreover, day length and light intensity may influence natural enemy-prey interactions. For example, the aphid predator Aphidoletes aphidimyza enters a resting (diapause) stage when experiencing short daylengths (less than12 hours). The predatory mite Neoseiulus cucumeris enters a diapause stage during the winter when temperatures are less than 21°C (70°F) and the day length is less than 12 hours. An option to overcome the direct effects of day length is to extend the photoperiod by using supplemental lighting, which can, in most cases, prevent the induction of diapause in certain biological control agents that enter a reproductive diapause under short daylengths.

Low light intensity may decrease the activity of certain biological control agents. For example, Encarsia formosa may be less effective during the fall months due to the direct effects of lower light intensities. However, reports indicate that the parasitoid Eretmocerus eremicus is more effective than Encarsia formosa in regulating populations of the greenhouse whitefly under lower light intensities. Therefore, more frequent release rates of Encarsia formosa may be required in order to compensate for this condition. Although, costs associated with purchasing the biological control agent and making frequent releases may be too expensive. Light intensity may also indirectly affect the performance of biological control agents. For instance, the heat generated from high light intensity (especially during the summer months) associated with sunlight may force Phytoseiulus persimilis to inhabit the leaf underside on the lower leaves of plants. This allows twospotted spider mite populations to escape exposure to the predatory mite because twospotted spider mites will feed on the upper portions of the plant.

When implementing a biological control program, be sure to take into consideration the existing environmental parameters in order to be successful when releasing parasitoids or predators. If you have any questions or comments regarding the appropriate environmental conditions for specific biological control agents, always contact your local supplier/distributor.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.

References

Stenseth, C. 1979. Effect of temperature and humidity on the development of Phytoseiulus persimilis and its ability to regulate populations of Tetranychus urticae (Acarina: Phytoseiidae, Tetranychidae). Entomophaga 24: 311-317.