Predatory mites provide natural control of pests

Growers should recognize the significance of biological control in agricultural systems, said long-time integrated pest management proponent Tom Quick, a speaker at the annual Sustainable/Organic Production seminar held in Tulare.

Quick, an entomologist who works for the organic fertilizer and natural pesticide company Grow More, emphasized that growers who are not in organic production can still cultivate and encourage populations of pest predators and parasitoids to control plant pests.

Biological control is the result of predators, parasites, pathogens and competitors overwhelming insect pests and diseases and their damage. These natural enemies are present in most agricultural systems and are being recognized as a component of pest control. They can be especially important in reducing the numbers of pest insects and mites, Quick said.

One of the more high-profile pest predators currently is tamarixia radiata, a wasp that has been introduced in Southern California citrus to parasitize the Asian citrus psyllid, an invasive pest that can spread citrus greening disease. High-profile and perennial pests include mites, mealybugs, peach twig borer, leafrollers and navel orangeworm.

While there are alternative controls, including chemicals, the pests that are perennial are more likely to be controlled biologically because they provide their predators with a steady supply of food, Quick said.

Chemical control of crop pests limits the development of pest predator populations, but Quick said he does see plenty of crop systems where biocontrol plays an integral part in pest control.

Predators, parasites and pathogens are the primary groups associated with biological control, according to the University of California IPM program. Pathogens include bacteria, fungi, nematodes and viruses that can infect and kill the host insect. Quick said populations of some aphids, caterpillars and mites can be reduced by naturally occurring pathogens. Commercially available pathogens include BT, bacillus thuringiensis—the original organic pesticide—Quick noted. Crown gall in walnuts is another biocontrol agent.

Parasites live in or feed on a larger host. Also known as parasitoids, they are smaller and develop inside or attach to the outside of the host's body—usually in immature stages. An example would be egg parasitic wasps on glassy-winged sharpshooters.

Predators use pest insects for food, often when they are the same species. Predatory mites prey primarily on spider mites.

Quick said the complex relationship between pests and their natural enemies depends on a number of factors. The relationship is driven by time and susceptibility in the prey-predator cycle, he said. A time lag between the build-up of pest populations and the biocontrol agents prevents dependable control.

"The lag phase is an important issue," Quick said. "Growers need to take action when the pest population is increasing and before it reaches the economic injury level."

Introduction of laboratory-raised biological control is one method to overcome the lag phase. Scientists raising parasitic wasps to control Asian citrus psyllid is one example. Quick said insectaries are also producing predators to control mites in strawberries.

Disease control in organic production is more of a matter of keeping nutrients, soil organisms and pathogens in balance.

Larry Parker with Westbridge Agricultural Products said disease happens with a susceptible host, the presence of a pathogen and a conducive environment. Increasing plant resistance to disease can be achieved by using resistant varieties, planting disease-free seed or increasing a plant's natural defense system. Soils that have a better mineral balance are important and can help plants resist diseases, Parker said. Two examples are zinc and manganese, which can be added to soils to reduce pathogen virulence.

Manganese fertilization has been shown to control powdery mildew of cereals and downy mildew of sorghum. It also inhibits fungal pathogens at levels necessary for plant growth, he said.

Zinc has direct effects on fungal growth and metabolism and can reduce severity of plant disease, inhibit production of phytotoxins at high concentrations and also enhances efficacy of biological control agents.

Parker explained that plant pathogens and pests produce chemicals that can be recognized by the plant and trigger immunizing responses. There are two types of response—a "systemic acquired resistance," which is a direct response to infection, and an "induced systemic resistance," which is a response to any material that is present and active in an individual organism or living cell, but that originated outside of that organism.

With a systemic acquired resistance, a plant will try to protect itself by killing the tissue surrounding the infection to isolate it. In the case of an induced response, there is a physical barrier to plant pathogens.

Environment modification can also be a factor in reducing plant disease in organic or conventional systems. Pathogens generally start on the lower leaf surface where they are protected from sunlight. Parker said that pruning, leafing or removing laterals to increase sunlight penetration and reduce humidity assists in disease prevention.

Applying specific materials has also proved beneficial in helping plants resist disease.

Raising or lowering leaf surface pH with organic acids or bases can inhibit bacteria. Light summer oils can act as a suffocating agent. Dormant oil sprays should only be applied after winter hardening, Parker said, as they can burn leaves.

Irrigation management also plays a part in control, as Parker advised using sprinklers only in the morning to reduce humidity.

(Cecilia Parsons is a reporter in Ducor. She may be contacted at ceciliaparsons8@gmail.com.)