Contemporary biological control

Biological control of insects and mites in greenhouse crops began in England during the late 1920s when Encarsia formosa was used to control greenhouse whitefly on tomatoes.The use of Encarsia stopped, however, after the development of synthetic organic pesticides in the 1940s. Using natural enemies for pest control in greenhouses became popular again in the 1960s when twospotted spider mite populations in European greenhouses became resistant to many pesticides and devastated cucumber crops. Growers introduced predatory mites to control the spider mites, and because pesticide use had to be limited, Encarsia was again utilized for whitefly control on cucumbers and tomatoes. Researchers developed most of the early techniques necessary for biological control in greenhouses at the Glasshouse Crops Research Institute in Littlehampton, England and the Research Station for Vegetables and Fruit Under Glass in Naaldwijk,The Netherlands.Today, researchers in North America also play an important role in developing and implementing biological control strategies for greenhouse production.

Increasingly, greenhouse growers around the world are replacing pesticides with biologically based controls. Why? Biological control offers a way to control pests that are resistant to insecticides, may reduce the cost of insect control, often achieves better control of the pests, and usually increases yields by eliminating the subtle deleterious effects of repeated applications of chemicals. In addition, public demand for pesticide-free produce is growing and governmental regulation is reducing the options for agricultural pesticides. Pesticides are still the main means of insect control in most U.S. greenhouses, but the loss of effective chemicals through regulation, increasing pest resistance, and the escalating cost of developing new pesticides is encouraging greater adoption of biological control methods.

Integrated pest management is now used in many European and Canadian commercial vegetable greenhouses. Biological control has been very successful on vegetable crops, such as tomatoes and cucumbers, because these crops can tolerate some insect or mite damage without affecting yield. Vegetable growers also use biological control because it is easier to maintain harvest schedules without pesticide reentry restrictions. Bumble bees can be used for more cost-effective pollination when pesticides are not used, and produce without pesticide residues may be more marketable or sold for higher prices.

However, certain obstacles must be overcome for the continued adoption of biological control in greenhouses.The greenhouse industry is shifting from seasonal production to nearly year-round production in the same green-houses.This may influence the effectiveness of biological control.There is no longer a host-free period in which pests starve to death, so newly introduced plant material may become infested almost immediately. Natural enemies may move from one crop to the next along with the pests, but there will likely be a lag time and hence some damage to the new crop.Worse, some of the available natural enemies enter diapause under winter greenhouse conditions—short days and cooler nights— and will not provide adequate control during this time.To prevent natural enemies from entering diapause, you can increase night temperatures or extend daylength by using artificial lighting. Increasing release rates, using nondiapausing natural enemies or integrating selective pesticides can also help to control pests during this period.

Biological control may not be practical in the production of certain ornamental plants or in multicropping.On ornamentals the presence of any insects or mites, whether pests or natural enemies, reduces the aesthetic value and salabil-ity of a plant. An IPM or biological control program on ornamentals must be modified to eliminate both the pests and the natural enemies prior to sale. In addition, since the entire ornamental plant is often sold, it can tolerate very little vegetative damage. Control measures must reduce pest populations before they damage the marketable portion of the crop. But there is usually a delay between natural enemy introduction and the reduction of pest populations, during which plant damage can occur. Effective commercial natural enemies may not be available and specific recommendations may not be developed for biological control of certain pests on many ornamentals. Multicropping in the same greenhouse also presents a challenge: maintaining biological control on several different plant species simultaneously. Certain plant production programs, or crop mixes within a greenhouse, may prohibit or severely restrict biological control.

Among the biggest obstacles to the implementation of biological control in greenhouses are the negative attitudes of some growers or the unrealistic expectations of others. Many growers are unwilling to invest adequate time to learn how to succeed with biological control. It takes a fair amount of time and effort to convert from reliance on pesticides to using natural enemies. Every situation is unique, and specific protocols must be developed for each greenhouse if biological control is to be effective. Experimentation may be required to adapt recommended release rates and timing to each situation. Sometimes growers'first attempts to implement biological control fail. But these failures often lead to a greater understanding of how to utilize natural enemies successfully—if the grower sticks to it.

Some growers believe that pesticides are the only way to provide sufficient control for high plant quality. Certainly there are excellent pesticides and application methods that can sometimes provide virtually pest-free plants. However, even ornamental crops produced under conventional chemical control—particularly those seen in mass-marketing outlets—are not pestfree. Biological control can be used during at least a portion of the production of many ornamental plants to produce plants that are equal to those produced with chemical pesticides. Pesticides may be necessary just for the final clean-up.

Some of the best candidates of floricul-tural crops for biological control are rose, alstroemeria, chrysanthemum and gerbera for fresh cut-flowers, plus poin-settia and numerous foliage plants. These plants are attacked by a limited number of pest species or can tolerate some pest injury, although aphids and thrips can be difficult to control in flori-cultural crops. Biological control is most likely to succeed on cut flowers, such as chrysanthemums or gerbera daisies, because only the top portion of the plant or flower stalk is harvested. About the top 32 inches of the chrysanthemum plant is marketed and the bottom 5-16 inches of each stem is stripped of all leaves. Damage to the lower portion of the plant when young (the first 4-6 weeks of plant growth) does not affect the marketable portion when harvested. With gerbera daisies grown for cut flowers, only flower stalks that totally lack foliage are harvested.

Even for crops where biological control of foliage and flower pests is difficult, there exist excellent opportunities for biological control of soil pests such as fungus gnat larvae.

Permanent plant collections, such as conservatories for public display or horticultural greenhouses for research or educational purposes, are also excellent places for implementing biological control. Marketability of conservatory plants is not a factor, so tolerance for injury may be higher.

0 0

Post a comment