Using Biocontrol Agents in the Commercial Greenhouse

Linda A. Topliff, Extension Associate - Greenhouse IPM
Kenneth N. Pinkston, Extension Entomologist
Sharon L. von Broembsen, Extension Plant Pathologist
Michael A. Schnelle, Extension Ornamentals/Floriculture Specialist
Michael D. Smolen, Extension Water Quality Specialist

Pest control in the commercial greenhouse has evolved from a fairly routine and unimaginative chore to a complex, multi-faceted one. Environmental concerns, restricted labeling of pesticides for greenhouse use and pest resistance have caused growers to seek alternative forms of pest management. Integrated pest management (IPM) offers growers alternatives to pesticides for pest control. Total abandonment of pesticides is not the focus of an IPM program. IPM is a holistic approach to managing diseases, insects and mites in the greenhouse. It combines many components, including judicious pesticide usage, that when employed, will decrease pesticide usage. Some of these components include sanitation, cultural practices, scouting, “softer” chemicals, physical barriers, record keeping and biological control agents.

One component of IPM that is receiving increasing attention is the use of biological control agents, also called biocontrol agents. Simply put, a biocontrol agent is any life form used to control or eradicate another. Biocontrol agents used in the greenhouse include insects, mites, bacteria, fungi and nematodes. These bioagents may be predatory, parasitic, pathogenic or work by antibiosis. Predaceous insects and mites function by consuming several prey over the course of their development. They are free living, and the predators are usually as large or larger than their prey (the pest species). Predators may be generalists (feeding on a wide variety of prey) or specialists (feeding on only one or a few related species). Common predators include lady beetles, lacewings, true bugs such as Podisus (Spined Soldier Bug) and Orius (Minute Pirate Bug), preying mantids, spiders, midges including Aphidoletes, mites such as Phytoseiulus and Amblyseius, and beneficial nematodes such as Steinernema carpocapsae.

True insect parasites are generally much smaller than their hosts. As they develop, parasites usually weaken but rarely kill their hosts. The majority of parasite-like forms being used in biocontrol programs are most appropriately referred to as parasitoids. In contrast to true parasites, many parasitoids are almost the same size as their hosts, and their development always kills the host insect. In contrast to predators, parasitoids develop on or within a single host during the course of their development.

Most parasitoids are highly host-specific, laying their eggs on or into a single developmental stage of only one or a few closely related host species. They are often described in terms of the host stage(s) within which they develop. For example, there are egg parasitoids, larval parasitoids, larval-pupal parasitoids (eggs are laid on or into the larval stage of the host, and the host pupates before it dies), pupal parasitoids and a few species that parasitize adult insects.

The vast majority of parasitoids are small to minute wasps, but a few species of flies and beetles are also parasitoids. Some of the parasitoids/parasitic wasps utilized in greenhouses include Trichogramma, Encarsia, Leptomastix, and Aphelinus.

Pathogens (disease causing organisms) such as bacteria, fungi and protozoans can also be used to manage greenhouse pests. These organisms either infect the pests directly or are ingested by the pests. The pests become diseased and stop feeding, fail to reproduce and/or die. Bacillus thuringiensis, a bacterial pathogen used to control insects such as leaf feeding Lepidoptera and fungus gnats, and Nosema locustae, a protozoan used to control insects such as grasshoppers outside greenhouses, are two examples of pathogenic biocontrol agents. Some fungal biocontrol agents which are incorporated into potting mixes to control soilborne plant pathogens not only parasitize the target fungi but also produce antibiotic compounds that inhibit them and plant pathogenic bacteria. Gliocladium virens (GL-21) is a fungal biocontrol agent that has recently become available for greenhouse use.

Many growers try to achieve “zero pest tolerance” by using pesticides. The development of resistance to pesticides within a traditional chemical control program is making this less of a possibility. Growers should be aware that zero pest tolerance is not practical using biocontrol agents; however, pest populations can be kept below economic thresholds. Once the biocontrol agents are well established, outbreaks rarely occur because the agents and pests are in balance at low levels. If the pest population becomes too low or is eradicated, biocontrol agents starve or move on and must be replaced.

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Encarsia formosa

This tiny parasitic wasp is used primarily to control greenhouse whitefly. It should be introduced as a preventative or at low pest population levels for best results. The wasps lay their eggs in the third and fourth instars of whitefly larvae. Encarsia are sold as parasitized whitefly pupae stuck to small cards. The cards are easy to hang on plants below the canopy, out of direct sunlight. Release rates should average 1 wasp for every 1 to 4 plants per week for 4 to 6 weeks. Higher release rates may be needed for older, and thus larger, plants.

Encarsia has also been tried for sweet potato whitefly control but with less success. Release rates are higher, 4 to 5 Encarsia per plant for the first 3 weeks, then 1 per plant for 6 weeks. Greenhouse whitefly pupae that have been parasitized turn black, and sweet potato whitefly pupae turn a straw color. When 70 to 80% of the plants contain parasitized pupae, releases may be discontinued.

The wasps do best when daily temperatures exceed 72° F. At lower temperatures, the whiteflies reproduce too fast for the Encarsia to control them, so it is not advisable to release them when day temperatures are below 72° F and the night temperatures are below 59° F, as often is the case in winter. April releases have been successful for some growers. Encarsia lay more eggs in bright light; however, sodium halide lights may kill Encarsia that fly into them. The wasps tend to migrate to drier parts of the greenhouse. Encarsia are readily available from numerous suppliers. To check viability of the wasps, a small portion of each shipment should be set aside in small containers, such as sealed plastic bags or yogurt cups, and emergence observed.

Delphastus pusillus

Another biocontrol agent effective against whiteflies is Delphastus pusillus. Both the larvae and adults of this small black lady beetle feed on whitefly eggs, larvae and adults. It should be used in conjunction with Encarsia. It has been effective against Bemisia tabaci, the sweet potato whitefly. Availability, however, is limited.

Phytoseiulus persimilis

This predatory mite is widely available for the control of spider mites. They are shipped mixed with vermiculite or bran. Check quality by shaking some vermiculite on white paper and looking for tiny orange mites moving around. Better results are generally obtained using locally produced predators, as these often arrive in better condition than imported ones due to shorter shipping time.

Care must be taken when releasing the Phytoseiulus. Sprinkle vermiculite on every infested leaf. Order enough for one mite per infested leaf. They become established in the crop in about one week. Beneficial mites prefer moderate temperatures and humid conditions (60-90% relative humidity).

Spider mites have been successfully controlled by releasing 10 predators per plant. Phytoseiulus has also been used on Gypsophila (Baby’s Breath) and has been effective from two to seven months.

Exercise care when using miticides near Phytoseiulus. Abamectin (Avid ®) will indirectly kill beneficial mites that feed on treated spider mites.

Other beneficial mites that are available include: Phytoseiulus longipes, Amblyseius californicus and Galendromus occidentalis. Each of these species is suited to a different environmental condition that should be considered when ordering. For example, Phytoseiulus persimilus does best in fairly humid conditions under 85° F. There is a strain of P. persimilus adapted to high temperatures that tolerate temperatures over 100° F with lower humidity. P. longipes can handle temperatures over 100° F and has the lowest relative humidity requirement of commercially available predatory mites—40% at 70° F (higher relative humidity required at higher temperatures). Amblyseius californicus tolerates slightly warmer temperatures than Phytoseiulus persimilus with lower relative humidity requirements. Most dealers are willing to assist growers in selecting the proper biological control agent for their greenhouse environment.

Steinernema (=Neoaplectana) carpocapsae

These beneficial nematodes attack numerous insects in the soil. A moist environment and appropriate application rates are the two main requirements for success. The nematodes can be supplied in a spray concentrate or a moist granular carrier. A smaller hybrid strain of Steinernema is especially suited to the warmer environment of the greenhouse. Apply 70,000 per sq. ft. of soil surface, preferably in the evening. Some growers apply them using injector systems or diluting them with water and using a pump sprayer, hose-end sprayer, watering can or pail. They will survive in oxygen-rich water, such as produced by nutrient film technique systems, but may “drown” if left in standing water for more than a few hours.

Amblyseius cucumeris and A. barkeri (=mckenziei)

These thrips predators are widely available. They are tan-orange mites that are similar in appearance to spider mites. They prey on Western flower, onion, broad and cyclamen thrips. They are best used as a preventative measure in ornamentals, before any evidence of thrips occurs. They prey only on the larval stages of the thrips, and they cannot control an established thrips population.

These predators are relatively inexpensive. They are received in containers of loose bran which is easily sprinkled on the foliage of the plant. A suggested release rate is 10 to 100 per m² weekly. Other sources suggest an introduction rate of one predator per ft², followed on a monthly schedule.

Amblyseius spp. preform best in warm, dry weather. They enter diapause and are not effective between September and March unless night temperatures remain above 70°F.

Hippodamia convergens

Both the adult and immature ladybug (ladybird) beetles feed on aphids and spider mites. Ladybugs do not always provide successful, long-term control; however, they are useful to “knock down” an infestation. Availability is good, except during the early summer months when supplies may be sold out.

Ladybugs may be kept in the refrigerator for several weeks and released one handful at a time. They are usually thirsty when released, so plants should be misted prior to release. Since ladybugs have a strong dispersal instinct, spraying them with diluted soda pop will cause their wings to stick together and keep them from flying. Releasing them in the evening is also helpful.

Aphidoletes aphidimyza

Aphidoletes aphidimyza is a predatory midge commonly known as the aphid predator. The tiny orange larvae of this predator attack and kill aphids. The adults resemble tiny mosquitoes. They are effective against many species of aphids, but not against the melon aphid.

The midges are shipped in the pupal stage, mixed in either small bags of vermiculite or peat moss. Cut a one inch hole in the top of the bag and place it at the base of an aphid-infested plant. Keep the vermiculite or peat damp but not saturated. Adults will emerge in seven to ten days. Adults fly at night and lay eggs in aphid colonies. Since only the larval stage attacks aphids, overlapping generations must be maintained. Repeat the application in two weeks. Suggested release rate is one cocoon per 10 ft² of greenhouse or a target level of 1 larvae per 15 to 20 aphids.

Short days will cause A. aphidimyza to enter diapause. Lighting with 80 to 100-watt bulbs at night will prevent diapause. The optimum temperature for the midge is 73° F.

This predator works especially well on long season crops such as lilies, roses and chrysanthemums and in greenhouses with in-ground beds or gravel floors because it pupates in the soil.

Cryptolaemus montrouzieri

This beneficial lady beetle is known as the mealybug destroyer or mealybug predator. It prefers warm temperatures (72-77° F) and is not effective during the winter months. The adult and larva are predaceous and attack mealybugs and scales. Release rates vary with pest population, approximately 2 per m² or 2 to 5 per infested plant. Release at various infestation sites. Repeat releases may be necessary. Citrus mealybugs may also be controlled by Leptomastix dactylopii. This parasitic wasp prefers sunny environments with warm temperatures of 74 to 80° F. Control takes one to three months.

Amblyseius barkeri, a predatory mite sold for control of thrips in greenhouses. (Photo by: Marion Herbut)	Enlarged view of Amblyseius, predatory mite with thrips.	Enlarged view of Phytoseiulus persimilis, a spider mite predator. (Photo by: Jack Scott)
View of sweet potato whitefly pupa parasitized by Encarsia formosa. (Photo by: Marion Herbut)	View of greenhouse whitefly pupa parasitized (black) by Encarsia formosa.	Greatly enlarged view of Encarsia formosa.
Enlarged view of predatory midge, Aphidoletes aphidimyza and aphid.	Tiny parasitic wasp, Aphidius matricariae, and parasitized aphids that have turned into hard, brown shells (mummies). Note holes/flaps at rear of mummies where the wasps have emerged.	Adult mealybug destroyer, Cryptolaemus montrouzieri, and enlarged view of the larva of this lady beetle. Note how much the larva resembles a mealybug.
Parasitic wasp, Metaphycus helvolus, that has been used to control some species of scale insects.	Enlarged view of Steinernema carpocapsae.

Chrysoperla (=Chrysopa) carnea and C. rufilabris

Lacewings are generalist predators. They will attack almost any soft-bodied insect. They are best known for attacking aphids and each larva can kill 30 to 50 aphids per day. In some species adults are predaceous, while in other species only the larvae are predaceous. Only the larvae of the two species commonly available, Chrysoperla carnea and C. rufilabris, are predaceous.

Lacewings are usually shipped in the egg stage, packed in bran or rice hulls. At 80° F they will hatch in about four days. They hatch with a voracious appetite and will often resort to cannibalism if no other food source is available. They will attack and eat other biological control agents. Each larva will feed for about three weeks, pupate and emerge as an adult about one week later.

Conclusions

Biological control agents are an important part of integrated pest management systems. Their use must be coordinated with other types of pest management, chemical and mechanical. Care must be taken when considering which agents to utilize. Pest species must be carefully identified; pest populations must be low enough so that bioagents are not overwhelmed; temperature, humidity and lighting of the greenhouse must be considered; and previous pesticide usage must be taken into account before biocontrol agents are introduced. Strict attention to these requirements will help make integrated pest management a reality in the commercial greenhouse.

Department of Horticulture and Landscape Architecture
P*E*A*R*L (Publications Database)
Oklahoma Cooperative Extension Service
Division of Agricultural Sciences and Natural Resources
Oklahoma State University