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EAP Publication - 52
An increasing number of commercial greenhouse growers around the world employ beneficial insects to control Pests. This is known as biological control. In 1982, this method was used by Dutch growers on 550 hectares of glasshouse tomatoes and 600 hectares of cucumbers (Ramakers, in press). In the Leamington area of Ontario' where one tenth of the greenhouses in Canada are concentrated, 40% of the tomato growers now use a Parasitic wasp, Encarsia formosa, to control whitefly. Biological control isn't a new idea--in fact over 50 years ago, Encarsia was used extensively in England and Australia in commercial greenhouses (Farr, et se. 1976) With the introduction of DDT in the 1940's, the use of biological control agents virtually ceased. The recent return to biological methods was prompted by the development of pesticide-resistant pest populations' the high cost of pesticides' the difficulty in observing pesticide withdrawal times while maintaining harvest schedules' and the reductions in yield due to the phytotoxic effects of chemicals. Increasing concern over the exposure of greenhouse workers to pesticides, improved monitoring of pesticide residues in food and stricter regulations governing the use of pesticides on food crops also contributed to renewed interest in a nontoxic alternative to chemical Pest control. A 1981 survey of 106 commercial growers in western Canada using biological control s on tomato and cucumbers,, found that they reported a 60% to 80% reduction in time spent on spraying for whitefly or spider mites 23% reported increased crop yields when biological controls were used, and 38% reported a reduced expenditure on pest control (Elliott' 1982).
This paper begins with a brief review of the principles of biological control as they apply to greenhouse pests. The application of the most commonly used biological control organisms is described and the paper concludes with a review of beneficial species that may soon be available to Canadian growers.
Presented at a Symposium on Perspectives of Development of the Greenhouse Industry in Quebec, Laval University, Ste. Foy, Quebec. October 1984.
The introduction of natural enemies in greenhouse cultures has several advantages over their use outdoors. The annual removal of all crop debris and the disinfection of the soil results in a clean start each year , with pest populations at a predictable, low level. The number of pest species involved is also usually low especially in monocultures. It is easier to monitor pest populations and control the environment under glass than it is outdoors. Unlike the outdoor grower, the greenhouse manager can apply biological controls without interference from treatments to neighboring crops.(van Lenteren, et al., 1980).
How does a grower implement a biological control program? Steiner and Elliott (1983) suggest seven basic steps that should be taken before natural enemies are released:
Natural enemies are applied in three main ways: conservation, inoculation and inundation. Conservation is the attraction and preservation, of naturally occurring populations of beneficial inserts. It is most applicable to integrated control programs outdoors, but for some pests' such as the tomato leaf miner, Liriomyza bryoniae, conservation of the native parasitic wasp complex has been found to provide adequate control for many Dutch growers (Woets and van der Linden, l982).
Inoculative releases are introductions of small numbers of biological control agents, early in the season, in order to establish reproducing populations inside the greenhouse. To be successful, the natural enemies must be released while the number of pests is still low. This gives natural enemy populations time to build up before the pests reach economically damaging levels. The advantage of this method is that the cost is low, since fewer insects are needed and pest control often continues all season. without further intervention by the grower.
Inundative releases are mass introductions of predators or parasites aimed at eliminating the pests immediately, during the first generation of the control agent. In this method. the beneficial insects are used in much the same way as a pesticide. While inundation is useful in cases where pest populations are high, it is more expensive than inoculative releases. Since the beneficial organisms usually die out once they have eliminated the pest, later introductions will be required if the pest reoccurs.
Most growers are well aware of the growth stages and the changes or deficiencies in their plants, but they are not as knowledgeable about either the biology of the pests or the beneficial insects. Familiarity with insect lifecycles is essential, because timing the releases of natural enemies, so that they will have the maximum impact on the pest population, is the key to success. Ideally, beneficial species are released at the first sign of the pest (Fig. la). In order to detect the earliest signs of an infestation, however the grower must know where and when the pests are likely to occur on the plants. For example, whitefly adults rest on the underside of the tip leaves on tomato. while the immature stages are found halfway down the plant. A new infestation of whitefly will be quickly detected by examination of top leaves, whereas the buildup of an Encarsia population would be best monitored where the whiteflies are pupating further down the plants. In some cases, inundative releases of large numbers of natural enemies can be used to control high population densities of pests (Fig. 1b), but biological control cannot succeed if 4 predators or parasites are released after the pests have reached economically damaging levels (Fig. lc). lt often takes growers one or more seasons to perfect the timing of introductions (Elliott, pers.comm.).
Once the biological control agents have been released, it is very important to have patience, keep good written records, and act promptly if "hot spots" (pest outbreak areas) occur. An accurately timed, carefully applied, local application of an appropriate non-residual pesticide, can often prevent a localized pest build-up, yet may do little damage to the natural enemy populations. Some chemicals, particularly fungicides, are only slightly toxic to beneficial organisms, and may be used with care. Pesticides with a short- period of residual toxicity can be used to reduce pest populations to low levels before the introduction of natural enemies. An alternative approach involves the selection of pesticide resistant biological control agents. The use of organophosphorus-resistant predatory mites is the best known example of this.
The following section describes the main greenhouse crop pests and their biological controls, some of which are currently sold in Canada, and others that may be marketed in the near future.
The spider mite is a major pest in commercial greenhouses. Shortly after organophosphorus pesticides were introduced, pesticide-resistant spider mite populations appeared. This prompted researchers in the Netherlands to begin investigating integrated control of this pest in the 1950's. In 1962, the discovery that the predatory mite, Phytoseiulus persimilis, could be used to control spider mites, stimulated research into biological control and mass production methods. By 1969 a few Dutch cucumber growers were using the predator to control spider mites. Acceptance of this predatory mite was hastened by the registration of a systemic fungicide for powdery mildew which, when combined with P. persimilis, eliminated the need for frequent fungicide-acaracide sprays (Ramakers, in press). Today about 2/3 of the Dutch cucumber growers use predatory mites on their crops. The widespread application of this predator was stimulated by the genetic selection of an organophosphorus-resistant strain of P.persimilis (Schulten, 1980). Growers thus have the option of using some pesticides on their crop, without destroying the investment in a biological control program.
The greenhouse whitefly is a common pest of tomatoes, cucumbers and some ornamentals under glass (Costello, et al., 1984). Chemical control is difficult because the eggs and immature larvae have a waxy coating that resists chemicals, Whitefly populations have also developed resistance to commonly used pesticides, even to the recently introduced synthetic pyrethrins, which it was hoped would replace the organophosphorus and organochlorine insecticides.
The first specimens of the tiny, parasitic chalcid wasp, Encarsia formosa, were found parasitizing greenhouse whitefly in the U.S.A . in 1924 (Gahan, 1924). From 1927 to 1953, prior to the widespread use of chemical pesticides, the wasp was used extensively to control whitefly. In the early 1960s, there was a revival of interest in Encarsia when it was found that pesticides used to control whitefly on cucumber interfered with biological control of the spider mite (Farr, et al., 1976). In 1983, a quarter of the commercial tomato growers in the Netherlands use Encarsia, There, the wasp pupae are sold glued to cards, with a prepunched hole at the top so they can be quickly fixed to a leaf stem. Growers routinely introduce the parasites, at the rate of one wasp per plant, two weeks after the crop is set out, or they wait until the first whitefly is seen and introduce them at the higher rate of 1.5-2 wasps per plant. Subsequent releases are made in 10 to 14 days until control is achieved. In the Leamington area of Ontario a system of "banker" plants is used quite successfully by tomato growers. In this approach, the first shipment of Encarsia is released onto four cucumber plants, planted at the head of each house' where they become established on whiteflies before the young tomatoes become infested. From there the wasps disperse throughout the crop. In this area, the local extension agent often delivers the parasites directly from the insectary near Windsor, thus eliminating the uncertainties of mail delivery. Aside from a fungicide spray early in the Year, growers usually apply no other pesticides to their tomatoes, and some report a 60% saving over their previous costs for chemical controls.
Suppression of whitefly by Encarsia is not always as successful in the winter as it is in the summer. This is because the wasps require a higher light intensity and warmer temperatures than the whitefly for maximum reproduction. In Europe, a highly specific fungal disease of whiteflies, Ascheronia aleyrodis is being tested for commercial application. Because of its specificity it may be useful for restoring a balance between parasite and host in the winter (Ramakers, 1984a).
Various species of aphids are a problem in greenhouses on both vegetable and ornamental crops, particularly between late fall and early spring on young plants. They are one of the most difficult insects to control with sprays because of their remarkable reproductive ability. Females do not mate or lay eggs during the summer. Instead, they are parthenogenic and give birth to live young--as many as 5 per day. If even one aphid survives a pesticide application (some always do) then she can generate a new colony and reinfest the crop.
An effective aphid predator or parasite that can he integrated with the other main biological controls has been sought for many years. Lady beetles (Hippodamia convergens), lacewings (Chrysopa carnea), and various species of parasitic wasps have been investigated, but the most promising has been the aphid midge, Aphidoletes aphidimyza. This is a native species, common to most of the northern regions of the world, and its small orange larvae are voracious predators of aphids (Gilkeson & Klein, 1981). Workers in Finland began investigating this insect in the early 1970's when the native populations persistently decimated greenhouse aphid cultures being kept for research. It was obvious then that the Aphidoletes were more effective than the insects under investigation. Growers in Finland now routinely use the aphid midge, which they obtain from a commercial company in Helsinki. One or two introductions of a thousand pupae in the early spring are usually enough to provide control of aphids in any size of greenhouse. Aphids are not a severe problem in the Netherlands, where the midge is used mostly on green peppers, which are very susceptible to green peach aphids. In Canada, midge pupae are now available from suppliers, but more work on recommended release rates is necessary. In my own research, I have genetically selected a strain of this midge that does not enter diapause (the insect equivalent of hibernation), under the short days of winter. Aphidoletes populations stay in the soil in diapause, even in a greenhouse, and thus are not present when aphids may still be reproducing on winter crops. This is not a problem in a very northern country, such as Finland, where winter days are much too short for plants, but in Canada it would be an advantage to have non-diapausing predators for use on winter crops. I am also investigating the effectiveness of the midge against aphids in cool greenhouses. We know that it is extremely effective during the spring and summer, even at low release rates, however, more work must be done to determine its usefulness on winter crops, such as lettuce, that are grown at lower temperatures .
Several other greenhouse pests, such as thrips, tomato and vegetable leaf miners, scales, and mealybugs may now be controlled by natural enemies, although at the present time their use in Canada is still experimental. In the Netherlands, the two predatory mites, Amblyseius mckenziei and A. cumeris, are now sold for control of Thrips tabaci on peppers and cucumber crops. The development of highly efficient mass-production methods for these mites, by culturing them on bran mites, has made their use economical at high rates of release (Ramakers, 1984b). Culture of A. mckenziei is now being studied at the Agriculture Canada Research Station in Harrow, Ontario and they may become available to growers in the near future.
Dutch growers saw leaf miners become a problem in tomato crops when they stopped spraying for whitefly. Some growers, however, observed that the leaf miners were naturally controlled by a complex of native parasites that entered through the greenhouse vents (Woets and van der Linden, 1982). Different combinations of parasites are mass-produced and sold in the Netherlands to control either the recently introduced American leaf miner, Liriomyza trifolii or the native species, L. bryoniae Researchers in Canada investigating the parasite complex of the vegetable leaf miner, L. sativae, found that although the parasites were difficult to culture efficiently, once they were released, they seemed to do well in greenhouses, although it was difficult to determine how much of the control was due to the influx of parasites from outdoors (McClanahan, pers. comm.). With better sanitation and the build up of natural parasite populations, the leaf miner is no longer considered a serious problem on greenhouse tomatoes in Ontario, although it is a problem for chrysanthemum growers.
Natural enemies are also available for controlling of the scale and mealybug species that occur in conservatories or greenhouses with specialty ornament crops. The parasite, Metaphycus helvolus, is effective against the hemispherical scale, Saissetia coffeae and useful against soft brown scale, Coccus hesperidum. The combination of the Australian lady beetle. Cryptolaemus montrouzieri and the parasitic wasp, Leptomastix dactyopii is effective against the most common mealybug under glass, Planoccoccus citri.
It is hoped that the examples reviewed in this paper will stimulate interest in biological control among commercial greenhouse growers. Demand from growers is essential in ensuring that research is conducted into the practical application of beneficial organisms. More specific information on release rates, timing of releases, and compatible pesticides, is available from insect suppliers, experimentation research station personnel and published papers. Further experimentation by researchers and growers is necessary however, to adapt this information to local conditions, taking into consideration local crops and market requirements.
Because the timing of releases is so critical, the availability of beneficial organisms is an important aspect of biological control in such a large country as Canada. One of the main obstacles to the use of biological controls in Ontario was the lack of a large scale production facility for Encarsia (McClanahan, 1980), a problem which was solved this year with the construction of just such a facility near Windsor, Ontario. The supply and distribution of beneficial organisms is a problem for those growers outside of the local distribution areas of our two main suppliers (one in Ontario and the other near Victoria, B.C.). This might be solved by cooperative ordering and distribution of predators and parasites, or by the institution of seasonal, grower-supported rearing facilities in local areas.
Biological control is most successful in areas where a professional pest manager is available to advise growers and assist with monitoring programs. An extension agent or agronome from a government supported research station may fill this role, or the manager could be employed by a grower cooperative on behalf of all members.
The main point to remember is that, while biological control methods may not be applicable to every crop, thousands of commercial growers around the world already employ such methods with success. With more research under Canadian conditions, better supply and distribution of beneficial organisms, and the employment of trained pest managers to work with growers, biological control of greenhouse pests can benefit an increasing number of Canadian growers.
Costello , R.A. , N.V. Tonks and D.P. Elliott. 1984. Integrated control of mites and whiteflies in greenhouses. 2nd ed. Pub. # 84:2. Ministry of Agriculture and Food, British Colombia. 17 pp.
Elliott, D. 1982. Biological control progress in western Canada. Sting: newsletter on biological control in greenhouses. 5:3-4. Glasshouse Crops Research and Experimental Station, Naaldwijk, the Netherlands.
Gahan, A.B. 1924. Some new parasitic Hymenoptera with notes on several described forms. Proceedings of the
United States National Museum. No. 2517. 65(4):1-24.
Gilkeson, L. and M. Klein. 1981. A guide to the biological control of greenhouse aphids. Ark Project, Institute of Man and Resources, Charlottetown, P.E.I. 25 pp.
Jordan , W.H. 1977. Windowsill Ecology. Rodale Press, Emmaus, PA. 229 pp .
McClanahan, R.J. 1980. Why has integrated control practice in the greenhouse leveled off in Canada? Working Group on Integrated Control in Glasshouses, Proceedings of the 4th Meeting. Bulletin S.R.O.P./W.P.R.S. III/3:141-144.
Parr, W.J., J.J. Gould, N.H. Jessop and F.A. Ludham. 1976. Progress towards a biological control programme for glasshouse whitefly (Trialeurodes vaporariorum) on tomatoes. Ann. Appl. Biol. 83:349-363.
Ramakers, P.J.M. In press. Application of and research on biological pest control methods on glasshouse vegetables in the Netherlands. Proceedings of the International Conference on Integrated Plant Protection. 1983. Budapest, Hungary.
Ramakers, P.J.M. 1984a. Ascheronia aleyrodis, a selective biological insecticide. Pub.#319. Glasshouse Crops Research and Experimental Station. Naaldwijk, the Netherlands.
Ramakers, P.J.M. 1984b. Mass production and introduction of Amblysieus mckenziei and A. cucumeris.
Pub.#320. Glasshouse Crops Research and Experimental Station, the Netherlands.
Schulten, G.G.M. 1980. A strain of Phytoseiulus persimilis (Acari: Phytoseiidae) resistant to organophosphorus compounds. 119-120. In: A.K. Minks and P. Gruys, eds. Integrated control of insect pests in the Netherlands. Centre for Agricultural Publishing and Documentation, Wageningen, the Netherlands. 304 pp.
Steiner, M.Y. and D.P. Elliott. 1983. Biological pest management for interior plantscapes. Pub.#AECV83-E1. Alberta Environmental Centre, Vegreville, AB. 304 pp.
Woets, J. and A. van der Linden. 1982. On the occurrence of Opius pallipes Wesmael and Dacnusa sibirica Telenga (Braconidae) in cases of natural control of the tomato leafminer Liriomyza bryoniae Kalt. (Agromyzidae) in some large greenhouses in the Netherlands. Med. Fac. Landbouww. Rijksuniv. Gent, 47(2):533-540.
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