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In this section, we will summarize some of the most common problems encountered during implementation of biological control for twospotted spider mite in the greenhouses. These are (1) improper timing of the release of P. persimilis; (2) impatience on the part of the grower; (3) lack of adequate pest suppression achieved by the control agent; and (4) cultural or chemical practices which adversely affect the natural enemy.
The need for releasing predators at the proper time has already been mentioned, but will be repeated because of its overall importance. To effectively use biological control, the grower must initiate the program when the pests are at relatively low levels. In this regard, we have observed that many growers opt for biological control when it is too late, i.e., after a major pest problem has developed and is on the verge of destroying the crop. Clearly, growers must use some forethought and plan to initiate a control program when the pests are first found. This requires careful monitoring of the greenhouse crop(s). Growers must also realize that, unlike effective chemical control, biological control does not produce instant results. This may be true even when massive numbers of natural enemies are released in the greenhouse. Nonetheless, releasing predators at the proper time (and in adequate numbers) will shorten the period before control is attained and improve considerably the likelihood that biology control will be successful.
Impatience on the part of the grower can often lead to failure of a biological control program. Such impatience usually results in the resumption of the chemical control program before there is any need to do so. The basic cause of this is probably a lack of appropriate experience with biological control. It must be remembered that mite populations--regardless of whether predator or pest--require time to develop and increase. On the other hand, "too much patience" can also lead to problems. For example, in certain instances P. persimilis may fail to establish, and long delays before taking corrective action may allow the mite population to build up to very high densities. This problem can be minimized by closely monitoring the pest population which is critical to any pest control program. A monitoring program should be designed to identify potential problems. It should also be sensitive enough to provide the grower with enough warning to allow time for remedial actions. This lead time is important and is determined by grower- defined measurer such as the relative densities of predators and prey and the presence of other pests. The level at which other control measures should be initiated must be determined prior to implementing a biological control program.
As an example, Hassen (1983) developed a program for cucumbers. Forty cucumber plants in each of three different greenhouses were chosen at random. These 40 plants were ca. 3, 6, and 20% of the total number of plants in each of the greenhouses. Each plant was examined weekly for the presence or absence of any stage of T. urticae or P. persimilis. At the same time, these plants were observed for other potential problems. If such a program is to be useful, the grower should look for areas where damage is about to exceed a critical damage threshold (Anonymous 1976a). Secondly, the grower should look for "hot spots" where predators are scarce. The remedial actions that could be taken would depend on what was found. Hot spots could be controlled with a selective pesticide such as soap, fenbutatin oxide, or oil. If the potential for severe damage exists the grower would have to consider treating the entire crop.
Another problem is that natural enemies may not control their hosts at all times or under all conditions. Many failures can be attributed to improper environmental conditions within the greenhouse. As we have explained previously, P. persimilis is adapted to certain temperature regimes, outside of which their performance is reduced measurably. Certain cultural practices can also be devastating to a biological control program. One practice that can cause problems is the movement of plants in and out of the greenhouse. Obviously, when the plants one is trying to protect are held in the greenhouse for only a short period of time, biological control agents will often have a minor impact on the pest population. Another common problem is the movement of plants from one greenhouse to another, or when plants are brought in from out- of-doors. These plants are seldom inspected for the presence of pests and, invariably, there will be one plant that is infested with some kind of pest. Furthermore, these pests generally enter the greenhouse undetected and without their effective natural enemies. Subsequently, they multiply unchecked. There are methods to control a few of the occasional pest species without the disruption of the biological control program. However, in many cases pesticides may have to be used, which may then kill or severely disrupt a P. persimilis population. These disturbances can be minimized through sanitation and careful inspection of all plant materials before they are brought into the greenhouse.
Plants contaminated with occasional pests may be present in the greenhouse when the grower, in preparation for a biological control program, discontinues the use of broad-spectrum pesticides. These pests are then free to multiply, unchecked, and usually require chemical control which, again, jeopardizes the survival of existing biological control agents. Perhaps the most effective way of eliminating this problem is to treat the greenhouse with the appropriate pesticide prior to releasing the predator.
This leads us to the quickest and most common way of causing a biological control program to fail, that is, the application of nonselective pesticides. As we have emphasized, P. persimilis is very susceptible to many chemicals (Table 3). However, there appears to be some discrepancy as to the susceptibility of P. persimilis to specific pesticides. Some of the confusion is probably due to differences between strains. Efforts are being made toward identifying and/or developing strains resistant to various pesticides (Schulten et al., 1976; Schulten, 1980). Resistance or tolerance to diazinon, demeton-S-methyl, malathion, mevinphos, fenbutatin-oxide, tetradifon, cyhexatin, azinphosmethyl , carbaryl, methidathion, pirimicarb, pyrazophos, and triforine has been reported (Woets and van Lenteren, 1983). Because important differences exist among strains, growers are advised to consult with their suppliers as to the sensitivity of their specific predators.
Table 3. Safety of commonly used pesticides to Phytoseiulus persimilis eggs and adults*. Chemical Method of
Eggs*** Adults Insecticides & Acaricides Acephate HV - H Aldicarb GRANULES - H Bacillus
HV S S Carbaryl HV - H Cyhexatin HV - H Diazinon HV - H Diazinon DRENCH - S Diazinon ULV - H Dicofol HV - H Dienochlor HV I H Dimethoate HV H H Dimethoate Drench H - Endosulfan HV - H Fenbutatin-oxide HV - S Insecticidal Soap HV S H Kinoprene HV S S Lindane HV H H Lindane DRENCH - I Lindane SMOKE S H Malathion HV H H Methomyl HV - H Nicotine HV - I Nicotine SMOKE S S Oxamyl GRANULES S S Oxamyl HV - H Oxydemetron-methyl HV - H Oxythioquinox HV H H Oxythioquinox SMOKE H H Parathion HV H H Parathion DRENCH H H Parathion SMOKE H H Permethrin HV - H Permethrin FOG - H Pirimicarb HV S S Pyrethrin HV H H Propargite HV H - Resmethrin HV H H Rotenoe HV - H Tetradifon HV S S
Fungicides Chemical Method of
Eggs *** Adults Benomyl HV H H Benomyl DRENCH I I Captafol HV S S Captan HV S S Chlorothalonil HV S S Dinocap HV - S Iprodione HV S S Mancozeb HV - S Maneb HV - S Metalaxyl HV - H Thiophanate-Methyl HV H S Thiram HV I S Triforine HV S S Vinclozolin HV - S Zineb HV - S
- *These data were compiled from Hassen and Oomen 1985, Ledieu 1985, and Steiner and Elliott 1983.
**HV = High volume and ULV = Ultra low volume.
***Key to symbols: S = Safe; I = Intermediate; H = Harmful; and - = Unknown.
OBTAINING NATURAL ENEMIES
Natural enemies for use in the greenhouse can be obtained from commercial insectaries. A number of such companies sell biological control agents. The problem for the grower is finding these companies. There are a few ways in which this can be accomplished. The first would be to contact the Florida Cooperative Extension Service. Offices are located within each county, with the main entomology office located at the University of Florida in Gainesville. There are a few publications that list companies that sell biological control agents (Hickman, 1984; Olkowski and Redmond, 1983). Interested individuals might also consult recent books dealing with organic methods of pest control as many of these will list sources of biological control agents.
Once an individual obtains P. persimilis, it is also possible to maintain a colony without much difficulty. There are various advantages to the grower in doing so: (1) it would provide a constant supply of the predators; (2) they would be readily available; and (3) it might be less expensive to propagate the predators than to periodically purchase them from commercial sources, especially if growers form a cooperative to maintain such a colony. The efficient production of P. persimilis depends on the availability of three isolated areas: (1) an area for growing plants (usually Henderson bush lima beans) without any pests present; (2) an area for maintaining pure colonies of the pest; and (3) an area for raising the predators. In this manner, noninfested plants can be placed in the pest colony. After a sufficient infestation is obtained, they can then be transfered to the final location where the infested plants are exposed to the predators. Although it is possible to maintain pests and predators in one place, this practice is not usually conducive to mass production. If this method is necessary (e.g., because of space shortage), the movement of mites among areas can be reduced by placing the plants on platforms which rest over containers of water. A surfactant or detergent should be added to the water to reduce the surface tension. In this way, both predator and prey mites which drop off the plants will fall into the water and die. A second recommendation is to locate the plants containing the predator colony "down wind" from the prey colony to minimize contamination by air currents.
The method of rearing P. persimilis at the Agricultural Research and Education Center in Apopka is quite simple. First we sow bean seeds into a commercially prepared soil mix which is contained within a 15x3Ox33 cm plastic dish washing tub. These tubs are kept in a greenhouse until the first true leaves are fully expanded. At this time they are moved into a room maintained at 25°C and lighted with four, 4-tube, 40 W Cool White fluorescent light fixtures. Infested leaves from about 10 plants are placed on the beans in one tub. The beans in this tub are grown until a heavy mite population develops. Each tub is then placed in close contact to a tub that is infested with predators. The appropriate time to harvest the predators depends on when the two populations reach the desirable ratio. This ratio (predator:prey) could be any where from 1:0 (no prey) to 1:50 depending on need. Harvesting predators is the next step. This is accomplished in a number of ways. We place leaves cut from one tub into paper bags, fold the top over about four times, and staple the bags shut. This prevents the accumulation of excess moisture as the foliage dries, and it allows the predators to feed on the remaining spider mites. Leaves are then removed from the bag as needed and placed throughout the crop. After most of the leaves have been removed, the bag is then placed in close proximity to the most heavily infested plants. Another method of harvesting predators is to place leaves in a plastic bucket which is sealed with a lid. A large hole should be cut into the lid and covered with nylon organdy to allow ventilation. After 24 to 48 hours predators can be found running around the lid and upper portion of the bucket. These individuals can be dislodged directly onto plants within the greenhouse or they can be collected for later use with a simple suction device such as an aspirator or with a soft camel-hair brush. Parr (1968) has shown that predators and prey can be stored. He placed 5 predators plus 15 prey in a tube and stored them at 8.3°C for 3 weeks to 4 months with ca. 60% survival regardless of the period of storage.
Additional aspects of culturing the twospotted spider mite and its predator can be found in the following: Anonymous (1975), Gilstrap (1977), Hoy et al., (1982), Kamburov (1966), McMurtry and Scriven (1965), Scopes (1968), Scriven and McMurtry (1971), Theaker and Tonks (1977).
We especially thank M. J. Tauber, Cornell University, for his expert advice and helpful comments during the preparation of the manuscript. We also thank the following individuals for critically reviewing the manuscript: W. W. Allen, R. G. Helgesen, M. A. Hoy, N. W. Hussey, J. E. Laing, R. K. Lindquist, R. J. McClanahan, J. A. McMurtry, R. F. Mizell, J. C. van Lenteren and J. F. Price. Preparation of portions of this Bulletin was financed by a grant from the Elvenia J. Slosson-University of California Endowment Fund for Environmental Horticulture. Photographs were taken by Jack K. Clark.
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