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Growers who are using or contemplating the use of "living insecticides" may be confused when those involved in biological control frequently speak of predators and parasitoids in the same breath. Although these types of natural enemies share many common characteristics they pursue lifestyles that are significantly different, as we shall see. Practitioners of integrated pest management, particularly those attempting to control insect pests in greenhouses, should be informed about these differences in order to ensure that natural enemies are able to perform as expected. As with chemical controls it is expected that the grower will follow instructions provided with any biological control product and thereby realize the results promised for the money invested.
In order to better manage insect natural enemies it is probably worth reviewing some basic insect biology. All insects pass through some type of metamorphosis, passing through a larval stage which may bear very little resemblance to the ultimate adult insect. (Adult insects are distinguished by sexual maturity and, for flying insects, by the presence of wings.) Larval insects' principle concern is feeding-- which is why herbivorous insects can be so problematic, causing substantial damage to ornamental or other crop plants, and why insect natural enemies can be such effective checks on pest species, which they search out and destroy. Predators and parasitoids are distinguished on the basis of the feeding habits of their immature stages.
The larval stages of predators are voracious in comparison to parasitoids. A predator will consume many prey individuals before attaining maturity. A parasitoid requires only a single host to complete its larval development. (Parasitoids are distinguished from parasites in that the former usually kill their host and the latter do not.) Because they require numerous prey items, larvae of predatory insects are mobile and equipped to attack and subdue their food species and are often quite a bit larger than their prey. Parasitoids are smaller than the insects they attack.
In contrast to the active hunting of immature predators, the task of locating the host of a larval parasitoid is (usually) accomplished by the adult female, who oviposits on or inside the host insect. Thereafter, the larval parasitoid absorbs nutrients from the body fluids of its host.
Once they have reached adulthood, both predators and parasitoids may exploit prey species as a food source -- often an advantage for the goals of biological control. For example, as predatory ladybird beetle larvae grow, they are able to attack and consume progressively larger stages of the aphids whiteflies and other small insects on which they prey. Similarly, Michael Parrella and Kevin Heinz at UC/Davis are studying Diglyphus wasps which depend on their hosts as food throughout their lives: larval leafmining flies serve as both food and shelter for the immature wasps and the adult wasps host- feed-- that is, they sting leafminers they cannot use for oviposition and lap up the blood from the wounded maggots, thereby increasing mortality of leafminers. Encarsia wasps that attack whiteflies, under investigation by Judy Nelson at UCD, usually indulge both in host-feeding and in drinking the sweet honeydew excreted by the sap-sucking insects they parasitize.
However, adult parasitoids and predators may have dietary habits entirely different from their larval stages , requiring that alternate food sources be present if persistence of the natural enemies is desired. This can also be advantageous for biocontrol if natural enemies can survive on other food items when prey becomes scarce. For instance, predatory lacewing larvae, available commercially, are voracious predators of aphids-- but the adults feed on pollen, nectar and honeydew. Likewise, many hymenopteran parasitoid adults feed on nectar and pollen and these must be available for them to breed and oviposit , which, of course initiates the parasitic, pest- controlling aspect of their life histories.
These differences in dietary habits, as well as other biological distinctions between predators and parasitoids, greatly affect the tactics of biological control in which they can be applied. Remember that parasitoids and predators may be encouraged to thrive in an ecosystem in a variety of ways:
* Conservation of natural enemies already present can be accomplished by providing to natural enemies habitat or provisions for alternate food sources, or by altering harvesting practices which might destroy these resources. For example, a mixture of yeast and wheat proteins combined with a dilute molasses solution sprayed on the foliage of crops can serve as an attractant to lacewing adults whose larva are near insatiable predators of aphids. Border plantings that yield pollen and nectar may be effective in promoting the activity of parasitic wasps, and strip mowing rather than mowing in blocks can conserve the natural enemies that occur in forage crops such as alfalfa.
* Classical introductions are those in which natural enemies become established permanently in an environment after being released and no further introductions are required. The vedalia beetle, Rodalia cardinalis , was introduced to California over a hundred years ago and continues to exert effective control over its prey species the cottony cushion scale, Icerya purchasi . In the same way, infestations of the ash whitefly, Siphoninus phillyreae have recently been reduced in California by the introduction of a miniwasp, Encarsia partenopea, reared at the University of California Riverside in a program coordinated by the California Department of Food and Agriculture.
* Augmentative introductions are those in which natural enemies are bolstered or renewed with periodic re- introductions to environments in which they do not persist for one reason or another. Because plant material is constantly moving through greenhouses the composition of insect fauna is usually very unstable-- which can contribute to the outbreak of pest species. Thus, augmentative releases are those most appropriate to greenhouse management. Inocculative releases "seed" an ecosystem with natural enemies, such that the progeny of the natural enemies introduced achieve control of the target species. Encarsia formosa is often introduced to greenhouses in an inocculative fashion to control greenhouse whitefly, Trialeurodes vapororiarum on vegetables. Inundative releases, as the name implies, flood an ecosystem with a preponderance of natural enemies, usually predators, such that immediate mortality to target species is attained through the action of insects released rather than their offspring. In a fashion similar to that for the Encarsia wasps, predaceous mites (which are not insects but rather belong to the related class of arachnids) are often released in greenhouses for the control of plant- feeding spider mites.
Basic biological differences can affect production costs and, therefore, applicabiligy of a natural enemy for use in biological control. Predators, in general, are less selective about the prey they will attack than are parasitoids. This is problematic in several ways. A predator may attack beneficial as well as target insects. For instance, ants are important predators of many arthropod pests, particularly caterpillars. However, ants are attracted to the honeydew produced by sap- sucking insects such as aphids, whiteflies and scales. When collecting honeydew, ants may disturb and attack parasitoids of these sap-sucking insect pests and, thus, discourage the egg- laying of the parasitoids. Therefore, ants must be controlled in situations in which parasitoids are expected to control honeydew- producing insects.
Parasitoids usually have a narrow host-range and generally only attack certain stages of those particular hosts. Therefore the practitioner of integrated pest management must ensure, through appropriate monitoring that stages of the pest susceptible to attack by parasitoids are in evidence at the release site. For instance, many parasitoid adults are short-lived. If they are released before the host instars are capable of supporting parasitization, adult parasitoids may all die off before they have a chance to begin egg-laying.
Take care to make releases early enough that infestations of host species do not outstrip the parasitoids ability to control them . Action thresholds vary from system to system. In poinsettia production, monitor both greenhouse and sweet potato whiteflies numbers using yellow sticky cards. Ten adult whiteflies caught over a week on any one yellow sticky card (3 x 5 inches placed at one per 400 square feet) may be a high enough count to warrant introducing Encarsia wasps (only for greenhouse whitefly) at a rate of one to three wasps per plant. This should be done only after foliage samples reveal the presence of second- to early fourth-stage immature whiteflies, which are susceptible to parasitization.
The very voraciousness that makes predators so attractive as control agents often causes complications in the insectary rearing facilities. Predatory insects, such as lacewings and ladybird beetles, are often cannibalistic in their larval stages , and may begin to eat one another soon after hatching. This can increase labor and equipment costs in the insectary if the insects have to be separated and reared in individual receptacles. Even aside from cannibalistic tendencies, predators' rapacious appetites require that a great number of prey insects must be produced to feed the predators. Some ladybird beetles require approximately 500 aphids to attain maturity and thereafter need to eat from 100 to 300 aphids per clutch of eggs they produce. Insectary-reared predators may therefore be more expensive to produce, and therefore to purchase, than parasitoids with equal killing power.
Increasing concern over the safety, efficacy and sustainability of chemical control of insect pests has spurred research in alternate tactics in an strategy of integrated pest management. Much interest has been generated in regard to biological control, especially tactics using insect natural enemies as allies in our endeavors to control pest arthropods. To best capitalize on the potential of insect natural enemies the greenhouse manager should:
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