Parasites and predators

The development of criteria for sound economic thresholds in order to determine when-to-treat is highly complex. Establishing the guidelines for an effective economic threshold for a single pest must take into account the following interacting factors 1) density of the pest, 2) densities of its parasites and predators,... 

Biological and Natural Controls. Parasites and predators are effective in limiting the numbers of pest Insects and plant pathogens both in nature and for crops (35). This basic fact led to the development of biological controls. For example, the vedalia beetle, which was Introduced for control of cottony cushion scale on citrus in California, has provided continuous effective control of this pest for many decades. Worldwide only approximately 1% of the pests have been effectively controlled by Introduced biological control agents (43). 

However, not all natural enemies are fully effective. For example, the gypsy moth has approximately 100 parasites and predators attacking it but the pest reaches outbreak levels periodically (35). Nearly 40 biological control agents were introduced from Europe and Asia to control the moth and 11 of these became established (44). Yet not one of the 11 blocontrol agents is providing fully effective control, although each contributes to some limitation of this pest. 

Dowden, P. B. Parasites and Predators of Forest Insects Liberated in the United States through 1960 U.S. Dep. Agr., Agr. Handbk. 226. 

The relative toxicity, especially of the newer compounds, to various pests and species is of practical importance, as are also differences in species tolerance to DDT and other sprays. This is also of direct concern in relation to parasites and predators. There is interest in synergistic action and the effect of spreaders, stickers, and related products. The effect and use of repellents and attractants have received attention. 

The same sort of balance must be maintained in the use of insecticides. DDT furnished an excellent example in Florida. It killed the young scale insects, but it also killed so many of the parasites and predators that terrific scale populations resulted, which were difficult to bring under control. Obviously, selective insecticides are needed, which kill only injurious insects. Lacking this perfect answer, however, insecticides are used which are relatively less damaging to the friendly insects as compared to their effect on deleterious insects. 

Brominated compounds widely occur in marine organisms, particularly seaweeds and invertebrates. Many of them play a defensive role against parasites and predators, and are incorporated by opisthobranch mollusks to this scope from the diet. Bromine is uptaken by seaweeds and invertebrates from bromide dissolved at sparingly 1 mM concentration in seawater. The process is catalyzed by haloperoxidases, which have been characterized both as structure and function (Butler 1997). 

The major role of chemical defenses in plants is hypothesized to be increasing the impact of insect diseases, parasites, and predators. None of these factors alone provides an explanation of why evolutionarily labile insects rarely defoliate their long-lived hosts. However, interactions among all of them could increase the useful evolutionary lifetime of each and the effectiveness of all. In particular, chemical variability is observed to place insects in compromise situations which increase their exposure and susceptibility to natural enemies. 

An insect host s exposure to parasites and predators may be increased by variable plant defenses in three ways. First, by restricting feeding activity to certain tissue types or portions of the host plant, the position of insect hosts becomes more predictable. Parasites (24,46,42) or predators (48) able to recognize physical plant traits such as tissue color or form, or those capable of employing the unique chemistry of the preferred tissues as cues (47,49) would be able to locate their hosts more readily by focusing their search on these traits. 

Several species of mites (order Acarina) are effectively controlled by oil sprays. The European red mite, a major pest of fruit in the northeastern United Stat, is a member of this group. This pest appears to have become more severe as a result of the general use of DDT. Such a situation is explained by the fact that mites are not controlled by DDT, while certain parasites and predators of mites are killed by it. The continued use of DDT under these circumstances favors the build-up of mites. 

Another disadvantage of the trap-tree system is that the trap -trees remove large numbers of parasites and predators of the bark beetle. By selectively trapping bark beetles in pheromone baited traps, the ratio of natural enemies to bark beetles is increased. It is probable that this altered survivorship in turn may result in higher bark beetle mortality rates. 

Additional control measures include use of Bacillus thurlngiensis formulations and the release of parasites and predators. There is a continuing program to find new species that can be imported and released. A virus has also been registered for use against the insect. Work on release of sterile males is in an early stage. 

There are many other scolytid species that utilize these and similar compounds to effect intraspecific communication. Francke et al. (18) have discovered several spiroketals as active components of scolytid pheromones. Additionally, several parasites and predators utilize the pheromones produced by their scolytid prey as kairomones. For example, the predatory beetle, Temnochlla chlorodla, responds specifically to exo-brevicomin produced by female D. brevlcomls (19). This same phenomenon has been demonstrated recently in Lepidoptera the egg parasite Trlchogramma sp. uses the Hellothls zea (Boddie) sex pheromone to locate the H. zea eggs (20). 

Several seasons of field testing have shown a number of advantages in the use of the antifeeding approach to insect control. First, it is selective. Antifeeding compounds affect only pests which feed on the crop protected. Parasites and predators which walk over the treated foliage or feed on the affected insects are not killed, as with conventional insecticides. Honey bees and other pollinators are not affected by toxic deposits. 

Our experience did show, however, that the antifeeding approach to insect control appears both practical and acceptable. The compound was harmless to all but the pests being controlled, and it fitted in well with the integrated control type of program in which parasites and predators are disturbed as little as possible. 

Breeding field crops for disease resistance has had similar successes. While the examples in which parasites and predators have been used to achieve pest control are numerous, it is evident that not enough support has been given here in the past and it is virtually certain that this will receive far more exploration in the future. Other biological... 

Predators and Parasites—Organisms known to attack insert (and insect-like) pests in their native environment can be imported or reared in labomto-ries and released in infested areas. This is done only after the parasites or predators are determined to be harmless to man, animals, plants, and other beneficial organisms. For example, several kinds of parasites and predators of the alfalfa weevil have been imported from Europe and Asia and released in the infested areas in this country. Several species have become established and are helping to reduce pest numbers. However, they do not always prevent serious outbreaks and the resultant damage. 

Drift from the target site may injure wildlife, livestock, pets, sensitive plants, and people. For example, drift of herbicides can damage sensitive nearby plants, including crops, forests, or ornamental plantings. Drift also can kill beneficial parasites and predators that are near the target site. 

Control Cover crops with floating row cover attract and conserve native parasites and predators, including birds spray BTK to kill larvae. 

The aforementioned parasites and predators are all somewhat tolerant of the pyrethroids, an attribute that gives some hope for at least a modicum of natural-enemy conservation when these compounds are used in mid-season. It seems reasonable that careful evaluations of comparative toxicity data for Heliothis and some key major natural enemies could lead to identification of nonpyrethroid insecticides that if needed could be used with at least some degree of selectivity against the first two generations of the pest. However, such data are scarce, and more work is needed to provide an adequate data base that would allow more flexibility in choosing an appropriate chemical. Given the history of resistance in the tobacco budworm to various classes of insecticides, it would be advisable to alternate classes of insecticides early in the season to avoid selecting populations of the pest for multiple resistance. 

Zehr El. Evaluation of parasites and predators of plant parasitic nematodes. J Agric Entomol 2 130-134, 1985. 

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