Natural enemy effectiveness

Glandular trichomes have also been associated with reduced natural enemy effectiveness. They have been shown to entrap several species of h3nnenopterous parasitoids (26-28) and to reduce the mobility of some species of coccinellids and chrysopids, but not others (, 29-31). 

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. 

Webb, R.E., M. Shapiro, J.D. Podgwaite, R.C. Reardon, K.M. Taffnan, I. Venables, and D.M. Kolodny-Hirsch. 1989. Effect of aerial spraying with dimilin, dipel, or gypchek on two natural enemies of the gypsy moth (lepidoptera lymantriidae). Jour. Econ. Entomol. 82 1695-1701. 

In sustainable agricultural systems, biodiversity has fundamental importance by providing a range of biological services including natural enemies. In conventional farming systems, these services are effectively substituted by external inputs. 

We also found that the response of the plant to the caterpillar spit is systemic (31). Thus, not only the damaged leaves but the entire plant produces and releases volatile compounds when one or more leaves are attacked by caterpillars. Dicke et al. (7) had earlier found a similar effect in that undamaged leaves of a spider mite-injured plant attracted predatory mites. This systemic effect could be very significant in terms of enabling the natural enemies to locate their victims. It makes the plant under attack stand out from its neighbors and act as a beacon to foraging natural enemies. 

Effect of phloem moisture and thickness on natural enemies of bark beetles... 

Insect resistance and environmental pollution due to the repeated application of persistent synthetic chemical insecticides have led to an Increased interest in the discovery of new chemicals with which to control Insect pests. Synthetic insecticides, including chlorinated hydrocarbons, organophosphorus esters, carbamates, and synthetic pyrethroids, will continue to contribute greatly to the increases in the world food production realized over the past few decades. The dollar benefit of these chemicals has been estimated at about 4 per 1 cost (JJ. Nevertheless, the repeated and continuous annual use in the United States of almost 400 million pounds of these chemicals, predominantly in the mass agricultural insecticide market (2), has become problematic. Many key species of insect pests have become resistant to these chemicals, while a number of secondary species now thrive due to the decimation of their natural enemies by these nonspecific neurotoxic insecticides. Additionally, these compounds sometimes persist in the environment as toxic residues, well beyond the time of their Intended use. New chemicals are therefore needed which are not only effective pest... 

Insect attractants are predominantly used for population monitoring and for control. It is necessary to know the degree of infestation to initiate the most effective control methods. There are two ways to prevent insecticides from entering the food chain by utilizing effective insect attractants. The first method is to attract the destructive insects themselves into a trap by an effective lure, and the second is by attracting their natural enemies so that the insects can be annihilated before they can cause much damage. 

Another millipede, Polyzonium rosalbum, contains two alkaloids, poly-zonimine (19) and nitropolyzonamine (49), in its defensive secretion (Tables II and IV). Polyzonimine (19) is repellent to such natural enemies as ants and might effect the deterrence of various insects by acting as a topical irritant 113). 

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. 

Resource restriction. If chemical defenses vary quantitatively within or between individual plants, then some tissues may be defended while others are not. As a result, insects have available to them the evolutionary option of avoidance they may develop the ability to recognize poor quality food and avoid it, rather than evolving detoxication mechanisms (12,18). This should result in feeding activity concentrated on a restricted set of tissues or plant individuals. There are two important consequences of this. First, contact rates with defenses can be lowered by avoiding them. Hence, the evolution of detoxication is less li)cely or less rapid (18). Second, and perhaps more important, the effectiveness of natural enemies may be enhanced (below). 

Multiple-factor Interactions. Each potential regulatory factor may Interact synergistically with the others and enhance their effectiveness. For example, plant chemistry can influence the effectiveness of predators, parasitoids and diseases in a variety of ways (, , ). However, the selective pressure exerted by uniform chemical defenses should be strengthened by interactions with natural enemies, and their useful life will be shortened. 

Consequently, although the ways in which plant chemistry can influence the effectiveness of natural enemies are dizerse, they can remain effective through evolutionary time only if variability is part of the picture as well. In fact, although reviews have tended to focus on chemical enhancement of natural enemy regulation (23), there are probctbly as many ways in which uniform plant chemistry can interfere with the actions of these enemies as there... 

I suggest that variable plant chemistry, by restricting resource availability and focusing the activities of herbivores on a few tissues, promotes compromises between food-finding and risks from natural enemies which are not readily countered by most insects. The spatial and temporal heterogeneity which appears to be common in forest trees is the most important part of the tree s defensive system, and is the only way a plant s chemical defenses can remain effective over evolutionary time. This variable impact on natural enemies may be more important in regulating consumption than any single factor can be. 

Although chemical variability may not alter all of the potential effects of plant chemistry on the effectiveness of natural enemies, there are a number of important qualitative differences in the kinds of interactions possible. In some cases the impact variable chemistry may have on an insect s susceptibility to risks is simply greater than it would be were plant chemistry uniform. In other cases wholly different relationships are possible. 

Some aspects of plant variation could interfere with the impact of natural enemies. Some enemies may be unable to associate microhabitat cues (e.g., chemical, physical, color, position) with prey or host location. For these enemies, prey or host feeding on restricted tissues will tend to appear widely spaced and they may not be readily encountered. It appears to me that many, if not most, parasitoids and predators can be found to use one or more cues. This negative effect could be counteracted by increased encounter rates during herbivore searching movements. 

Schiiler, T. H., Poppy, G. M., Kerry, B. R. and Denholm, I. (1999a). Potential side effects of insect-resistant transgenic plants on arthropod natural enemies. Trends in Biotechnology 17 210-216. 

Hummel, R.L., Walgenbach, J.F., Hoyt, G.D. and Kennedy, G.G. 2002. Effects of production system on vegetable arthropods and their natural enemies. Agriculture, Ecosystems and Environment 93 165-176. 

---