Caterpillar attack

Fritzsche Hoballah, M. E. F. and Turlings, T. C. J. (2001). Experimental evidence that plants under caterpillar attack may benefit from attracting parasitoids. Evolutionary Ecology Research 3 553-565. 

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One result of pesticide use is the growth in the number of target species ( pests ) that have become resistant to pesticides. From 1970 to 1984, the number of resistant arthropod species doubled - from 224 to 447. These include 25 species of beetles, mites, and caterpillars that attack... 

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. 

Field experiments have shown that ants indeed defend their caterpillars effectively. Among the caterpillars most persistent foes are marauding social wasps. Although adult wasps eat only plant products, they spend much of their time hunting caterpillars to kill, cut up, and feed to their carnivorous larvae. Caterpillars guarded by ants withstand wasp attacks much better than those without protection. 

Responses of Alder and Willow to Attack by Tent Caterpillars and Webworms ... 

Figure 1- A Average biomass and survivorship of groups of western tent caterpillar larvae fed leaves in the laboratory from red alder trees under attack by tent caterpillars in the field (---) compared to those fed leaves from unattacked con-...

Thus, in contrast to the results obtained the same season with alder, no differences were found in tent caterpillar growth or survival when fed leaves from attacked versus control willows. 

Figure 2. Top normalized average relative growth rates of groups of western tent caterpillar larvae raised in the laboratory of leaves from test, nearby control, and far control Sitka willow trees. The test trees were loaded with tent caterpillar colonies on the indicated date. Bottom density of tent caterpillar larvae attacking the...

On the other hand, no differences were found in growth or mortality of tent caterpillars fed leaves from unattacked willows compared to those fed leaves from willows attacked by tent caterpillars (Figure IB). Reload of the attacked trees with additional tent caterpillars coincided with a rapid decrease in biomass and an increased mortality of Insects fed leaves from both attacked and unattacked control trees. This could have been by chance, but it suggested the possibility that both attacked and control trees exhibited a rapid decrease in food quality in response to the addition of more insects to the attacked trees. If so, this suggested that unattacked willows were sensitive to signals from nearby attacked willows or the attacking Insects. An experiment to test this hypothesis produced positive results (Figure 2). 

Tent caterpillars fed leaves from willows attacked by tent caterpillars grew more slowly than larvae fed leaves from nearby and far controls 11.5 days after the initiation of attack. Three days later, larvae fed leaves from both attacked willows and nearby controls grew more slowly than larvae fed leaves from the far controls. No evidence was found for root connections between willows of the same age as the study trees at the same site. 

The burden of proof for such an unprecedented effect should be high, and the foregoing experiments with willows and tent caterpillars cannot be considered to constitute such proof. However, at the very least, they show that the results of experiments designed to test for changes in leaf quality of attacked plants should be Interpreted with caution, particularly if control plants are near attacked ones. 

The ethylene-insensitive plants also showed reduced defense protein synthesis and were susceptible to soil pathogens to which they were normally fully resistant. In connection with the third trophic level, Kahl et al. (2000) found that attack by Manduca caterpillars on wild tobacco plants causes an ethylene burst that suppressed induced nicotine production but stimulated volatile emissions. They argued that the plant chooses to employ an indirect defense (the attraction of natural enemies) rather than a direct defense to which the attacker could adapt (Kahl et al, 2000 Winz and Baldwin, 2001). This implies that the plant is capable of identifying its attacker. We discuss this possibility in more detail in the discussion of specificity. 

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