Repellents, herbivores, effectiveness

Before delving into ways the living world uses its special chemicals, we should note that these compounds touch our own lives in important ways. For millennia, humans have been borrowing natural chemicals for their own purposes, most often as drugs. Our oldest medicine is opium, which we prepare from the opium poppy (Papaver somniferum) today much as Mediterranean peoples did four thousand years ago. Just as we do, these early communities valued opium for its ability to kill pain and impart a sense of well-being. The principal constituent responsible for these effects is a chemical compound called morphine, which remains unsurpassed in its ability to control severe pain. In poppies, morphine s toxicity and bitterness presumably repel herbivores looking for a tasty meal. 

Allelopathic compounds act as repellents for herbivorous pests, so the same strategy used in weed control could be effective against pests and pathogens. Only allelopathy is not possible to use the complete control of weeds, pests or diseases it is necessary to combine it with other methods of plant protection. 

The trans-chrysanthemic acid 88 is an essential component of naturally occurring pyrethrin esters which are present in the flower of Chrysanthenum cinera-riaefolium and has a defense function in these plants [122]. Very effective as an antifeedant for herbivores, it presents a broad spectrum as an insect repellent. 

Most insect herbivores appear to be rather effectively repelled by furanocoumarin-containing plants (21-24). A notable exception to this generalization occurs among some butterflies of the family Paplllonldae, whose caterpillars are adapted to feed successfully and in fact preferentially on plants that contain linear, but not angular, furanocoumarlns (22). These circumstances prompted us to undertake studies with the black swallowtail butterfly (Papilio polyxenes) and radiolabeled furanocoumarlns in attempts to elucidate the nature of the Insect/furanocoumarln Interactions Involved. 

One reason that dominant plants are successful in semi-arid and arid ecosystems is due to the high concentration of terpenoids and prenylated phenolics that repel and/or deter feeding by herbivorous insects. Compounds like sesquiterpene lactones and benzopyrans are present in high quantities in the leaves of species of Pjajl t h ij jn, E n c e l i a and D i c o r i a and have been demonstrated to be effective feeding deterrents against known economic insect pests. Ijt vivo experiments are still needed in the field with bioactive constituents to better understand their effects on native phytophagous insects. 

Lack of alternative food during late winter may have enhanced the acceptance of some or all samples. During the growing season, by contrast, the repellent effect may be considerably stronger because the herbivores have so many more choices. 

The suitability of arthropods as prey or as hosts of insect predators and parasitoids has been repeatedly shown to be affected by host plant chemistry. The mechanisms by which plant chemical factors mediate suitability of herbivores as hosts (prey) for their natural enemies fall into two broadly overlapping categories. The first involves host plant compounds ingested, and in some cases sequestered, by the herbivores that are toxic or distasteful to parasitoids or predators (4-6). Such compounds may be active as repellents or as acute or chronic toxins ( 5, 7-10). The second category involves the nutritional quality of the host plant mediating herbivore utilization by parasitoids and predators (11-14). Changes in herbivore size due to host plant effects, for example, have been associated with differences in size and sex ratio of parasitoids (13, 12), and differences in functional responses of parasitoids and predators (16). 

Leaf cutter ants, abundant from Texas to Argentina, are polyphagous herbivores, but will not attack several plants. The ant, Atta cephalotes, for example, does not feed on Lasi-anthaea fruticosa (Asteraceae). The active repellent substance is lasidiol angelate (51) (Wiemer and Ales, 1981). Three of four other inhibitory compounds to ant feeding, nerolidol (7), caryophyllene epoxide (52), kolavenol (a diter-pene), and caiyophyllene (not active), either produced dramatic effects on the ants or the fungus cultivated by the ants (Howard et al., 1988). 

In other words methlocarb reduced bird damage, as measured by observations of missing and damaged sprouts, but also may have carried with It some phytotoxic effects. Thus, It may be that, as with some plants that have developed their own chemical defenses, the addition of a chemical repellent to a crop may only be advantageous under conditions of high levels of herbivore predation. 

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