Host plant deterrents

The selection or avoidance of potential host plants by phytophagous Insects Is guided by a complex combination of physical and chemical stimuli. Color, shape and olfactory cues may play a role In the Initial orientation, whereas acceptance or rejection of a plant depends on texture as well as chemical stimulants or deterrents. Initiation of feeding Is stimulated or deterred by the presence or absence of specific chemicals or groups of chemicals, many of which have been Identified. The selection of a suitable plant for ovlposltlon Is also crucial for survival of the progeny of most herbivorous Insects, but the chemical factors Involved are known In relatively few cases. Ovlposltlon stimulants and deterrents often appear to be quite different from the chemicals that elicit or Inhibit feeding responses of larvae. 

The behavior of Insects In selecting a host plant for food and shelter is affected by a wide array of physical and chemical stimuli. Chemicals that play a role in resistance mechanisms may interfere with an Insect s orientation, inhibit feeding, or deter ovipositlon. Most of the known mechanisms of resistance involve feeding deterrents, but the most vulnerable phase of the Insect life cycle may prove to be ovipositlon. Environmental factors may influence the ability... 

Ingested alkaloids can also be utilized for defense even if these compounds are still present in the intestine. For example, grasshoppers and the larvae of many species of butterflies and moths regurgitate when tactually disturbed. The enteric fluid, fortified with alkaloids such as PAs, can function as a highly effective topical deterrent against small invertebrate predators.33 Thus the plant has adapted its host plant s allelochemicals to function as a secretion that can be accurately applied in aggressive confrontations. 

Sequestration of plant natural products by herbivorous insects is widespread. This is not surprising, since most insects are herbivorous and it is estimated that there are between two and six million species. While these sequestrators obviously exploit their alkaloid-rich host plants, such specialists represent a very minor component of the total herbivorous population. In essence, these nitrogenous compounds are highly effective deterrents for most of the herbivorous species with which they share their world. On the other hand, a brief discussion of some of the alkaloids and their specialist herbivores illustrates the versatiliity of these insects as exploiters of toxic compounds This alkaloidal treatment has been derived from the tables of Brown and Trigo10 and emphasizes... 

Therefore, NPA may be an allomone protecting crownvetch acting as a deterrent to herbivores. Meadow voles are certainly considered herbivores that could encounter crownvetch in the field. However, cabbage loopers may not adequately represent insect herbivores on crownvetch because they use Cruclferae and other plant families as host plants rather than Legumlnosae. 

The host range of the diamondback moth, Plutella xylos-tella, appears to be correlated with the presence of glucosinolates in the host plants, but the role of stimulants and attrac-tants in oviposition seem to be important. Extracts of Brassica oleracea (cabbage), B.juncea (mustard), and Erysimum cheiranthoides stimulated oviposition by gravid females on bean plants. Extracts of Erysimum cheiranthoides, Tropaeolum majus, and Capsella bursa-pastoris which deterred oviposition by the cabbage butterfly Pieris rapae, were not deterrent to Plutella xylostella (Renwick and Radke, 1990). 

Figure 1.4 shows examples of recordings from the tarsal, D-hairs of the carrot rust fly (P. rosae) in response to leaf surface extracts. The spike patterns show clear differences which may aUow this fly to discriminate between its respective host and non-host plant. The same extracts also allow discrimination in an oviposition bioassay in the carrot rust fly (Stadler, 1977). Although the non-host leaf extracts are not repellent or deterrent (compared with solvent), these extracts are also perceived. The different cell types observed in the recordings remain to be identified using isolated compounds. 

Inhibitory compounds may be at first unexpected in host plants, but such examples are known (Schoonhoven and Jermy, 1977 Schoonhoven, 1981). Stadler and Hanson (1978) isolated fractions from tomato leaves which were deterrent to M. sexta although tomato is a preferred host plant. The compound or compounds for this deterrency and the corresponding receptors have not yet been identified. Further indirect evidence for inhibitory compounds occurring naturally in host plants may be found in examples of successful breeding of plants showing a non-preference type of resistance (Beck and Schoonhoven, 1980 Schoonhoven, 1981). 

The adaptive value of deterrent receptor cells in preventing toxication by allelochemics in non-host plants seem to be evident. Other examples which, in terms of our present knowledge, are more difficult to explain on the basis of selective value, are deterrent cells sensitive to allelochemics that do not occur in the insects normal habitat (Schoonhoven, 1981). Possible explanations could be (i) a redundancy from earlier phylogeny and, (ii) a still unknown chemical similarity of the used stimulating deterrent with compounds present in the insect s natural environment. 

The second is the cabbage looper, Trichoplusia ni, where compounds in crucifers and other non-cruciferous hosts, released through the feeding activities of the larvae and present also in the larval frass, deter adult egg-laying (Ren-wick and Radke 1980, 1981). As in the case of D. oleae, one wonders about other sorts of biotic and abiotic agents which may cause release of oviposition deterrents from a host plant, thereby indicating the plant may be incapable of supporting larvae of O. nubilalis or T. ni to maturity. 

It should also be remembered that the defensive role of alkaloids in host protection is probably not as simple as the presence or absence of a single functional group on an alkaloid or even whether the alkaloid is in the host plant at all. In many cases, there may be additive effects of feeding deterrent compounds based on the entire chemical composition of the host plant. For example, when the alkaloids nicotine and tomatine, each of which have been shown to exhibit feeding deterrences of 25%, were fed in artificial diets to Locusta migratoria the overall feeding deterrence was approximately 50% (19). In this case the alkaloids acted in an additive fashion to produce a more effective chemical defensive mechanism. Adams and Bernays (19) went on to show that this additive characteristic of feeding deterrence was not restricted to alkaloids but was equally effective with alkaloids and various combinations of phenolics, terpenoids, and other miscellaneous secondary plant compounds. 

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