Tobacco hornworm, insecticidal

The 29-fluorophytosterols, members of a new class of selective pro-insecticides, have been synthesized and examined m vivo in tobacco hornworms. Dealkylation at C-24 of the steroid side chain by insects releases the latent poison fluoroacetate, resulting in dose-dependent reductions in growth rate, maximum weight, and survival when fed at 1 to 100 ppm to Manduca sexta. 

Grasshoppers, however, wreak their damage as adults, while the great majority of agricultural damage is done by larval stage insects. The tobacco hornworm has served as a laboratory model for insecticidal screening (15). 

Plants that produce "specific toxins may be plagued by Insects that develop a tolerance to these toxins in much the same way as Insects develop tolerance to synthetic insecticides. Two examples from this chapter are the tobacco hornworm and the boll weevil which have developed a high tolerance to nicotine and gossypol, respectively. Some occurrence in the distant past may have placed sufficiently high selection pressure on these Insects that they developed tolerance to these compounds. Alternatively, the same effect could have occurred by a low selection pressure applied over a very long period time. Other plants protect themselves by employing general" toxins. 

The host range of the tobacco hornworm (Manduca sexta) is limited to selected members of the family Solanaceae. In an effort to better understand the chemical basis for the host plant selection process, we have undertaken an examination of both hornworm preferred and non-preferred members of the Solanaceae. Our investigations have shown this tc be a complex system involving the subtle interaction between such behavioral modulators as (1) Ovipositional stimulants (2) Feeding stimulants and imprinters (3) Anti-feedants (A) Repel-lants (5) Insecticides. The results of these investigations will be discussed. 

Insects use camouflage coloration as a means of avoiding predation. The green color of the tobacco hornworm larvae, (Manduca sexta) can be separated into constituent blue and yellow components. The water soluble blue component is the biliprotein, insecticyanin. The yellow color is derived from lipoprotein bound carotenes. This lipoprotein, lipophorin, is the major lipid transport vehicle in insect hemolymph. In addition to transporting dietary lipid, lipophorin is also involved in the transport of lipophilic insecticides. Nearly all the recovered radioactivity in hemolymph from topically applied [14c] ddt is associated with lipophorin. Lipophorin of adult M. sexta is larger, less dense and is associated with small amounts of a third, adult specific, apoprotein. Alterations in adult lipophorin density, lipid content and apoprotein stoichiometry can be caused by injection of the decapeptide, adipokinetic hormone. 

Differential insecticidal activity for a-terthienyl in different herbivorous lepidoptera has been noted. Under fluorescent daylight tubes, the topical LD50 values for the tobacco hornworm Manduca sexta, the tobacco budworm Heliothis virescens, and the European corn borer Ostrinia nubilalis were in the ratios 1 47 70 (755). The toxicokinetics of tritiated a-terthienyl were studied in these three insects. Following either oral or topical administration, larvae of 77. virescens and O. nubilalis rapidly cleared the chemical from the body via the feces, but M. sexta did more slowly. For example, after 48 h of feeding on treated diet, the ratio of radiolabel in the body to that in feces was 16 84 for O. nubilalis, 32 68 for 77. virescens, and 58 42 for M. sexta. Likewise, the half-time values for elimination of after topical application in O. nubilalis, 77. virescens, and M. sexta were approximately in the ratios 1 3 6. Unfortunately,... 

All the available evidence suggests that plants make alkaloids to deter predators. Some, like the tobacco alkaloids are strongly toxic to insects. Nicotine, anabasine and other related alkaloids are produced in the roots of the tobacco plant and translocated to the leaves. Nicotine is certainly toxic to most insects. Formerly, a crude preparation of nicotine was used commercially as an insecticide, but the tobacco hornworm (Manduca sexto) (Plate 15) has adapted itself so that its larvae feed only on tobacco... 

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