Electroantennographic and behavioral responses of the sphinx moth Manduca sexta to host plant headspace volatiles

Coupled gas chromatography with electroantennographic detection (GC-EAD) using antennae of adult female Manduca sexta was employed to screen for olfactory stimulants present in headspace collections from four species of larval host plants belonging to two families: Solanaceae—Lycopersicon esculentum (tomato), Capiscum annuum (bell pepper), and Datura wrightii; and Martyniaceae—Proboscidea parviflora. Headspace volatiles were collected from undamaged foliage of potted, living plants. GC–EAD revealed 23 EAD-active compounds, of which 15 were identified by GC-mass spectrometry. Identified compounds included aliphatic, aromatic, and terpenoid compounds bearing a range of functional groups. Nine EAD-active compounds were common to all four host plant species: (Z)-3-hexenyl acetate, nonanal, decanal, phenylacetaldehyde, methyl salicylate, benzyl alcohol, geranyl acetone, (E)-nerolidol, and one unidentified compound. Behavioral responses of female moths to an eight-component synthetic blend of selected tomato headspace volatiles were tested in a laboratory wind tunnel. Females were attracted to the blend. A comparison of responses from antennae of males and females to bell pepper headspace volatiles revealed that males responded to the same suite of volatiles as females, except for (Z)-3-hexenyl benzoate. EAD responses of males also were lower for (Z)- and (E)-nerolidol and one unidentified compound. Electroantennogram EAG dose–response curves for the 15 identified EAD-active volatiles were recorded. At the higher test doses (10–100 g), female antennae yielded larger EAG responses to terpenoids and to aliphatic and aromatic esters. Male antennae did respond to the higher doses of (Z)-3-hexenyl benzoate, indicating that they can detect this compound. On the basis of ubiquity of the EAD-active volatiles identified to date in host plant headspace collections, we suggest that M. sexta uses a suite of volatiles to locate and identify appropriate host plants.     

Feeding stimulatory and inhibitory chemicals from an acceptable nonhost plant forManduca sexta: Improved detection by larvae deprived of selected chemosensory organs

The chemical basis of feeding responses to the acceptable nonhost plantVigna sinensis (cowpea) by larvae ofManduca sexta was investigated using chemical isolation techniques directed by a novel chemosensory-based bioassay. The presence of feeding stimulatory and inhibitory compounds in leaves or leaf extracts was determined in a two-choice preference test using leaf disks or glass fiber filter paper disks laced with leaf extract as test substrate and filter paper disks laced with water as control. Larvae strongly prefer the control disks over leaf disks, indicating the presence of feeding inhibitory compounds in the leaf. An ethanol extract of both fresh and dried leaves neither stimulated nor inhibited feeding. The cause of this inactivity was examined by using larvae that respond strongly to either feeding stimulatory or inhibitory compounds due to selective chemosensory deprivation. Larvae having chemosensory organs remaining only on the maxillary palps are stimulated to feed by whole leaf disks and by the ethanol extracts. In contrast, larvae having only the medial and lateral maxillary sensilla styloconica and the epipharyngeal sensilla remaining are strongly inhibited by whole leaf disks and the ethanol extract of fresh leaves. Thus, the ethanol extract contains both feeding stimulatory and inhibitory compounds, which elicit opposite behavioral effects in unoperated larvae, therefore nullifying any stimulatory and inhibitory activity. These compounds can only be demonstrated by using discrimination-enhanced larvae in the choice tests. Further isolation of the feeding stimulatory principle inV. sinensis yielded two separate fractions of neutral compounds, suggesting at least two different chemicals belonging to two different classes: nonpolar and polar lipids. Feeding inhibitory chemicals have apparently polar properties because strong activity was found in the ethyl acetate and methanol extracts of dried leaves. The role of feeding stimulatory and inhibitory compounds in food selection ofM. sexta larvae is discussed.     

Effects of comsumption of high and low nicotine tobacco byManduca sexta (Lepidoptera: Sphingidae) on survival of gregarious endoparasitoidCotesia congregata (Hymenoptera: Braconidae)

The significance of nicotine in the three trophic level interaction involving tobacco (Nicotiana tabacum), the tobacco hornworm (Manduca sexta), and the parasitoidCotesia congregata was investigated in field plots of two varieties of tobacco which had about a 10-fold difference in their nicotine content. WhileM. sexta mortality, rates of parasitism byC. congregata, and the total number ofC. congregata larvae produced per host were similar on each of the two varieties, the number of parasitoids reaching adult-hood on the low nicotine treatment was nearly twice that on the high nicotine treatment. This difference was due to the significantly greater proportion of parasitoid larvae which failed to emerge from the host or that died prior to pupation after emerging from hosts which fed on the high nicotine variety. A greater proportion of larvae from hosts which fed on the low nicotine tobacco died as pupae. No treatment differences occurred for either sex of the parasitoid in individual dry weight, longevity, or pupal development time, except that female pupal duration was prolonged in the high nicotine treatment. These results support the suggestion that plant allelochemicals, which may function to provide plant resistance against pest herbivores, can be detrimental to natural enemies of the pest.Scientific article No. A4148 of the Maryland Agricultural Experiment Station, Department of Entomology. Research supported by NSF grant BSR-84-00614.     

Plant allelochemicals and insect parasitoids effects of nicotine onCotesia congregata (say) (Hymenoptera: Braconidae) andHyposoter annulipes (Cresson) (Hymenoptera: Ichneumonidae)

Parasitoids developing within tobacco hornworms or fall army-worms exhibit significant differences in development and survival depending on whether their hosts fed on nicotine-free or nicotine-containing diets. The effects of nicotine were more severe on the relatively less adapted parasitoid,H. annulipes than the specialist parasitoid,Cotesia congregata. Labeled alkaloid originally placed in herbivore diet was incorporated in several parasitoid tissues. These results suggest that interactions between plant allelochemicals and parasitoids should be considered in the development of theory on insect herbivory and plant defense.Scientific article No. A4408, Contrib. No. 7397 of the Maryland Agricultural Experiment Station, Department of Entomology. Research supported by USDA. Competitive Grant No. 59-2241-1-1-749-0 and NSF grant BSR-84-00614.Mention of a commercial product does not constitute an endorsement.     

Field tests of syntheticManduca sexta sex pheromone

In field experiments traps were baited with live females or with a two-, four-, or eight-component blend of the 16-carbon aldehydes previously identified as components of the sex pheromone emitted by femaleManduca sexta moths. The blends were formulated on rubber septa. Traps baited with a blend of all eight aldehydes captured moreM. sexta males than any other treatment. Septa loaded with 600 g of the eight-component blend were attractive to males for about seven days in the field. Septa loaded with the eight-component blend and stored in a refrigerator at 4°C for a year released the conjugated diene and triene aldehydes at the same rate as freshly prepared septa and were equally attractive in the field.     

The role of leaf lipids in food selection by larvae of the tobacco hornworm,Manduca sexta

The role of leaf lipids in food plant selection by larvae ofManduca sexta was investigated by measuring preference responses in a two-choice preference test using glass fiber filter paper disks laced with extract (test) or water (control). The larvae respond to the petroleum ether extract of whole leaves of the host-plantLycopersicon esculentum (tomato) extract in a concentration-dependent manner. At natural concentration it is the most strongly stimulating extract or compound yet tested using the disk test. This response is affected by food plant experience of the larvae, suggesting stimulation by plant-specific compounds in the extract. The extract contains volatile compounds that attract the larvae. In contrast, it does not promote continued feeding on an agar-cellulose diet that incorporates the extract. Also stimulating are the extracts of leaf surfaces of two hosts,L. esculentum andSolanum pseudocapsicum, and two acceptable nonhosts,Brassicae napus andVigna sinensis, indicating the presence of nonpolar feeding stimulants at the leaf surface. However, similar leaf-surface extracts of the unacceptable plantCanna generalis were inactive, although the surface extraction process renders this plant acceptable. Leaf-surface extracts ofL. esculentum, S. pseudocapsicum, andB. napus evoke feeding responses that are qualitatively comparable to those of their corresponding leaves. However, no such parallel is found for surface extracts ofV. sinensis andC. generalis. Thus, nonpolar compounds at the leaf surface of host and some acceptable non-host-plant species strongly stimulate feeding and hence must play an important role in food selection by the tobacco hornworm.     

Sex pheromone ofManduca Sexta (L) Stereoselective synthesis of (10E,12E,14Z)-10,12,14-Hexadecatrienal and Isomers

Three isomeric conjugated triene aldehydes,(10E,12E,14Z)-10, 12-14-hexadecatrienal (IX), (10E,12Z,14E)-10,12,14-hexadecatrienal (XX), and (10E,12E,14E)-10,12,14-hexadecatrienal (X), two of which are components of theManduca sexta (L.) sex pheromone, have been stereoselectively synthesized and spectroscopically characterized.This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or the recommendation for its use by USDA.     

The protective effect of antioxidants to a phototoxin-sensitive insect herbivore,Manduca sexta

Photo-activated plant secondary compounds have been shown to be toxic to many organisms including insects. Insect defenses include behavioral mechanisms such as light avoidance, as well as specific biochemical defenses such as antioxidants and antioxidant enzymes. These antioxidant defenses eliminate or quench the deleterious singlet oxygen and free radicals formed by these phototoxins. In this paper we examined the role of dietary antioxidants in protecting the phototoxin-sensitive insect herbivoreManduca sexta. Elevated dietary levels of the lipid-soluble antioxidants-carotene and vitamin E resulted in a concentration-dependent reduction in the mortality associated with treatment ofM. sexta larvae with the phototoxic thiophene-terthienyl. Elevated levels of dietary ascorbic acid had no effect, whereas reduced levels greatly increased the toxicity of-terthienyl. Tissue levels of antioxidants were shown to increase substantially in larvae fed antioxidant-supplemented diets. The results suggest that the ability to absorb and utilize plant-derived antioxidants could be an important defense against photo-activated plant secondary compounds and may have allowed some insects to exploit phototoxic plants.     

Protective action of midgut catalase in lepidopteran larvae against oxidative plant defenses

Catalase activity was detected in the midgut tissues and regurgitate of several lepidopteran pests of the tomato plant. Greatest activity in the midgut was detected in larvalHelicoverpa zea, followed bySpodoptera exigua, Manduca sexta, andHeliothis virescens. We present evidence that catalase, in addition to removing toxic hydrogen peroxide, may inhibit the oxidation of plant phenolics mediated by plant peroxidases. Small amounts of larval regurgitate significantly inhibited foliar peroxidase activity via removal of hydrogen peroxide. Treatment of foliage with purified catalase nearly eliminated peroxidase activity and was superior as a larval food source compared to untreated foliage. Tomato foliar peroxidases oxidize an array of endogenous compounds including caffeic acid, chlorogenic acid, rutin, coumaric acid, cinnamic acid, and guaiacol. The oxidized forms of these compounds are potent alkylators of dietary and/or cellular nucleophiles (e.g., thiol and amino functions of proteins, peptides, and amines). When tomato foliar protein was pretreated with peroxidase and chlorogenic acid and incorporated in artificial diet, larval growth was reduced compared to larvae fed untreated protein. Thus, the diminution of peroxidase activity and removal of hydrogen peroxide by catalase may represent an important adaptation to leaf-feeding. The secretion of catalase in salivary fluid during insect feeding is also suggested to be a potential mechanism for reducing hydrogen peroxide formation as an elicitor of inducible plant defenses.     

Survival and Development of Tobacco Hornworm Larvae on Tobacco Plants Grown Under Elevated Levels of Ozone

Tobacco plants, Nicotiana tabacum were grown under different levels of ozone (O₃) in open-top chambers. Ozone concentrations were established by charcoal filtration, which reduced O₃ to approximately one-half ambient, or by the addition of O₃ to unfiltered air to increase concentrations to approximately 1.4 or 1.7 times ambient O₃. Survival of tobacco hornworm, Manduca sexta, larvae was increased when second instars were fed tobacco leaves grown in chambers with elevated levels of O₃. Second instars also gained significantly more weight when they were fed for one week on plants exposed to elevated levels of O₃ than when they were fed plants grown in charcoal-filtered air. Ozone-treated tobacco plants had higher levels of total nitrogen (primarily reduced nitrogen) and soluble carbohydrates (sugars), and lower levels of leaf-surface components, starch, nicotine, and rutin. Increased survival and growth response of hornworm larvae to elevated O₃ levels in these experiments suggests that similar responses could occur in the southeastern US tobacco production areas where O₃ levels can be high enough to injure tobacco plants.