Feeding responses ofChilo partellus (Swinhoe) (Lepidoptera: Pyralidae) larvae to sorghum plant phenolics and their analogs

Phagostimulatory responses of third-instar larvae ofChilo Partellus to phenolic components identified in an ethyl acetate extract of the leaf whorls of 3-week-old plants ofSorghum bicolor cultivar IS 18363 were studied in no-choice bioassays. The major components in the extract were identified as 4-hydroxybenzaldehyde and 4-hydroxybenzoic acid, with 4-hydroxy-3-methoxycinnamic acid, 3,4-dihydroxycinnamic acid, and 4-hydroxycinnamic acid present in minor amounts. All but 4-hydroxycinnamic acid were stimulatory at the doses tested. 4-Hydroxybenzaldehyde was more stimulatory than other potential biogenetic analogs. Hydroxybenzoic acids generally elicited greater feeding response than cinnamic acids, and the pattern of oxygen substitution in the benzene ring was related to bioactivity.     

Feeding Stimulating Signal in Labial Gland Secretion of the Subterranean Termite Reticulitermes santonensis

The paired labial glands of the French subterranean termite Reticulitermes santonensis Feytaud are located in the thorax. In the head, the glandular ducts join with those of the water sacs. In feeding choice tests with two semicircles of moist filter paper as food, workers of R. santonensis preferred the semicircle treated with labial gland extract compared to the semicircle treated with water (control). The labial gland secretion carries a signal that stimulates gnawing and feeding by termite workers during food exploitation. The extract of the labial gland even elicited feeding behavior when applied without food (on glass plates). The content of the water sacs was not effective as a feeding stimulus and neither were different body parts except for the fat-body. Water sacs are thus not a reservoir for the labial gland secretion, but their contents may serve as a solvent or carrier for the phagostimulant signal. The signal is highly polar, heat-resistant, nonvolatile, and thus very persistent. There is evidence that the signal from the labial gland is not species specific. Instead, it might be part of a general strategy by termites to exploit food sources.     

6,10,14-Trimethylpentadecan-2-one: A Bermuda grass phagostimulant to fall armyworm (lepidoptera: Noctuidae)

A phagostimulant, 6,10,14-trimethylpentadecan-1-one (phytone), was isolated and identified from Bermuda grass,Cynodon dactylon (L.). The phagostimulant activity of this isoprenoid ketone was established from bioassays of fall armyworm larvae,Spodoptera frugiperda (J.E. Smith). Larvae displayed increased body mass accumulation as well as preference to diet supplemented with this molecule. Neonate larvae fed diet supplemented with chromatographic isolates of phytone-containing fractions from six Bermuda grass cultivars showed a 10–40% increase in body mass accumulation as compared with controls. This variation in larval body mass accumulation seems attributable to a differential concentration of phytone in the cultivars, which ranged from 0.5 to 43 ppm. Additionally, first-instar larvae responded preferentially to diet pellets topically treated with phytone in concentrations as low as 0.1 ppm.     

Cucurbitacins fromCucurbita texana: Evidence for the role of isocucurbitacins

In addition to cucurbitacins E andI, cucurbitacins D,1, 3-epi-isocucurbitacin D,2, and B,3, were isolated from the fruits ofCucurbita texana and structurally identified by UV, IR,¹H NMR,¹³C NMR, and MS, and 2-O--glucopyranosylcucurbitacin I was identified. These compounds have not been reported previously as constituents of this species. The isolation of 3-epi-isocucurbitacin D2 together with normal cucurbitacins suggests that isocucurbitacins occur naturally. Evidence is also discussed that isocucurbitacins are biosynthesized one step ahead of normal cucurbitacin.     

Isolation and Identification of Primary Metabolite Feeding Stimulants for Adult Western Corn Rootworm, Diabrotica virgifera virgifera Leconte, from Host Pollens

Consumption of pollens from several host plants was quantified for adult western corn rootworm, Diabrotica virgifera virgifera, with significantly more sweet corn and winter squash pollen consumed than that of common sunflower and Canada goldenrod. Sequential solvent extraction and chromatographic fractionation of host pollens, guided by a feeding bioassay of respective fractions, isolated dominant phagostimulatory activity in pools of free amino acids. Amino acid profiles were determined by TLC and HPLC analysis. For sweet corn pollen, proline was predominant, accounting for 70% of a total 95.3 mol free amino acid/g pollen; serine, alanine, -aminobutyric acid (GABA), and asparagine occurred in decreasing order of concentration. In squash pollen, -alanine predominated (30% of 111 mol amino acid/g pollen), followed by asparagine, alanine, proline, and GABA. Histidine (42% of 83.8 mol amino acid/g pollen), proline, aspartic acid, asparagine, and alanine were dominant for sunflower pollen; and for goldenrod, proline (51% of 50.6 mol amino acid/g pollen), trans-hydroxyproline, serine, histidine, and alanine predominated. The five dominant amino acids from each pollen accounted for 80–90% of total amino acid content. Simple mixtures of some dominant amino acids, approximating levels found in sweet corn and squash pollen, gave phagostimulation similar to that of the crude extracts.     

Chemical basis of egg cannibalism in a caterpillar (Utetheisa ornatrix)

Larvae of the mothUtetheisa ornatrix are shown to cannibalize eggs in the laboratory. They proved most cannibalistic if they were systemically deficient in pyrrolizidine alkaloid (PA), the defensive agent that protectsUtetheisa at all stages of development against predation, and whichUtetheisa acquire as larvae from their food plant. In exercising cannibalistic choice,Utetheisa larvae feed preferentially on eggs that are PA-Iaden rather than PA-free. Egg cannibalism can therefore provideUtetheisa with a supplemental means of PA procurement. Moreover, presence of PA in the egg, while providing the egg with defense against predation, can increase its vulnerability to cannibalism. Although evidence is presented thatUtetheisa larvae cannibalize eggs in nature, it is argued that such feeding may occur only opportunistically in the wild, rather than as a matter of course.Paper No. 104 of the series Defense Mechanisms of Arthropods, No. 103 in Eisner and Eisner, 1991.Psyche 98:111–118.     

Food Exploitation in Termites: Indication for a General Feeding-Stimulating Signal in Labial Gland Secretion of Isoptera

The paired labial glands are located in all termite species in the thorax. During food exploitation workers of the French termite Reticulitermes santonensis and the African termite Schedorhinotermes lamanianus release the secretion of their labial glands directly onto the food. The secretion carries a water-soluble, heat-resistant, nonvolatile signal that stimulates gnawing and feeding and leads to aggregations of feeding workers. In a feeding bioassay, extracts of the labial glands of 11 termite species from five families all proved to have this feeding-stimulating effect both on R. santonensis and S. lamanianus. The heat resistance of the feeding-stimulating signal also could be shown for selected species from all termite families tested. A combined thin layer chromatography–feeding bioassay on cellulose TLC plates showed that after chromatography of labial gland secretion, the feeding-stimulating signal is located in all 11 species in the same area from R_f 0.46 to 0.88. An extract of labial glands of cockroaches stimulated feeding in R. santonensis and S. lamanianus as well, but was not active after heat treatment and after TLC. This points towards a general feeding-stimulating signal having evolved only in the labial gland secretion of termites.     

An Alternate Route to Insect Pharmacophagy: The Loose Receptor Hypothesis

The phagostimulatory response of some diabroticite cucumber beetles toward triterpene cucurbitacins is used as a model in support of an alternative hypothesis explaining the evolution of pharmacophagous feeding behavior in insects. Whereas the use of noxious compounds from nonhost sources for purposes other than nutrition or host-plant recognition (pharmacophagy) has historically been explained in terms of the ancestral host hypothesis, we suggest that the less than perfect specificity of the binding properties of some peripheral receptors provides an opportunity for novel compounds sharing the configuration and polarity of target molecules to elicit a feeding response by coincidence rather than adaptive design.     

Identification of sensilla involved in taste mediation in adult western corn rootworm (Diabrotica virgifera virgifera LeConte)

A group of sensilla present on the maxillary galea of adult western corn rootworm,Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) beetles has been identified morphologically and physiologically to be involved in taste mediation. There are approximately 15 chemosensory hairs on each galea. Bilateral removal of these structures resulted in a significantly reduced consumption of a strongly phagostimulant triterpenoid, cucurbitacin B, and led to increased ingestion of a phagodeterrent alkaloid, strychnine. Electrophysiological responses obtained via tip-recording of galeal chemosensilla with submillimolar concentrations of host and nonhost plant compounds resulted in dose responses overlapping with the effective behavioral ranges. Cucurbitacin B was found to evoke chemosensory responses at levels as low as 0.1µM. Since-aminobutyric acid (GABA) is an agonist. (-)--hydrastine and strychnine are antagonists, and cucurbitacin B has been proposed to act at a separate modulatory site of classical synaptic GABA and glycine receptor-channel complexes, results reported here raise the possibility that there are peripheral chemosensory receptor sites that may resemble, functionally and structurally, synaptic receptor sites in the central nervous system.     

A new cucurbitacin profile forCucurbita andreana: A candidate for cucurbitacin tissue culture

In addition to previously reported cucurbitacins B,1, and D,2, cucurbitacin E,3, and I,4, aglycones and their glucosides 2-O--glucopyranosyl-cucurbitacin E,5, and 2-O--glucopyranosyl-cucurbitacin I,6, were isolated and identified as constituents ofCucurbita andreana on the basis of MS, FD-MS, 1 H NMR and¹³C NMR spectroscopy. Also, 2-O--glucopy-ranosyl-cucurbitacin B, 7, cucurbitacin B glucoside was isolated and identified.