Host-plant acceptance by geographic populations of the colorado potato beetle,Leptinotarsa decemlineata

We used a detailed analysis of feeding behavior to investigate the role of solanaceous alkaloids as sensory-based feeding deterrents for the Colorado potato beetle,Leptinotarsa decemlineata (Say). Experiments were conducted on three geographic, host-adapted populations of beetles to determine whether evolutionary changes in host use have been accompanied by behavioral adaptations to alkaloids. Solanine and tomatine, steroidal glycoalkaloids found in two regional host plants, did not reduce leaf consumption or significantly alter behavior patterns of newly emerged beetles, including those from populations that normally will not feed on plants containing the compounds. Atropine, a tropane alkaloid found in several taxonomically related nonhost species, caused a significant increase in sampling behavior (indicating direct action on the sensory system) and reduced acceptance of treated potato leaves. We propose that variable acceptance of host plants among regional populations ofL. decemlineata has evolved independently of adaptations to alkaloids at the sensory level. To establish that secondary compounds such as atropine influence host choice in nature, field observations are needed to confirm that beetles routinely encounter, and sample, nonhost species.     

Colorado Potato Beetle Toxins Revisited: Evidence the Beetle Does Not Sequester Host Plant Glycoalkaloids

The Colorado potato beetle feeds only on glycoalkaloid-laden solanaceous plants, appears to be toxic to predators, and has aposematic coloration, suggesting the beetle may sequester alkaloids from its host plants. This study tested 4th instars and adults, as well as isolated hemolymph and excrement, to determine if the beetles sequester, metabolize, or excrete alkaloids ingested from their host plants. HPLC analysis showed: that neither the larvae nor the adults sequestered either solanine or chaconine from potato foliage; that any alkaloids in the beetles were at concentrations well below 1 ppm; and that alkaloids were found in the excrement of larvae at approximately the same concentrations as in foliage. Analysis of alkaloids in the remains of fed-upon leaflet halves plus excreta during 24 hr feeding by 4th instars, as compared to alkaloids in the uneaten halves of the leaflets, showed that equal amounts of alkaloids were excreted as were ingested. The aposematic coloration probably warns of a previously-identified toxic dipeptide instead of a plant-derived alkaloid, as the Colorado potato beetle appears to excrete, rather than sequester or metabolize, the alkaloids from its host plants.     

Bioassays of segregating plants

Solanum chacoense is a wild potato species resistant to the Colorado potato beetle,Leptinotarsa decemlineata. Most genotypes ofS. chacoense synthesize the glycoalkaloids solanine (sol) and chaconine (chac) and are hosts of the beetle. A few rare genotypes have a gene(s) for acetylation of carbon-23 of the steroid aglycone of sol and chac. Laboratory bioassays and replicated field tests of clones differing in the presence or absence of the acetyl moiety showed that acetylation of sol and chac markedly affects the response of both adults and larvae to the foliage. Adult feeding deterrency conferred by acetylated forms of sol and chac (leptines) in leaf-disk preference tests was consistent with the degree of antixenosis measured in the field. Development of larvae on foliage of clones with leptines was also inhibited. The studies support the validity of using laboratory bioassays of plants segregating for levels of a suspected defense compound to determine the role the compound has in defending the plant from attack by an insect predator in the field.     

Response of Colorado potato beetles,Leptinotarsa decemlineata (Say), to volatile components of tansy,Tanacetum vulgare

The responses of Colorado potato beetle,Leptinotarsa decemlineata (Say), to volatile components of tansy,Tanacetum vulgare L., were investigated in order to establish a chemical basis for observed reduction in beetle populations when potatoes,Solanum tuberosum L., were interplanted with tansy. Colorado potato beetles exhibited avoidance behavior to tansy oil, volatiles from intact tansy plants, a hydrocarbon fraction of tansy oil, obtained by fractionation on alumina, and five of the 13 known components of tansy oil that were tested. One constituent of tansy oil, -pinene, attracted beetles.     

Antifeedant Effects of Some Novel Terpenoids on Chrysomelidae Beetles: Comparisons with Alkaloids on an Alkaloid-Adapted and Nonadapted Species

Structure–dose–feeding deterrency relationships were compared between the Colorado potato beetle, Leptinotarsa decemlineata (Say), and the western corn rootworm, Diabrotica virgifera virgifera LeConte, using 15 alkaloids, terpenoids, and phenolic derivatives. The former species, a specialist herbivore on selected alkaloid-rich Solanaceae species, was on average 100-times less sensitive to the antifeedant effects of alkaloids, but more similarly sensitive to the terpenoids and phenolics than the latter species, a generalist flower herbivore predominantly on Graminae, Cucurbitaceae, and Compositae species. Antifeedant ED₅₀ values for the potato beetle and corn rootworm, each from closely related subfamilies of Chrysomelidae, ranged over four orders of dose magnitude among the 15 compounds with major species differences in stereosensitivity to -hydrastines and analog sensitivity with the silphinenes. Extremes in sensitivity ranged from silphinene, a rare tricyclic sesquiterpene that is 53 times more active on the potato beetle to aconitine, which is 430 times more antifeedant to the corn rootworm. Among silphinene and its two hydrolysis derivatives, there was not a strong correlation between antifeedant potency and injected toxicity for the two beetle species, but there was correlation between behavioral activity and galeal taste cell electrophysiological threshold and frequency responses. That all of the established GABA- and glycinergic compounds tested were antifeedant for both species suggests a shared molecular mechanism for antifeedant taste chemoreception in these divergent Chrysomelidae species. Moreover, the wide differences in antifeedant sensitivities among these and other chrysomelids to a suite of ligand-gated ion channel antagonists implicate a common protein neuroreceptor type with extraordinary heterogeneity in beetle taste.     

Interactions of alkaloids with galeal chemosensory cells of colorado potato beetle

The galeal chemosensory system of the adult Colorado potato beetle was used as a model to study the effects of alkaloids on insect gustation. Nine alkaloids, representing a wide range of structural types, were used. Their ability to stimulate chemosensory cells when presented in isolation and their ability to interfere with normal chemosensory processes were emphasized. None of the alkaloids stimulated chemosensory cells in a dose-dependent manner, although a few stimulated low-level activity from some cells. There was no evidence for a general deterrent receptor in these beetles. Some of the alkaloids had a marked inhibitory effect on normal chemosensory responses. Tomatine, solanine, papaverine, and sparteine significantly inhibited responses to amino acids (represented by GABA) while quinine and papaverine inhibited responses to sucrose. An attempt was made to correlate neurophysiological action of some alkaloids with their effects on feeding behavior. It was clear from this correlation that even a dramatic inhibition of sensory input by an alkaloid does not necessarily lead to measurable effects on behavior. The results are discussed in the context of current theories on the mode of action of alkaloids and other secondary plant compounds which may be involved in host recognition by phytophagous insects.     

Tasting green leaf volatiles by larvae and adults of Colorado potato beetle,Leptinotarsa decemlineata

Larvae and adults of the Colorado potato beetle,Leptinotarsa decemlineata (Say), are shown to have galeal gustatory cells that are highly sensitive to distillate of potato leaf extracts, (E)-2-hexen-1-ol, (E)-2-hexenal, and other saturated and unsaturated six-carbon alcohols. In larvae and adults, the sensory response patterns elicited by leaf homogenate, leaf distillate and a mixture of these two extracts differ in subtle ways. Beetle larvae feed most readily on Millipore disks treated with leaf homogenate and the mixture, but they did not feed on disks treated with leaf distillate. The differences in behavioral response and sensory input are used to derive a potential gustatory code that may stimulate different levels of feeding. This code may be disrupted by compounds present in nonhost leaves, thus leading to reduced feeding. Possible interactions of sapid leaf volatiles, amino acids, sugars, and potentially deterrent plant compounds are discussed.     

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.     

Effect of trichome B exudate ofSolanum berthaultii Hawkes on consumption by the colorado potato beetle,Leptinotarsa decemlineata (Say)

The leaf extract fromS. berthaultii Hawkes (PI 473340) contains exudate from trichomes (type B). Consumption ofS. tuberosum var. Norchip foliage by the Colorado potato beetleLeptinotarsa decemlineata (Say) was reduced when treated with the leaf extract. The leaf extract from a resistantS. berthaultii clone without type B trichome had no antifeedant activity. It suggests that more than one mechanism of resistance to the Colorado potato beetle exists inS. berthaultii.     

Chemically mediated behavior in Acari: Adapations for finding hosts and mates

Ticks and mites respond to a limited spectrum of stimuli in their search for hosts and mates. Airborne chemical signals include carbon dioxide, ammonia, organic acids, terpenoids, 2,6-dichlorophenol, and other phenolic compounds. These are detected primarily by sensilla in and adjacent to Haller's organ. Most ixodid species examined have one or more multiporose sensilla that detect such volatiles. These olfactoreceptors enable the ticks to respond to remote volatile chemicals from hosts and from the other ticks, e.g., sex pheromones. Other sensilla, probably mechanogustatory in function, also occur on the tarsi. Gustatory sensilla on the palps detect assembly pheromones that enable ticks and mites to respond to conspecific or heterospecific chemical stimuli in their environment. Responses to those stimuli in ticks result in clustering, i.e., arrestant behavior. Arrestant behavior also occurs in certain mites. Finally, cheliceral chemosensilla enable ticks to recognize specific phagostimulants in host blood, e.g., ATP and glutathione, that stimulate feeding. InDennacentor variabilis andD. andersoni, these same cheliceral chemosensilla recognize species-specific genital sex pheromones in the vulvae of conspecific mates, without which they do not copulate.     

Avoidance of antinutritive plant defense: Role of midgut pH in Colorado potato beetle

The fate of the tomato foliar phenolic, chlorogenic acid, in the digestive systems of Colorado potato beetleLeptinotarsa decemlineata (Coleoptera: Chrysomelidae) andHelicoverpa tea (Lepidoptera: Noctuidae) is compared. In larvalH. zea and other lepidopteran species previously examined, approximately 35–50% of the ingested chlorogenic acid was oxidized in the digestive system by foliar phenolic oxidases (i.e., polyphenol oxidase and peroxidase) from the tomato plant. The oxidized form of chlorogenic acid, chlorogenoquinone, is a potent alkylator of dietary protein and can exert a strong antinutritive effect upon larvae through chemical degradation of essential amino acids. In contrast, inL. decemlineata less than 4% of the ingested dose of chlorogenic acid was bound to protein. In vitro experiments to determine the influence of pH on covalent binding of chlorogenic acid to protein showed that 30–45% less chlorogenic acid bound to protein at pHs representative of the beetle midgut (pH 5.5–6.5) than at a pH representing the lepidopteran midgut (pH 8.5). At an acidic pH, considerably more of the alkylatable functional groups of amino acids (–NH₂, –SH) are in the nonreactive, protonated state. Hence, polyphenol oxidases are unlikely to have significant antinutritive effects against the Colorado potato beetle and may not be a useful biochemical source of resistance against this insect. The influence of feeding by larval Colorado potato beetle on foliar polyphenol oxidase activity in tomato foliage and its possible significance to interspecific competition is also considered.Approved by the Director of the Arkansas Agricultural Experiment Station.     

Effects of Colorado potato beetle and potato leafhopper on amino acid profile of potato foliage

Feeding by Colorado potato beetles and artificial defoliation had a minor effect on the profile of amino acids in potato foliage, whereas feeding by potato leafhoppers caused an increase in about one half of the amino acids measured.     

Responses of parasiteEdovum puttleri to kairomone from eggs of colorado potato beetle,Leptinotarsa decemlineata

When females of the eulophid parasiteEdovum puttleri were exposed to ann-hexane wash of eggs of the Colorado potato beetle,Leptinotarsa decemlineata, their responses indicated that the eggs contained a kairomone which elicited host-finding and host-acceptance behaviors. The hostfinding responses included an increased amount of time searching glass beads treated with kairomone, and drumming on the beads with the flagellum of the antennae. Host-acceptance responses included grasping of the beads with the legs, raising of the abdomen, and probing the kairomone-treated surface with the ovipositor. None of these responses were noted in females on beads treated only with hexane, or in males exposed to kairomone-treated beads. When kairomone was applied to a flat surface, filter paper disks, few episodes of drumming and no episodes of probing with the ovipositor were noted. Responses ofE.puttleri to eggs of Colorado potato beetle were similar to those on kairomone-treated beads, but females normally drilled into eggs and fed on host fluids after retracting the ovipositor. The kairomone is volatile, and responsiveness was reduced in parasites exposed to beads treated several hours previously.Hymenoptera: Eulophidae.Coleoptera: Chrysomelidae.     

Biological performance of Colorado potato beetle larvae on potato genotypes with differing levels of polyphenol oxidase

Two potato genotypes resistant to the Colorado potato beetle (CPB) and three susceptible genotypes were used to investigate the role of total foliar polyphenol oxidase (PPO) on the performance of CPB larvae in long-term feeding assays. A significant positive correlation was found between larval mortality and PPO content in potato foliage. Moreover, a significant negative correlation was also observed between PPO content and larval weight, fecundity, and relative larval growth rate. These results suggest a significant role of PPO in conferring potato resistance to the Colorado potato beetle at least in those clones where the PPO levels were above a certain threshold.     

Peripheral coding of sex pheromone and a behavioral antagonist in the Japanese beetle,Popillia japonica

Male antennae of the Japanese beetle, Popillia japonica, possess olfactory receptor neurons ORNs cocompartmentalized in the same sensilla placodea, one tuned to the sex pheromone, (R)-japonilure, and the other to the detection of a behavioral antagonist, (S)-japonilure. In-depth electrophysiological experiments revealed mutual inhibitory and synergistic effects in ORNs stimulated simultaneously with the two semiochemicals. The olfactory system of P. japonica exhibited a remarkable ability to discriminate completely coincident strands of pheromone and behavioral antagonist from strands of the two semiochemicals temporally isolated (by 1.5–3 msec). The mutual inhibition was reflected mainly by the delay of onset or total lack of spikes and by the significant increase in the rise time of potentials generated by blends of (R)- and (S)-japonilure. In contrast, synergist ORNs showed no neural activity (spikes) when stimulated with either the sex pheromone or the behavioral antagonist, but showed clear responses to blends of the two semiochemicals. Evidence for mixture-suppressed responses was observed not only in the Japanese beetle, but also in the Osaka beetle, Anomala osakana, and the Oriental beetle, Exomala orientalis, thus suggesting that it is a common feature in the sensory physiology of scarab beetles.     

Olfactory coding in the perception of semiochemicals

Information processing in the olfactory pathway underlying the perception of semiochemicals by insects is discussed. Both the chemical message for mates and the message for food consist of blends of chemicals. Olfactory receptors in an insect species are tuned to the detection of those compounds which comprise such chemical messages for that species. The classification of receptors as specialists or generalists coincides with two concepts of information processing, i.e., labeled lines and across-fiber patterns, respectively. The olfactory code coming from antennal receptors inPieris brassicae larvae is a combination of labeled lines and across-fiber patterning. When antennae of adult Colorado potato beetles,Leptinotarsa decemlineata, are stimulated by binary mixtures of leaf odor components, the pattern of neural activities in the olfactory receptors shows some separation into two channels, quantitative versus qualitative detection. The separation is complete in the antennal lobe of this beetle.Presented at the Symposium on Insect Chemical Communications: Unifying Concepts. ACS 194th National Meeting, New Orleans, Louisiana, August 31, 1987.     

Larval Beetles Form a Defense from Recycled Host-Plant Chemicals Discharged as Fecal Wastes

Larvae of the leaf-feeding beetles Neolema sexpunctata and Lema trilinea carry feces on their backs that form shields. We used the generalist predatory ant, Formica subsericea, in a bioassay to determine whether shields were a physical barrier or functioned as a chemical defense. Fecal shields protected both species against ant attack. Larvae of both species reared on lettuce produced fecal shields that failed to deter ants. Commelina communis, N. sexpunctata's host, lacks noxious secondary compounds but is rich in phytol and fatty acids, metabolites of which become incorporated into the fecal defense. In contrast, the host plant of L. trilinea, Solanum dulcamara, contains steroidal glycoalkaloids and saponins, whose partially deglycosylated metabolites, together with fatty acids, appear in Lema feces. Both beetle species make modifications to host-derived precursors before incorporating the metabolites into shields. Synthetic chemicals identified as shield metabolites were deterrent when applied to baits. This study provides experimental evidence that herbivorous beetles form a chemical defense by the elimination of both primary and secondary host-derived compounds. The use of host-derived compounds in waste-based defenses may be a more widely employed strategy than was hitherto recognized, especially in instances where host plants lack elaborate secondary compounds.     

Effects ofneo-clerodane diterpenes fromTeucrium on feeding behavior of colorado potato beetle larvae

Treatment of potato leaf disks with fourneo-clerodane diterpenes fromTeucrium (eriocephalin, teucrin-A, teucvin, and teuscorolide) significantly reduced feeding by larvae ofLeptinotarsa decemlineata (Colorado potato beetle). Choice and no-choice tests suggest that teuscorolide acts as a feeding deterrent, whereas the antifeedant activity of teucrin-A, teucvin, and eriocephalin is likely associated with a toxic mode of action. Nutritional tests, antifeedant simulation assays, and posttreatment studies confirmed that teucrin-A can be categorized by its mode of action as a toxin, rather than as a feeding deterrent.     

Taste receptors for pyrrolizidine alkaloids in a monophagous caterpillar

Caterpillars of Utetheisa ornatrix are monophagous on species of Crotalaria from which they obtain pyrrolizidine alkaloids (PAs) for defense and which the males convert to a pheromone. We show that a taste receptor neuron in each of the lateral and medial galeal styloconic sensilla responds to PAs of three different types. Monocrotaline, commonly present in Crotalaria species, is the most strongly stimulating with thresholds of response below 10^–11 and 10^–9 M in the two sensilla. These are among the lowest known taste thresholds in any insect and are similar to the thresholds for PAs in a polyphagous arctiid caterpillar that also sequesters PAs and uses them as the source of male pheromone. The receptors also respond to heliotrine, a type of PA that is probably never encountered by the insects. Monocrotaline and senecionine N-oxide are shown in behavioral assays to be phagostimulants. The data show that there is no tight link between taste receptor sensitivity to specific PAs and hostplant selection in these caterpillars. Perhaps the adults are primarily responsible for hostplant selection.     

Influence of aggregation inhibitors (verbenone and ipsidenol) on landing and attack behavior ofDendroctonus brevicomis (Coleoptera: Scolytidae)

The influence of the aggregation inhibitors verbenone and ipsdienol on the response of western pine beetle,Dendroctonus brevicomis, to attractive host trees was investigated. Paired ponderosa pine trees (Pinus ponderosa) were baited with aggregation semiochemicals to stimulate mass attack. One tree in each pair received an inhibitor treatment consisting of five sets of two verbenone and two ipsdienol dispensers spaced 1 m apart vertically along the tree bole. Beetle landing was monitored with sticky traps on the tree bole, and attack density was assessed from bark samples removed four or seven days after baiting. The inhibitor treatment resulted in a significant reduction of both the numbers of beetles landing on trees and the density of attacking beetles compared to control trees (without inhibitors). The ratios of beetle landing density to attacking density were not different between inhibitor-treated and control trees, nor were the vertical distributions of beetles landing or attacking, suggesting that beetle behavior was primarily influenced at a longer range, prior to landing on the tree. Although the application of verbenone and ipsdienol did not preventD. brevicomis from attacking baited trees, our results suggest that when applied to unattacked (and unbaited) trees, their effectiveness at reducing the attack pressure might allow trees having a certain amount of resistance to survive attack by pioneer beetles.