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.     

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.     

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.     

Leaf surface extracts ofSolanum berthaultii hawkes deter colorado potato beetle feeding

Leaf rinses ofS. berthaultii PI 473334 with methylene chloride were deterrent to feeding by the Colorado potato beetle when applied toS. tuberosum tuber and leaf disks. When the leaf rinse was separated into its nonvolatile and volatile fractions and applied to tuber disks, the nonvolatile fraction was highly deterrent, while the volatile fraction reduced consumption, but not significantly compared to the controls. A hexane leaf rinse was not deterrent to feeding, while an acetone rinse was approximately twofold more deterrent than the methylene chloride rinse when applied to leaf disks. Three cycles of bioassay-guided, reversed-phase open-column fractionation of an acetone leaf rinse yielded a relatively polar fraction with low deterrent activity, and two nonpolar fractions exhibiting higher specific activity. Reversed-phase preparative HPLC of these fractions yielded seven active fractions among the 10 assayed. Subsequent analytical HPLC indicated that two fractions each contained a single UV-absorbing compound, while another represented a mixture of at least four compounds. The remaining fractions were composed of complex mixtures of possibly ionic or polymeric compounds that were poorly resolved by HPLC.     

Isolation and identification of volatiles in the foliage of potato,Solanum tuberosum,a host plant of the colorado beetle,Leptinotarsa decemlineata

The volatile compounds ofSolanum tuberosum L., a host plant of the Colorado beetle,Leptinotarsa decemlineata Say, were isolated by successive vacuum steam distillation, freeze concentration, and extraction. The main components aretrans-2-hexen-1-ol, 1-hexanol,cis-3-hexen-1-ol,trans-2-hexenal, and linalool. The distribution of these compounds in a variety of plant families and their biosyntheses are reviewed. These leaf volatiles constitute a general green leaf volatile complex, being active in the olfactory orientation of the Colorado beetle and as such are probably of importance to various phytophagous insects.     

Differential neurosensory responses of adult Colorado potato beetle, Leptinotarsa decemlineata, to glycoalkaloids

Neurons from chemosensory hairs on the galeae of adult Colorado potato beetle (CPB), Leptinotarsa decemlineata(Say), were investigated for responses to glycoalkaloids of the family Solanaceae. While solanine and tomatine elicited irregular firing by multiple neurons and bursting activity at 1 mM concentration in most sensory hairs, stimulation with leptine I resulted in consistently high-frequency, slowly adapting responses with a dose-dependent effect between 0.03 and 0.3 mM concentrations. Responses to a mixture of solanine and leptine I suggested possible modification of the leptine I response by other glycoalkaloids, resulting in reduced neural activity relative to leptine I alone. These results establish a method for specifically evaluating leptine I and other glycoalkaloids for effects on feeding behavior of CPB and provide a sensory component for incorporating deterrent chemistry into biorational control methods for the CPB.     

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.     

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 and toxic effects of sesquiterpenes fromSenecio palmensis to colorado potato beetle

A bioassay-guided fractionation of the aerial parts ofSenecio palmensis resulted in the isolation of two sesquiterpenes, 2,10-bisaboladien-1-one and 11-acetoxy-5-angeloyloxy-silphinen-3-one. The bisabolene and the silphinene represented 0.012% and 0.024% of the plant dry weight, respectively. Both compounds showed antifeedant activity againstLeptinotarsa decemlineata larvae and adults in short-term choice and no-choice bioassays. Both compounds were also tested against different species of phytopathogenic fungi. The beetles were more sensitive to these compounds in choice than in no-choice assays, with a gradient of increasing sensitivity from second instars to adults. Bisabolene was 45 times less active as an antifeedant than juglone, which was tested as a positive control. The silphinen was more active than the bisabolene, with a range of activity similar to juglone. Furthermore, exposure of fourth instars to these compounds over a 24-hr period resulted in reduced feeding and growth rates. To distinguish between antifeedant and toxic effects, growth efficiencies were calculated as the slope of the regression of relative growth rate on relative consumption rate. The comparison of these results with those of antifeedant simulation and contact toxicity bioassays indicates that feeding inhibition is the primary mode of action of the bisabolene, while the silphinene shows both antifeedant and toxic effects.     

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.     

Electroantennogram Responses of a Predator, Perillus bioculatus, and its Prey, Leptinotarsa decemlineata, to Plant Volatiles

The two-spotted stinkbug, Perillus bioculatus, is a predator of the Colorado potato beetle (CPB), Leptinotarsa decemlineata. Behavioral tests revealed that P. bioculatus is attracted to potato plants, Solanum tuberosum L. (Solanaceae), infested by the CPB. Electroantennograms from the antennae of P. bioculatus were recorded in response to compounds present in the headspace of CPB-infested potato plants. (Z)-3-Hexen-1-ol and 2-phenylethanol elicited the highest EAG amplitudes. Linalool, 4,8-dimethyl-1,3(E),7-nonatriene, nonanal, decanal, and (R)-(+)-limonene evoked lower EAG amplitudes. The major headspace components -caryophyllene and -selinene produced only weak EAG responses. Antennal sensitivity of the CPB to (Z)-3-hexen-1-ol was higher than that of P. bioculatus, whereas the stinkburg was more sensitive to 2-phenylethanol, -caryophyllene, (R)-(+)-limonene, and decanal. Among these compounds, 2-phenylethanol is of special interest since it was observed to be emitted by potato foliage only after being damaged by CPBs.     

Sexual Maturation and Temporal Variation of Neural Responses in Adult Colorado Potato Beetles to Volatiles Emitted by Potato Plants

Neural responses of the Colorado potato beetle (CPB), Leptinotarsa decemlineata to volatiles emitted by potato plants, Solanum tuberosum L were investigated. Amplitudes of electroantennograms to measured amounts of a standard odorant, (Z)-3-hexenyl acetate, increased from day of emergence through at least six to eight days of adulthood. Among 20 potato volatiles examined, several constitutive compounds, e.g., the green leaf volatiles (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol, and (Z)-3-hexenyl butyrate, and systemic volatiles released primarily in response to insect feeding, e.g., (±)-linalool, nonanal, methyl salicylate, and indole, were the most effective stimuli. A statistic called linear age-skew (linear orthogonal polynomial) was used to examine differences in responses to potato volatiles between young and mature CPB. Based on plots of linear age-skew and overall neural responsiveness, 10 volatiles could be identified for which responses increased at a rate similar to or greater than the standard. The results are discussed with regard to the relationship of the CPB to its host plant and developmental studies of insect sensory responses to chemical signals.     

Misleading the Colorado potato beetle with an odor blend

Walking tracks of Colorado potato beetles,Leptinotarsa decemlineata Say, were recorded on a locomotion-compensator in response to wind, odors of host plantsSolanum tuberosum L. and nonhost plantsLycopersicon hirsutum f.glabratum C.H. Mull, and to mixtures of these plant species. Host-plant odor induced positive anemotactic responses in starved females, whereas odor of the nonhostL. hirsutum was neither repellent nor attractive. The attractiveness of host-plant odor, however, was neutralized in the odor blend of plant species. Masking the attractive host-plant odor will hinder the beetle's searching for host-plant patches, and this principle may be exploited in pest control by mixed cropping.The locomotion-compensator was constructed with financial support from the Foundation for Fundamental Biological Research (BION), which is subsidized by the Netherlands Organization for the Advancement of Pure Research (ZWO), grant 14-02-02. The first author was supported by a grant from the French Ministry of Industry and Research.     

Effects ofSolanum glycoalkaloids on chemosensilla in the Colorado potato beetle

Steroidal glycoalkaloids, found in species of the Solanaceae, elicit bursting activity in galeal and tarsal chemosensilla of adult Colorado potato beetles. The effect has an average latency of 6–12 sec, depending on the sensillum/alkaloid combination. A 20-sec alkaloid treatment is often suffficient to render galeal sensilla unresponsive to gamma-aminobutyric acid, normally an effective stimulant. The alkaloids have similar effects on galeal sensilla of larval Colorado potato beetles and on labellar chemosensilla of the blowfly. It is concluded that these compounds act independently of any specialized chemoreceptor in the Colorado potato beetle, and that association of the Colorado potato beetle with solanaceous plants has not led to evolution of a specific receptor forSolanum glycoalkaloids.     

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.     

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.     

Identification of branched alkanes in lipids ofLeptinotarsa decemlineata say andTribolium destructor by GC-MS: A comparison of main-beam and link-scanned spectra

Branched hydrocarbons were identified in the lipids ofLeptinotarsa decemlineata Say andTribolium destructor by gas chromatography, ordinary electron impact mass spectrometry, and linked, scanned, daughterion monitoring. This methodology allowed us to revise our earlier results based only on GC-MS data confirming the existence of only monomethyl-, dimethyl-, and trimethylalkanes in the hydrocarbons ofL. decemlineata Say. The hydrocarbons fromTribolium destructor consist ofn-alkanes, 3-methylalkanes, internally branched monomethylalkanes and dimethylalkanes. Daughter-ion monitoring can be particularly important for distinguishing between incidentally overlapped GC peaks of hydrocarbons from different series. A trace, for example, of dimethylalkane coeluating withn-alkane was easily identified in GC peak of hydrocarbon mixture ofT. destructor. Link scans confirmed also molecular weights for the compounds without molecular ions in the mass spectra. Structural assignment of the compounds were verified by comparison of the experimental and calculated values of the GC retention Kovats indexes (KI).     

Identification of food volatiles attractive to dusky sap beetle,Carpophilus lugubris (Coleoptera: Nitidulidae)

The chemical mediation of host-finding was investigated for the dusky sap beetle,Carpophilus lugubris Murray. GC-MS analysis of the headspace volatiles above whole-wheat bread dough inoculated with baker's yeast, a substrate previously determined to be an effective attractant, revealed seven major components in the following order of decreasing concentration: ethanol, acetaldehyde, 2-methylpropanol, 3-methylbutanol, propanol, 2-methylbutanol, and ethyl acetate. Solutions of these seven compounds blended so as to mimic the odor of whole-wheat bread dough elicited upwind orientation from 1.8 m in a wind tunnel byC. lugubris at a level comparable to that elicited by the bread dough. A series of bioassays investigating the role of individual components from the synthetic blend determined that all seven compounds contributed to behavioral activity; however, the simplest blend evoking attraction comparable to bread dough included acetaldehyde, ethyl acetate, 2-methylpropanol, and 3-methylbutanol. Of these compounds, acetaldehyde was essential, ethyl acetate was interchangeable with ethanol or partially replaceable with propanol, and 2-methylpropanol and 3-methylbutanol were partially replaceable with a combination of the other alcohols. Headspace volatiles above aseptic or fungus-inoculated tomato, banana, sweet corn, and strawberry were also qualitatively and quantitatively analyzed. This study suggests thatC. lugubris locates its food sources by response to variable blends of common volatile constituents of plants and fungi.     

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.