Changes in leaf mono- and sesquiterpene metabolism with nitrate availability and leaf age inHeterotheca subaxillaris

The concentration of leaf mono- and sesquiterpenes is greater in nitrate-limited than in nitrate-richHeterotheca subaxillaris plants and is highest in young leaves and declines with leaf age. To determine whether rates of volatile terpene synthesis and/or loss vary with nitrate availability and leaf age, incorporation of¹⁴C from photosynthetically fixed¹⁴CO₂ and the subsequent loss of label was measured in plants grown under nitrate-limited and nitrate-rich conditions.¹⁴C incorporation into mono- and sesquiterpenes was greater in nitrate-limited than in nitrate-rich plants and was highest in young leaves and declined with leaf age. Incorporation continued for several days after exposure, while loss of label was slow until leaves were 4–6 weeks old. These results suggest that the higher leaf volatile terpene content observed under nitrate limitation apparently results from increased synthesis per leaf and accumulation of mono- and sesquiterpenes in immature leaves of nitratepoor plants. Furthermore, volatile terpene synthesis is highest in young leaves, declines with leaf age, and is very low in older leaves. Carbon used for synthesis of this pool may be derived from both current photosynthesis as well as carbon transported to young leaves from older leaves. These data are consistent with hypotheses that predict that greater levels of carbon-based chemical defenses occur in plants under nutrient limitation. The apparent low metabolic cost of maintenance (i.e., slow turnover) of the accumulated terpenoid pool would limit the energetic cost of volatile terpenes as a chemical defense.     

Volatiles mediating plant-herbivore-natural enemy interactions: Electroantennogram responses of soybean looper,Pseudoplusia includens,and a parasitoid,Microplitis demolitor,to green leaf volatiles

Electroantennograms were recorded from an herbivore,Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), and a parasitoid,Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae), exposed to 5-through 12-carbon aliphatic compounds of several chemical classes. The response of the herbivore was higher for the 6- and/or 7-carbon hydrocarbons, alcohols, aldehydes, esters, and ketones. The response of the parasitoid was higher for the 7- and 8-carbon hydrocarbons, aldehydes, and ketones. Responses of the herbivore and the parasitoid to alcohols were similar. Both the herbivore and the parasitoid were most sensitive to aldehydes and ketones, and least sensitive to alcohols and hydrocarbons. Responses of the parasitoid to hydrocarbons, aldehydes, and ketones were numerically higher than those of the herbivore. The adaptive significance of differential olfactory sensitivity between the herbivore and the natural enemy is discussed in relation to tri-trophic interactions among plants, herbivores, and natural enemies.     

Specificity of male response to multicomponent pheromones in noctuid mothsTrichoplusia ni andPseudoplusia includens

The response of male cabbage looper (CL) and soybean iooper (SBL) moths was observed in the flight tunnel and measured in field tests to the six-component CL pheromone, the five-component SBL pheromone, and toZ7–12: OAc, the major component common to each pheromone. In both the flight tunnel and the field, male CL exhibited significantly greater levels of response to their six-component blend than toZ7–12: OAc alone. A low level of cross-attraction of male CL to the SBL pheromone was observed in both the flight tunnel and the field, but it was quantitatively and qualitatively similar to their response toZ7–12: OAc alone. Thus the minor components of the SBL blend did not appear to disrupt the flight behavior of male CL. With respect to SBL, in the flight tunnel males also exhibited a greater level of response to the five-component blend compared toZ7–12: OAc, but in the field their response was not significantly different to either treatment. There was also a low level of cross-attraction of male SBL to the CL blend, but this appeared to involve a significant arrestment effect on the upwind flight of males, as well as a difference in male sensitivity to the blend of components compared withZ7–12: OAc alone. The observed arrestment effect may have been due to male perception of one or more minor components of the CL pheromone. The results show that the multicomponent pheromones of these species function effectively as specific mating signals and that discrimination of odor quality by male moths can occur as the result of minor components affecting male sensitivity or their upwind flight response to the pheromone.     

Effect of qualitative and quantitative variation in allelochemicals on a generalist insect: Iridoid glycosides and the southern armyworm

The behavioral and physiological effects of plant allelochemicals have been difficult to demonstrate; it is not often clear whether the compounds are deterrent, toxic, or both. In this study, we compared the qualitative and quantitative effects of several iridoid glycosides on a generalist lepidopteran herbivore,Spodoptera eridania (Noctuidae). Larval growth and survivorship and larval preference or avoidance were measured on artificial diets containing different iridoid glycosides at different concentrations. We also tested the toxicity/deterrence of these compounds. We found that iridoid glycosides retarded larval growth significantly at relatively low concentrations and that they were usually avoided in preference tests. The toxicity/ deterrence test did not always reflect the results of these other tests. The merits of using a variety of methods for determining deterrence and/or toxicity of plant allelochemicals are discussed.     

Microsomal oxidation of allelochemicals in generalist (Spodoptera frugiperda) and semispecialist (Anticarsia gemmatalis) insect

Midgut microsomes prepared from larvae of the fall armyworm [Spodoptera frugiperda (J.E Smith)], a generalist insect, and the velvetbean caterpillar (Anticarsia gemmatalis Hubner), a semispecialist, were used to study their oxidative activity toward a variety of allelochemicals. Allelochemicals such as terpenoids, alkaloids, indoles, glucosinolates, flavonoids, coumarins, cardenolides, phenylpropenes, and a ketohydrocarbon were all metabolized by the microsomal cytochrome P-450 monooxygenases in both species. Fall armyworm microsomes oxidized monoterpenes more favorably than other types of terpenes, indicating a preference for these compounds. In all instances, the oxidative metabolism of these allelochemicals can be induced by dietary allelochemicals such as indole 3-carbinol, indole 3-acetonitrile, menthol, flavone, or peppermint oil ranging from 1.3- to 9.5-fold. In the case of certain triterpenes, tetraterpene, alkaloid, coumarin, and cardenolides, metabolic activity can only be observed after induction. The monooxygenase activities toward these allelochemicals were generally higher in the generalist than in the semispecialist. These findings provide strong evidence that microsomal monooxygenases play an important role in the detoxification of plant toxins and hence host-plant selections in herbivorous insects.     

Activity of volatile compounds in glandular trichomes ofLycopersicon species against two insect herbivores

Several major chemicals in the glandular heads of type VI trichomes ofLycopersicon species were identified and quantified by gas chromatography and mass spectrometry. Two normal odd-chained ketones, 2-undecanone (47 ng) and 2-tridecanone (146 ng), and one unknown sesquiterpene (5 ng), comprised approximately 95% of the contents of a gland ofL.hirsutum f.glabratum Mull. In a closely related plant,L.hirsutum Humb. & Bonpl. (LA 361), two unknown insecticidal sesquiterpenes accounted for 6% of the gland contents. Additionally, small amounts of one unknown monoterpene and another unknown sesquiterpene were found in type VI glands of a commercial tomato variety,L.esculentum Mill. Bioassays comparing the gland exudate (by direct contact) and isooctane extracts of glands to neonate larvae ofKeiferia lycopersicella (Walsingham) (Lepidoptera: Gelechiidae) andSpodoptera exigua (Hübner) (Lepidoptera: Noctuidae) indicated that: (1) 2-tridecanone and 2-undecanone were the major insecticidal compounds inL.hirsutum f.glabratum, (2) the two unknown sesquiterpenes inL.hirsutum were acutely toxic to both species, and (3) gland contents in the commercial tomato variety provided only a physical barrier toK.lycopersicella, and were not detrimental toS.exigua. In topical bioassay trials, synthetic mixtures of 2-tridecanone and 2-undecanone (3 1) demonstrated potentiation. Concentrations of these chemicals decreased as trichomes aged. Quantities of insecticidal chemicals and density of type VI trichomes varied with plant age and location within plants.     

Inhibition ofCampoletis sonorensis parasitism ofHeliothis zea and of parasitoid development by 2-tridecanone-mediated insect resistance of wild tomato

Field populations ofHeliothis spp. were sampled for levels of naturally occurring larval parasitism on six tomato lines varying in levels of 2-tridecanone-mediated resistance toManduca sexta (L.) andLeptinotarsa decemlineata (Say). Second and third instars were parasitized byCampoletis sonorensis (Cameron) (Hymenoptera: Ichneumonidae) andCotesia (=Apantales)marginiventris (Cresson) (Hymenoptera: Braconidae) in 1984 through 1986 and byMicropletis croceipes (Cresson) (Hymenoptera: Braconidae) in 1986. Differences in parasitism by individual and multiple species among host plants were not demonstrated. However, levels of parasitism were low and variable among replicates. Total larval parasitism averaged across all plant lines was less than 6% in 1984 and 1986 and approximately 11% in 1985. In laboratory cage studies,C. sonorensis parasitized fewerH. zea larvae on tomato foliage with high levels of 2-tridecanone than on foliage with low levels. RearingH. zea on diet containing 2-tridecanone and 2-undecanone did not alter incidence of parasitism byC, sonorensis; nor did rearing parasitizedH. zea larvae on chemically treated host diets precondition the parasitoid to higher or lower mortality when transferred to foliage as a substrate for cocoon spinning, regardless of the foliage genotype. However, parasitoid survival during cocoon spinning on foliage varied significantly among plant lines in a manner corresponding to the level of 2-tridecanone-mediated resistance of the foliage. Parasitoid mortality was greatest on highly resistant foliage and lowest on susceptible foliage.     

Effects of cotton leaf surface alkalinity on feeding ofSpodoptera littoralis larvae

The high leaf surface pH in cotton (Gossypium hirsutum) var. Acala SJ2 was bioassayed againstSpodoptera littoralis larvae. Weight gain and leaf consumption of the larvae feeding on leaves devoid of alkalinity, due to daily washing, were recorded. Untreated cotton, with a leaf surface pH of 9.5–10.0 was used as control. The gland exudates contained potassium and magnesium cations, and the gland surface and intergland leaf areas were rich in calcium and phosphorus and low in K or Mg. The role of this plant antibiosis in the insect-host-plant relationship is discussed.Contribution No. 1885-E 1986 series, from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel.     

Potential role of lipoxygenases in defense against insect herbivory

The potential role of the plant enzyme lipoxygenase in host resistance against the corn earwormHelicoverpa zea was examined. Lipoxygenase is present in most of the common host plants ofH. zea, with highest activity in the leguminous hosts such as soybean and redbean. Treatment of dietary proteins with linoleic acid and lipoxygenase significantly reduced the nutritive quality of soybean protein and soy foliar protein. Larval growth was reduced from 24 to 63% depending upon treatment. Feeding byH. zea on soybean plants caused damage-induced increases in foliar lipoxygenase and lipid peroxidation products. Larvae feeding on previously wounded plant tissue demonstrated decreased growth rates compared to larvae feeding on unwounded tissue. Midgut epithelium from larvae feeding on wounded tissues showed evidence of oxidative damage as indicated by significant increases in lipid peroxidation products and losses in free primary amines. The potential role of oxidative and nutritional stress as a plant defensive response to herbivory is discussed.     

Orientation ofMicroplitis croceipes (Hymenoptera: Braconidae) to green leaf volatiles: Dose-response curves

FemaleMicroplitis croceipes wasps were tested in a wind tunnel for their ability to orient to various concentrations of eight different green leaf volatile (GLV) substances [hexanal, (E)-2-hexenal, (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (E)-2-hexenyl acetate, (Z)-3-hexenyl acetate, (Z)-3-hexenyl propionate, and (Z)-3-hexenyl butyrate]. Overall, the esters elicited the greatest percentage of successful orientation flights, the alcohols elicited an intermediate response, and the aldehydes elicited a low response. The semilog dose-response curves were generally hill-shaped with high responses at medium release rates and low responses at high or low release rates. For the aldehydes, positive responses occurred at all GLV release rates between 0.01 and 100 nl/min. For some alcohols and esters, positive responses occurred at release rates as low as 1 pl/min and as high as 1μl/min. These data show thatM. croceipes wasps are strongly attracted to GLVs and are capable of orienting to GLV concentrations that would occur in nature when a caterpillar feeds on a green leaf. Hence, in nature, GLVs may be important clues, enablingM. croceipes to locate their hosts.     

Chemical and ultrastructural analysis of corn cuticular lipids and their effect on feeding by fall armyworm larvae

The cuticular lipid composition of lower and upper leaves of five genotypes of field-grown corn,Zea mays L., was determined by combined gas chromatography-mass spectrometry. Surface lipids of the upper leaves had a higher proportion ofn-alkanes (45–52%) than the lower leaves, while the lower leaves had higher percentages of fatty alcohols (12–18%) than the upper leaves. Scanning electron microscopy showed that the upper leaves of two corn genotypes, MpSWCB-4 and Cacahuacintle X's, had a smooth amorphous appearance, while the lower leaves had a dense array of wax crystals.Spodoptera frugiperda (J.E. Smith) larvae weighed more and developed more rapidly when they were reared on diet containing corn foliage from which the cuticular lipids had been removed than when they were fed untreated foliage. However, growth was not inhibited when larvae were fed diet containing the cuticular lipid extracts or individual cuticular lipid components.     

Analysis and field evaluation of volatile blend emitted by calling virgin females of beet armyworm moth,Spodoptera exigua (Hübner)

The volatile blend emitted by calling virgin beet armyworm females was collected and analyzed by capillary gas chromatography and mass spectrometry. Five compounds were detected and identified in this blend: (Z)-9-tetradecenyl acetate, (Z)-9-tetradecen-l-ol, (Z,E)-9,12-tetradecadienyl acetate (Z,Z)-9,12-tetradecadienyl acetate, and (Z)-l 1-hexadecenyl acetate. They were emitted in a ratio of 47.9 4.040.2 6.5 1.7, respectively. This five-component blend and several other blends with one or more components deleted were formulated on rubber septa to release the components in about the same ratios as the females. Field tests of these formulations in bucket traps indicated that a blend of (Z)-9 tetradecen-1-ol, (Z,E)-9,12-tetradecadienyl acetate, and (Z)-l 1-hexadecenyl acetate was most effective for trapping beet armyworm males and that (Z)-9-tetradecenyl acetate actually decreased trap captures. Blends that did not contain both (Z)-9-tetradecen-l-ol and (Z,E)-9,12-tetradecadienyl acetate were ineffective as trap baits.     

Identification of unique pheromone components for soybean looper mothPseudoplusia includens

Analysis of sex pheromone glands from individual female soybean looper moths showed that in addition to the previously identified main component (Z)-7-dodecenyl acetate, the compounds dodecyl acetate, 11-dodecenyl acetate, (Z)-7-dodecenyl propionate, and (Z)-7-dodecenyl butanoate were also produced. Two of the components, 12OAc and 11–12OAc, were not detected in a single analysis of female effluvium. Flight-tunnel tests showed that the five-component and three-component blends were equal to each other and to a female extract (> 80% source contacts) and that the mixtures were superior toZ7–12OAc alone. Field tests indicated that the five-component blend was significantly more attractive thanZ7–12OAc alone at a dosage of 1 mg, but that the blend was only slightly better at 3 mg.     

Developmental profile of sinalbin (p-hydroxybenzyl glucosinolate) in mustard seedlings,Sinapis alba L., and its relationship to insect resistance

Sinalbin was identified as a chemical component of insect anti-xenosis and antibiosis resistance mechanisms in seedlings ofSinapis alba by DEAE-Sephadex chromatography, HPLC, treatment with sulfatase and myrosinase, various feeding tests using artificial and natural substrates, and by measuring sinalbin concentrations in cotyledons and leaves during seedling development. The effects of sinaibin on feeding were dependent upon the insect species and upon the rapidly changing profile of sinaibin concentrations in the developing seedling. The high concentrations of sinalbin found in young cotyledons (up to 20 mM) and leaves (up to 10 mM) deterred the feeding of the flea beetle,Phyllotreta cruciferae Goeze and larvae of the bertha armyworm,Mamestra configurata Walker. The protection that sinalbin confers upon the vulnerable, newly emerged seedling (and upon tiny, young leaves) appears critical for the first few days of survival ofS. alba under feeding pressure from flea beetles in the field. The lower concentrations of sinaibin found in older cotyledons and leaves (2–3 mM) offer little or no protection againstP. cruciferae and may actually stimulate the feeding of this crucifer specialist. These concentrations of sinaibin, however, are still effective in reducing the level of feeding by larvae of the more generalist feederM. configurata.     

Response of generalist and specialist insects to qualitative allelochemical variation

We examined the effects of a set of four biosynthetically related iridoid glycosides, aucubin, catalpol, loganin, and asperuloside, on larvae of a generalist,Lymantria dispar (Lymantriidae), the gypsy moth, and an adapted specialist, the buckeye,Junonia coenia (Nymphalidae). In general,L. dispar grew and survived significantly less well on artificial diets containing iridoid glycoside, compared to a control diet without iridoid glycosides. In choice tests, previous exposure to a diet containing iridoid glycosides caused larvae subsequently to prefer iridoid glycoside-containing diets even though they were detrimental to growth and survival. In contrast,J coenia larvae grew and survived better on diets with aucubin and catalpol, the two iridoid glycosides found in the host plantPlantago lanceolata (Plantaginaceae), than on diets with no iridoid glycoside or with loganin and asperuloside. The results of choice tests of diets with and without iridoid glycosides and between diets with different iridoid glycosides reflected these differences as well. These results are discussed in terms of (1) differences between generalists and specialists in their response to qualitative variation in plant allelochemical content, (2) the induction of feeding preferences, and (3) the evolution of qualitative allelochemical variation as a plant defense.     

Host-plant protein and phenolic resin effects on larval growth and survival of a butterfly

Euphydryas chalcedona prediapause larvae were reared on fertilized and control shrubs of the host plant,Diplacus aurantiacus. Larval growth was enhanced by high leaf nitrogen content and inhibited by high leaf phenolic resin content. Larvae fed less on leaves near the branch tip which contained a higher leaf resin content. The results agree with prior laboratory investigation that the dietary content of nitrogen andD. aurantiacus leaf resin are major determinants ofE. chalcedona larval growth and suggest that the phenolic leaf resin ofDiplacus may both deter and inhibit leaf herbivores.     

Responsiveness ofPseudaletia unipuncta (Lepidoptera: Noctuidae) males,maintained as adults under different temperature and photoperiodic conditions,to female sex pheromone

Different-agedPseudaletia unipuncta (Haworth) males, maintained as adults at 10 and 25°C under 16∶8 and 12∶12 light-dark photoperiodic regimes, were flown to a 30-μg pheromone source in a wind tunnel at 20°C. While the responsiveness of individuals reared under both photoperiodic conditions at 25 °C was similar by day 5 (65–75%), response levels of younger males showed that those reared at 16∶8 light-dark matured more rapidly than those under 12∶12 light-dark. A significant delay in the onset of responsiveness was observed for males reared under 10°C with less than 25% of individuals responding to the pheromone, even at 25 days old. At this temperature, photoperiodic conditions had no significant effect on male receptivity. When males are transferred from 10°C to 25°C 16∶ 8 light-dark, their level of responsiveness after five days was similar to control 5-day-old males reared at 25°C, 16∶8 light-dark. These results are discussed relative to the hypothesis that this species migrates in response to seasonal cues that indicate a predictable habitat deterioration.     

Activation of plant foliar oxidases by insect feeding reduces nutritive quality of foliage for noctuid herbivores

The foliage and fruit of the tomato plantLycopersicon esculentum contains polyphenol oxidases (PPO) and peroxidases (POD) that are compartmentally separated from orthodihydroxyphenolic substrates in situ. However, when leaf tissue is damaged by insect feeding, the enzyme and phenolic substrates come in contact, resulting in the rapid oxidation of phenolics to orthoquinones. When the tomato fruitwormHeliothis zea or the beet army-wormSpodoptera exigua feed on tomato foliage, a substantial amount of the ingested chlorogenic acid is oxidized to chlorogenoquinone by PPO in the insect gut. Additionally, the digestive enzymes of the fruitworm have the potential to further activate foliar oxidase activity in the gut. Chlorogenoquinone is a highly reactive electrophilic molecule that readily binds cova-lently to nucleophilic groups of amino acids and proteins. In particular, the —SH and —NH₂ groups of amino acids are susceptible to binding or alkylation. In experiments with tomato foliage, the relative growth rate of the fruitworm was negatively correlated with PPO activity. As the tomato plant matures, foliar PPO activity may increase nearly 10-fold while the growth rate of the fruitworm is severely depressed. In tomato fruit, the levels of PPO are highest in small immature fruit but are essentially negligible in mature fruit. The growth rate of larvae on fruit was also negatively correlated with PPO activity, with the fastest larval growth rate occurring when larvae fed on mature fruit. The reduction in larval growth is proposed to result from the alkylation of amino acids/protein byo-quinones, and the subsequent reduction in the nutritive quality of foliage. This alkylation reduces the digestibility of dietary protein and the bioavailability of amino acids. We believe that this mechanism of digestibility reduction may be extrapolatable to other plant-insect systems because of the ubiquitous cooccurrence of PPO and phenolic substrates among vascular plant species.     

Maturation and senescence of an insect chemosensory response

Electroantennogram responses of male and femalePseudaletia unipuncta to the two major components of the presumed pheromone associated with the male anterior abdominal scent brushes vary with age. There is a postemergence period of maturation of responsiveness, followed by senescence. The age of maximum responsiveness differs according to the sex and to the compound tested.This research was supported in part by NSF grant BNS76-10268 and grants from the Killam Foundation and the Rockefeller Foundation.     

Detrimental effects ofCinchona leaf alkaloids on larvae of the polyphagous insectSpodoptera exigua

YoungCinchona ledgeriana plants contain two types of alkaloid: indole alkaloids in the leaves and quinoline alkaloids in the root. FromCinchona leaves, a crude alkaloid extract was made, containing the cinchophylline type of indole alkaloids and a small amount of 5-methoxytryptamine. The leaf alkaloid extract exerted a strong detrimental effect on the growth of larvae of the polyphagous beet armyworm,Spodoptera exigua (Lepidoptera). Feeding of larvae on an artificial diet containing the leaf alkaloids at the same concentrations as those found in the plant resulted in significant growth reduction, retardation in development, and mortality of the larvae. Cinchophyllines are composed of 5-methoxytryptamine coupled to a corynantheal unit. When incorporated into the artificial diet, 5-methoxytryptamine alone had no effect on the 5.exigua larvae. Corynantheal, however, had a strong detrimental effect on growth of the larvae, its effect being comparable to that of the leaf alkaloid extract. In contrast to the indole-type leaf alkaloids, the quinolinetypeCinchona root alkaloids did not affect growth and development of the larvae. These results suggest that the indole-type alkaloids, which inCinchona plants are present at the highest concentrations in the young, vulnerable leaflets, are involved in the chemical defense of the plant against herbivorous insects.