How contact foraging experiences affect preferences for host-related odors in the larval parasitoidCotesia marginiventris (Cresson) (Hymenoptera: Braconidae)

Responses of individual females of the parasitoidCotesia marginiventris to the odors of four different complexes of host larvae feeding on leaves were observed in a four-arm olfactometer. The plant-host complexes were composed of fall armyworm (FAW) larvae or cabbage looper (CL) larvae feeding on either corn or cotton seedlings. Prior to testing, each female was given a brief foraging experience on a plant-host complex and was then exposed to the odors of the same complex in the olfactometer. The experienced females responded to familiar odors in a dose-related manner, and these responses were virtually identical to all four complexes. Preferences for the odors of one of two plant-host complexes were tested in dual choice situations. Generally, FAW odors were preferred over CL odors and corn odors over cotton odors. A short foraging experience significantly affected the females' odor preferences in favor of the odors released by the experienced complex. Additional experiments revealed that neither longer bouts of experience nor bouts that included ovipositions resulted in a stronger change in preference. Experience affected preference in combinations where only the host species was varied as well as in combinations where only the plant species was varied. The results, therefore, strongly indicate that both the plants and the hosts somehow are involved in the production and/or release of the semiochemicals that attractC. marginiventris.Mention of a proprietary product does not constitute an endorsement or the recommendation for its use by the USDA.     

The Role of Fresh <Emphasis Type="Italic">versus</Emphasis> Old Leaf Damage in the Attraction of Parasitic Wasps to Herbivore-Induced Maize Volatiles

The odor produced by a plant under herbivore attack is often used by parasitic wasps to locate hosts. Any type of surface damage commonly causes plant leaves to release so-called green leaf volatiles, whereas blends of inducible compounds are more specific for herbivore attack and can vary considerably among plant genotypes. We compared the responses of naïve and experienced parasitoids of the species Cotesia marginiventris and Microplitis rufiventris to volatiles from maize leaves with fresh damage (mainly green leaf volatiles) vs. old damage (mainly terpenoids) in a six-arm olfactometer. These braconid wasps are both solitary endoparasitoids of lepidopteran larvae, but differ in geographical origin and host range. In choice experiments with odor blends from maize plants with fresh damage vs. blends from plants with old damage, inexperienced C. marginiventris showed a preference for the volatiles from freshly damaged leaves. No such preference was observed for inexperienced M. rufiventris. After an oviposition experience in hosts feeding on maize plants, C. marginiventris females were more attracted by a mixture of volatiles from fresh and old damage. Apparently, C. marginiventris has an innate preference for the odor of freshly damaged leaves, and this preference shifts in favor of a blend containing a mixture of green leaf volatiles plus terpenoids, after experiencing the latter blend in association with hosts. M. rufiventris responded poorly after experience and preferred fresh damage odors. Possibly, after associative learning, this species uses cues that are more directly related with the host presence, such as volatiles from host feces, which were not present in the odor sources offered in the olfactometer. The results demonstrate the complexity of the use of plant volatiles by parasitoids and show that different parasitoid species have evolved different strategies to exploit these signals.     

Comparison of larval and adult P-450 activity levels for alkaloid metabolism in desertDrosophila

The cytochrome P-450 monooxygenase system has been implicated in plant utilization by at least three species ofDrosophila (D. nigrospiracula, D. mettleri, andD. mojavensis) that are endemic to the Sonoran Desert of the southwestern United States and northwestern Mexico. Basal and induced levels of total cytochrome P-450 were determined for third-instar and decapitated 2- to 5-day post eclosion adults of the three desert species. Total P-450 levels, both basal and induced for all species assayed, were significantly higher for adults than for larvae by up to 20-fold. On a per organism basis, the levels of in vitro metabolism of the cactus alkaloid, carnegine, and patterns of response to induction by cactus tissue for adult desertDrosophila approximated those of larvae. Induction by phenobarbital, however, resulted in levels of in vitro carnegine metabolism that were up to 5.6-fold higher in adults than in larvae.     

Differential effect of tomatine and its alleviation by cholesterol on larval growth and efficiency of food utilization inHeliothis zea andSpodoptera exigua

The effect of tomatine on larval growth ofHeliothis zea andSpodoptera exigua was assessed by rearing larvae on diets with different concentrations of the chemical added. When reared from neonates, linear dose-response relationships were obtained for both species, withS. exigua being three times more sensitive to tomatine thanH. zea. Tomatine toxicity was completely alleviated inH. zea by the addition of equimolar cholesterol into the diet; however, inS. exigua some toxicity was maintained. Larvae ofS. exigua that were started on control diet were insensitive to tomatine after five days; larvae started on diet with an EC₅₀ of tomatine and then switched to control diet after five days failed to recover from toxicosis. Larval growth ofH. zea, on the other hand, was affected both at the neonate and third-instar stage, but normal growth resumed when the larvae were transferred to control diet. Tomatine had little or no affect on food consumption, assimilation, or dietary utilization of the food by third-instar larvae ofS. exigua, except at a concentration 10 times the EC₅₀. In contrast, the efficiency of food utilization ofH. zea larvae decreased with increasing tomatine concentrations. Assimilation of the food tended to increase, although not significantly, as tomatine levels increased. Food consumption ofH. zea larvae also increased when the tomatine concentration was greater than an EC₅₀. The addition of equimolar cholesterol to diets with an EC₅₀ of tomatine restored weight gain and nutritional indices values to control values. These results are related to the utility of using tomatine in host-plant resistance programs.     

Beneficial arthropod behavior mediated by airborne semiochemicals

Odor-mediated host-searching by femaleMicroplitis croceipes, a braconid endoparasitoid of larvalHeliothis species, was studied in a flight tunnel. Volatiles emitted by third-instarH. zea feeding on cowpea seedling leaves were released and carried with the wind, resulting either in an irregularly shaped plume, or injected with high velocity, resulting in a conically shaped jet plume. Flight maneuvers ofM. croceipes had a higher stereotype in jet plumes compared to irregularly shaped plumes. Variation in odor concentration in jet plumes due to intermittent feeding of the host larvae did not affect the number of flights. At the levels employed, odor concentration did not alter the rate of upwind progress in jet plumes. Because air velocity inside a jet increases in the upwind direction,M. crocepies regulates its ground-speed during its approach to the odor source.     

Systemic induction of feeding deterrents in cotton plants by feeding ofSpodoptera SPP. Larvae

Cotton,Gossypium hirsutum L., has been shown to exhibit systemic induced resistance to arthropods under certain conditions. We conducted experiments to determine the effects of previous feeding ofSpodoptera exigua Hübner andSpodoptera littoralis (Boisd.) larvae on feeding behavior, growth, and survival of larvae subsequently feeding on cotton. In one feeding choice test,S. exigua larvae preferred young leaves from undamaged control plants to undamaged young leaves from a previously damaged plant. Feeding deterrence was noticeable after only 6 hr of initial feeding damage by larvae, and there was almost complete deterrence after 30 and 54 hr of continuous feeding. In a second feeding choice test,S. littoralis larvae fed more on mature leaves from undamaged control plants than on undamaged mature leaves from previously damaged plants. In no-choice tests, third instars ofS. littoralis fed undamaged young leaves from damaged plants did not gain weight and died by the seventh day, whereas larvae fed young leaves from undamaged control plants gained weight and pupated within 11 days. Sixth instars ofS. littoralis fed either old damaged leaves, old undamaged leaves, or young undamaged leaves all from previously damaged plants gained weight slowly and took more than 12 days to pupate, whereas larvae fed young leaves from undamaged plants gained weight rapidly and pupated within five days of the beginning of the experiment.     

Crayfish feeding responses to zebra mussels depend on microorganisms and learning

Three species of crayfish (Orconectes virilis, O. rusticus, andCambarus robustus) were tested for feeding responses to potential food odors from mollusks (either zebra mussels,Dreissena polymorpha, or native gastropods). In all three crayfish species, feeding responses to odor cues were shown only by individuals experienced with feeding on a prey type. Individuals exposed to just the smell of prey organisms did not show feeding responses, indicating the role of associative learning in diet breadth. Establishment of a learned association took more than one feeding experience but once established lasted more than three weeks. When microbial enzymatic degradation of food protein was eliminated, either by UV radiation or microfiltration, feeding responses were eliminated even for crayfish experienced with a prey type.     

Relative importance of semiochemicals from first and second trophic levels in host foraging behavior ofAphidius ervi

The responses of femaleAphidius ervi to odors from a host food plant (Vicia faba), host aphids (Acyrthosiphon pisum), nonhost aphids (Aphis fabae), and aphid-plant complexes were investigated in a specially designed wind tunnel and a Y-tube olfactometer. In single-target (no-choice) and two-target (dual-choice) experiments, plant volatiles played a crucial role in the host foraging behavior ofA. ervi. The odor from theA. pisum-plant complex elicited the strongest responses byA. ervi females, followed by the odor from plants previously damaged by the feeding ofA. pisum. There was a significantly weaker response to odor fromA. pisum in the absence of the plant and to undamaged plants. Similarly, mechanically damaged plants and plants infested with the nonhost aphidA. fabae did not elicit strong responses. A plant that had been damaged byA. pisum and subsequently washed with distilled water was as attractive as an unwashed, previously infested plant.Aphidius ervi probably overcomes the reliability-detectability problem by selectively responding to herbivore-induced, volatile, semiochemical cues emitted by the first trophic level and by distinguishing between the volatiles induced by host and nonhost aphids.     

Induction of volatile emissions in maize by different larval instars of Spodoptera littoralis

Maize plants under attack by caterpillars emit a specific blend of volatiles that is highly attractive to parasitic wasps. The release of these signals is induced by elicitors in the caterpillar regurgitant. Studies suggest that plants respond differently to different herbivore species and even to different herbivore stages, thus providing parasitoids and predators with specific signals. We tested if this is the case for different larval instars of the noctuid moth Spodoptera littoralis when they feed on maize plants. Cut maize plants were incubated in diluted regurgitant from second, third, or fifth instar caterpillars. There were no differences in total amount released after these treatments, but there were small differences in the release of the minor compounds phenethyl acetate and -humulene. Regurgitant of all three instars contained the elicitor volicitin. To test the effect of actual feeding by the larvae, potted plants were infested with caterpillars of one of the three instars, and volatiles were collected the following day. The intensity of the emissions was correlated with the number of larvae feeding on a plant, and with the amount of damage inflicted, but was independent of the instar that caused the damage. We also used artificial damage to mimic the manner of feeding of each instar to test the importance of physical aspects of damages for the odor emission. The emission was highly variable, but no differences were found among the different types of damage. In olfactometer tests, Microplitis rufiventris, a parasitoid that can only successfully parasitize second and early third instar S. littoralis, did not differentiate among the odors of maize plants attacked by different instar larvae. The odor analyses as well as the parasitoid's responses indicate that maize odors induced by S. littoralis provide parasitoids with poor information on the larval developmental stage. We discuss the results in the context of variability and lack of specificity in odorous plant signals.     

Sexual behavior ofMatsucoccus josephi (Homoptera: Margarodidae)

The daily emergence patterns ofMatsucoccus josephi adults and third-instar male larvae raised on artificially infested saplings ofPinus halepensis were determined. A single peak of emergence was found for adult males between 0300 and 0500 hr. Adult females emerged throughout the day, with maximum emergence between 0500 and 0700 hr. Two daily emergence periods were observed in third-instar male larvae, one between 0300 and 0900 hr and the other between 1700 and 2100 hr. Airborne pheromone emitted by adult virgin females was collected using a flow system. No significant differences were recorded in the attraction of the male to crude pheromone collected at different times of day. A single female, virgin or half an hour after mating, was sufficient to attract the males in an olfactometer system. From minor differences in pheromone release throughout the day, and from the lesser degree of attraction by females half an hour after mating, it is assumed that there is no mechanism controlling the daily release of the female sex pheromone. Reduction of pheromone emission after mating is suggested.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 2743-E, 1989 series.     

Developmental stage of herbivorePseudaletia separata affects production of herbivore-induced synomone by corn plants

The female parasitic waspCotesia kariyai discriminated between the volatiles of corn leaves infested by younger host larvaePseudaletia separata (first to fourth instar) and uninfested leaves in a Y-tube olfactometer; the wasps were attracted to the infested leaves. In contrast, when corn plants were infested by the later stages (fifth and sixth instar) of the armyworm, the wasps did not distinguish between infested corn leaves and uninfested corn leaves in the olfactometer. Mechanically damaged leaves were no more attractive than undamaged leaves, and host larvae or their feces were not attractive to the parasitoid. Through chemical analysis, the herbivore-induced plant volatiles were identified in the headspace of infested corn leaves. The herbivore-induced volatiles (HIVs) constituted a larger proportion of the headspace of corn leaves infested by early instar armyworms than of corn leaves infested by late instar armyworms. Application of third-instar larval regurgitant onto artificially damaged sites of leaves resulted in emission of parasitoid attractants from the leaf, whereas leaves treated with sixth-instar regurgitant did not. The function of this herbivore-stage related specificity of herbivore-induced synomones is discussed in a tritrophic context.     

Specialist Leaf Beetle Larvae Use Volatiles from Willow Leaves Infested by Conspecifics for Reaggregation in a Tree

Young, gregariously living larvae of the willow leaf beetles Plagiodera versicolora are known to exhibit characteristic aggregation-dispersion-reaggregation behavior and local fidelity to a host tree. In this study, we investigated whether plant volatiles induced by feeding P. versicolora larvae were involved in the reaggregation behavior. Under laboratory conditions, we conducted dual-choice bioassays and found that the first and second instars discriminated between volatiles from leaves infested by larvae and volatiles from uninfested leaves. The discriminative behavior was dependent on both the time leaves were infested and the age of discriminating larvae. First and second instars preferred odor from 1-d-infested leaves to odor from uninfested leaves, whereas third instars (solitary stage) did not discriminate between these volatile blends. Odor from 2-d-infested leaves was preferred to odor from 1-d-infested leaves by first instars, whereas odor from leaves infested for 3 d was not attractive to these very young larvae. Neither was odor of leaves infested for 1 d and then left uninfested for 1 or 2 d attractive to young larvae. The data suggest that the first and second instars use volatiles from a leaf newly infested by conspecific larvae as one of the reaggregation cues. We detected several herbivore-induced compounds in the headspace of the attractive leaves. Among those, a mixture of synthetic (E)-β-ocimene, (Z)-β-ocimene, allo-ocimene, and linalool was found to attract the larvae.     

Rejection of host plant by larvae of cabbage butterfly: Diet-dependent sensitivity to an antifeedant

Garden nasturtium,Tropaeolum majus (Tropaeolaceae), is an acceptable host plant for the cabbage butterfly,Pieris rapae. Eggs are readily laid on the plant and hatching larvae feed and develop into normal pupae and adults. However, when second- to fifth-instar larvae were transferred from cabbage to nasturtium, they refused to feed and starved to death. Similar results were obtained when larvae were transferred from other host plants to nasturtium. However, larvae that were reared on nasturtium readily accepted cabbage as a new host plant. We have demonstrated the presence of strong antifeedants in nasturtium foliage and identified the most prominent active compound as chlorogenic acid. However, larvae reared on nasturtium had limited sensitivity, and larvae reared on a wheat germ diet were completely insensitive to the antifeedants. Larvae apparently develop sensitivity to the deterrent as a result of feeding on other host plants, whereas continuous exposure to the deterrent causes habituation or suppression of sensitivity development. The results demonstrate that dietary experience can dramatically affect the response of an insect to a potentially antifeedant compound in a plant.     

Growth inhibition ofMusca domestica L. andCulex quinquefasciatus (say) by (−)-3-epicaryoptin isolated from leaves ofClerodendron inerme (Gaertn) (Verbenaceae)

The petroleum ether extract ofClerodendron inerme leaves afforded a compound that matched the clerodane compound (–)-3-epicaryoptin in physical and spectral characteristics. The test compound inhibited the development ofMusca domestica andCulex quinquefasciatus larvae. First- and third-instar fly larvae reared on 3-epicaryoptin-treated diet pupariated later and earlier than their respective controls and adult emergence from puparia was inhibited. Apart from larval mortality, exposure of fourth-instar mosquito larvae to 3-epicaryoptin resulted in death at larval-pupal molt and pupaladult eclosion, indicating inhibition of the molting process.     

Generalization of a Habituated Feeding Deterrent Response to Unrelated Antifeedants Following Prolonged Exposure in a Generalist Herbivore, Trichoplusia ni

The possibility of generalization of habituated response to unrelated feeding deterrents following prolonged exposure was examined in third instar Trichoplusia ni (Lepidoptera: Noctuidae) larvae by rearing them on antifeedants and then testing with other unrelated antifeedants. We introduced neonate larvae (<24-hr old) onto cabbage leaves treated with crude seed extracts of Melia volkensii (Meliaceae) or oil of Origanum vulgare (“oregano”) (Lamiaceae) and allowed them to feed until early in the third instar. Naïve larvae were reared on cabbage leaves treated with carrier solvent alone. Both experienced and naïve larvae were tested for feeding deterrent response with the same and the different extracts in a leaf disc choice bioassay. Habituation was generalized to both M. volkensii and oregano following prolonged exposure to either plant extract and also to a pure allelochemical, thymol, following prolonged exposure to either digitoxin or xanthotoxin. However, there was no generalization of the habituated response to oregano following prolonged exposure to digitoxin or thymol, or to thymol or xanthotoxin following prolonged exposure to oregano or M. volkensii. Our results demonstrate that habituated response to feeding deterrents in a polyphagous insect herbivore can be generalized among and between plant extracts and pure allelochemicals, but not in all situations. The implications of such behavioral plasticity in herbivorous insects for the use of antifeedants as crop protectants or for host plant shifts is discussed.     

Odor learning and foraging success in the parasitoid,Leptopilina heterotoma

A brief 2-hr experience with hostDrosophila larvae in artificial apple-yeast or mushroom microhabitats had three effects on the foraging behavior of femaleLeptopilina heterotoma (Hymenoptera: Eucoilidae) parasitoids under field conditions. First, experienced females released at the center of circular arrays of apple-yeast and mushroom baits were more likely to find a microhabitat over the course of a daily census than naive ones. Second, for those females that found a microhabitat, experienced ones found it faster than naive ones (i.e., experience reduced travel times). Third, females experienced with a particular microhabitat were more likely to find that micro-habitat than an alternative one. Learned preferences were retained for at least one day and possibly as many as seven. Results generally did not depend on the host species (D. melanogaster orD. simulans) with which females were given experience. Females tended to arrive at baits upwind of the point of release, suggesting that odor is involved in finding host microhabitats and, in particular, in learning to find them more effectively. The implications of these results for the application of semiochemicals in biological control are discussed briefly.     

Similarity of Cuticular Lipids Between a Caterpillar and Its Host Plant: A Way to Make Prey Undetectable for Predatory Ants?

Ithomiine butterflies (Nymphalidae) have long-lived, aposematic, chemically protected adults. However, little is known about the defense mechanisms in larvae and other juvenile stages. We showed that larvae Mechanitis polymnia are defended from ants by a chemical similarity between their cuticular lipids and those of the host plant, Solanum tabacifolium (Solanaceae). This is a novel defense mechanism in phytophagous insects. A field survey during one season showed that larval survivorship was up to 80%, which is high when compared with other juvenile stages. In a laboratory bioassay, live larvae on their host plant were not attacked by the predatory ant Camponotus crassus (Formicidae). Two experiments showed that the similarity between the cuticular lipids of M. polymnia and S. tabacifolium protected the larvae from C. crassus: (a) when the caterpillar was switched from a host plant to a non-host plant, the predation rate increased, and (b) when a palatable larva (Spodoptera frugiperda, Noctuidae) was coated with the cuticular lipids of M. polymnia and placed on S. tabacifolium leaves, it no longer experienced a high predation rate. This defensive mechanism can be defined as chemical camouflage, and may have a double adaptive advantage, namely, protection against predation and a reduction in the cost of sequestering toxic compounds from the host plant.     

Insecticidal activity of hypericin towardsManduca sexta larvae

The toxic effect of hypericin (HYP), a photodynamic quinone that occurs in certain species of the genusHypericum, towardsManduca sexta third-instar larvae was investigated. The LD₅₀ of purified hypericin was 16 g/g larval initial fresh weight in constant light (22 W/m²). Reduced irradiance resulted in decreased mortality. Sublethal applications retarded larval growth (body fresh weight) in a dose-dependent manner. Toxicity had an absolute light dependence at the dose levels used, the active wavelengths being >500 nm. The phototoxic effect was rapidly lost when larvae were maintained in darkness (>8 hr) prior to irradiation. However, the potential for light-dependent mortality was retained if larvae were starved prior to irradiation.     

Orientation of the fiddler crab,Uca cumulanta: responses to chemical and visual cues

Behavioral responses of the fiddler crab Uca cumulanta to flat geometric shapes mimicking natural objects were measured in a circular arena by using zonal recovery as a behavioral measurement. Crabs were tested either in presence or absence of odors from two common predator species, the blue crab Callinectes sapidus, and the pufferfish Sphoeroides testudineus. The study tested the hypothesis that U. cumulanta have different behavioral responses to visual cues in the presence of chemical cues associated with predators. Escape direction tests demonstrated that U. cumulanta is able to show zonal recovery behavior based upon astronomical references. When tested in water lacking predator odor, crabs failed to exhibit a consistent orientation if a single silhouette target was interposed in the landward direction. However, when animals were tested in different predator odor concentrations, an orientation response was obtained at 10 and 20 g/liter/hr blue crab odor and 10 g/liter/hr pufferfish odor, demonstrating U. cumulanta ability to detect the potential presence of its natural predators by this odor. Thus, the hypothesis was supported, and the results suggest that behavioral responses to chemical and visual cues are involved in predator avoidance.     

Effect of DIMBOA,an aphid resistance factor in wheat,on the aphid predatorEriopis connexa Germar (Coleoptera: Coccinellidae)

DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a secondary metabolite found in cereal extracts, confers resistance in wheat to aphids. Its effect on beneficial organisms was tested on larvae of the aphid predatorEriopis connexa Germar. Larvae were fed until pupation on artificial diets to which different concentrations of DIMBOA (2–200g/g diet) were added, as well as on aphids that had been feeding on wheat seedlings with different DIMBOA levels (140–440 g/g fresh tissue). In diets, the effect of DIMBOA was greatest on survival of third-instar larvae and on the duration of the second and fourth instars. When aphids were provided as food, those that had fed on a wheat cultivar with an intermediate DIMBOA level led to a significantly longer larval duration in the predator than did those that fed on either low or high DIMBOA cultivars. Shortest predator development times were obtained with aphid prey that had fed on high DIMBOA seedlings. Higher DIMBOA levels in the plant appear to reduce aphid feeding rates (and rates of DIMBOA ingestion), decreasing aphid survival and minimizing the effect of the toxin on the predator.On leave from Departamento de Entomologia, Universidad Nacional Agraria La Molina, Apartado aéreo 456, Lima, Perú.