Plant wounding affects the oviposition of Hellula undalis(F.). The impact of leaf chemistry

Crucifer specialists, such as the tropical pest Hellula undalis (F.), developed a close association to their hosts and depend on characteristic secondary plant compounds, the glucosinolates (GS), in many cases. Following tissue damage, GS undergo hydrolysis catalyzed by the enzyme myrosinase to produce a complex array of products which include isothiocyantes, cyanides, and thiocyanates depending on reaction condition and the chemical nature of the side chain. We found that mechanical plant damage to the tropical crop plant Brassica campestris L. ssp. chinensis (pak-choi) leads to increased GS content, especially of indolyl GS and affected the oviposition behaviour of H. undalis negatively. GS were detected in methanolic leaf surface extracts from intact B chinensis and Sinapis alba (L.) plants, but no hydrolysis products of GS were found in leaf extracts with methylene chloride. Furthermore, the oviposition stimulant activity of GS extracts for H. undalis was greatly reduced after hydrolysis of GS by myrosinase. These results suggest the importance of GS for inducing oviposition, thus excluding hydrolysis products as active principle. Specific produced slightly volatile hydrolysis products of GS (3-indolylmethyl-cyanide and 5-vinyloxazolidine-2-thione) reduced oviposition, when applied to the host-plant. Leaf chemistry, including volatile and non-volatile compounds, may determine oviposition in H. undalis indicating a suitable host-plant.     

Physical and chemical cues affect oviposition by Neoleucinodes elegantalis

Recognition and acceptance of a suitable host plant by phytophagous insects requires the integration of visual, physical and chemical cues. The present study investigates the host cues that a specialist insect integrates to optimize oviposition decisions and whether these cues are weighted in a specific way. The study also determines whether the tomato fruit borer Neoleucinodes elegantalis (Guenée) (Lepidoptera: Crambidae), an important pest on Solanaceae in Brazil, shows a preference for oviposition sites that differ in physical and/or chemical cues. When styrofoam balls are provided as artificial fruits, N. elegantalis deposit significantly more eggs on rough artificial fruits than on smooth ones. Hexane fruit extracts applied to the artificial fruits stimulate female oviposition strongly. Physical and chemical cues also affect the oviposition of females when offered together. Furthermore, certain parts of the artificial fruits are prefered, irrespective of the presence of chemical cues. Both physical and chemical cues affect oviposition decisions; hence, the fruit borer relies on cues of different sensory modalities.     

Evaluating quantum yield and ultraviolet–visible reflectance in baby pumpkins: Implications for oviposition behavior of pumpkin fruit flies

Frugivorous insects utilize both olfactory and visual cues to locate their host plants. Although volatiles have been extensively studied for detecting infested fruits, the role of visual cues in oviposition site selection remains mostly unknown among frugivorous insects. To investigate physiological changes in a host plant, we measured the quantum yield and reflectance of three wavelengths of light (350, 450 and 520 nm) after puncturing the surfaces of commercially grown pumpkins using insect pins during two different seasons outdoors. Quantum yields did not show significant differences between undamaged and simulated oviposition sites. Two wavelengths within the visual spectrum were similar between the two treatments. However, photon counts at 350 nm, in the ultraviolet range, were 1.76 fold higher in simulated oviposition sites than in undamaged sites, which was consistently observed across three field seasons. Considering that frugivorous insects and other phytophagous insects recognize the 350 nm wavelength for host identification and oviposition, we conclude that the disparities in ultraviolet reflectance can serve as a baseline for assessing the actual oviposition response of fruit flies. This finding contributes to the role of visual cues in the oviposition behavior of frugivorous insects and the development of a nondestructive detection method for pumpkin fruit flies.     

Feeding damage by larvae of the mustard leaf beetle deters conspecific females from oviposition and feeding

Herbivorous insects may be informed about the presence of competitors on the same host plant by a variety of cues. These cues can derive from either the competitor itself or the damaged plant. In the mustard leaf beetle Phaedon cochleariae (Coleoptera, Chrysomelidae), adults are known to be deterred from feeding and oviposition by the exocrine glandular secretion of conspecific co-occurring larvae. We hypothesised that the exocrine larval secretion released by feeding larvae may adsorb to the surface of Chinese cabbage leaves, and thus, convey the information about their former or actual presence. Further experiments tested the influence of leaves damaged by conspecific larvae, mechanically damaged leaves, larval frass and regurgitant on the oviposition and feeding behaviour of P. cochleariae. Finally, the effect of previous conspecific herbivory on larval development and larval host selection was assessed. Our results show that (epi)chrysomelidial, the major component of the exocrine secretion from P. cochleariae larvae, was detectable by GC-MS in surface extracts from leaves upon which larvae had fed. However, leaves exposed to volatiles of the larval secretion were not avoided by female P. cochleariae for feeding or oviposition. Thus, we conclude that secretion volatiles did not adsorb in sufficient amounts on the leaf surface to display deterrent activity towards adults. By contrast, gravid females avoided to feed and lay their eggs on leaves damaged by second-instar larvae for three days when compared to undamaged leaves. Mechanical damage of leaves and treatment of artificially damaged leaves with larval frass or regurgitant did not affect oviposition and feeding of P. cochleariae. Since no adverse effects of previous herbivory on larval development were detected, we suggest that female P. cochleariae avoid Chinese cabbage leaves damaged by feeding larvae for other reasons than escape from competition or avoidance of direct negative effects that result from consuming induced plant material.     

Foliar form, colour and surface characteristics influence oviposition behaviour of the carrot fly

Various leaf models made of paper were presented to carrot flies, Psila rosae (F.) (Diptera: Psilidae) in choice assays to investigate the effect of non-chemical plant traits on oviposition behaviour. The surrogate leaves differed in colour, shape, surface coating, size and stem length. In the presence of host-plant extracts, physical factors strongly influenced oviposition. Green, yellow and orange three-dimensional models similar in shape to host-plant leaves (pinnately or ternately compound or dissected) and with a thin cover of paraffin wax were most acceptable to the females. Egg-laying was not affected by leaf size, but was negatively correlated with stem length. The results obtained by testing models with simple leaf silhouettes were confirmed in an experiment using more lifelike imitations of real host and non-host leaves. The findings are discussed by an extensive review of similar studies in three other phytophagous fly species (cabbage root fly, onion fly, Hessian fly).     

The decision to reject an oviposition site: sequential analysis of the post-alighting behaviour of Delia floralis

Abstract.The post-alighting behaviour of gravid female turnip root fly, Delia floralis (Fallén) (Diptera: Anthomyiidae), was observed in the laboratory, using four plant genotypes differing in antixenotic resistance, to stimulate a range of oviposition-related behavioural events. Analysis of the behavioural sequences of individual female flies suggested that during the post-alighting behaviour of D. floralis , the decision to reject a highly resistant plant was predominantly based on plant cues received during a stationary period immediately after landing on the leaf (the leaf contact phase). Female flies that rejected a plant immediately after the leaf contact phase did so after spending approximately twice as long in the leaf contact phase as those flies that continued to explore the plant. On a plant genotype of intermediate susceptibility, D. floralis females on the stem of the plants increased proboscis extension. The mechanisms involved in early host plant rejection decisions and the possible ecological advantages of such behaviour patterns to female flies seeking suitable oviposition sites are explored. The potential advantages of using detailed studies of individual behaviour sequences to focus studies seeking to elucidate chemical oviposition stimuli on the plant's surface are also discussed.     

Oviposition and chemosensory stimulation of the root flies Delia radicum and D. floralis in response to plants and leaf surface extracts from resistant and susceptible Brassica genotypes

In Brassica crops differences in susceptibility to root fly attack can be largely attributed to antixenotic resistance. Plants of four genotypes (two swedes and two kales) with widely differing resistance in field trials, were compared in laboratory choice assays for their susceptibility to oviposition by the root flies Delia radicum (L.) and D. floralis (Fallen) (Diptera, Anthomyiidae). For both species the preference among the genotypes corresponded to the susceptibility of the genotypes in the field. The preference ranking in response to surrogate leaves treated with methanolic surface extracts of the four genotypes was identical to the preference among potted plants, demonstrating that chemical factors on the leaf surface mediate host preference for oviposition in these species.For both species of fly, glucosinolates are major oviposition stimulants and for D. radicum an additional, nonglucosinolate oviposition stimulant, presently called CIF, is known. We describe a procedure for chromatographic separation of glucosinolates from CIF in leaf surface extracts. In oviposition-choice assays with D. radicum, the CIF-fractions of the two swede genotypes applied to surrogate leaves received a 1.8 and 4.6 times higher proportion of eggs than the respective glucosinolate-fractions, confirming the major importance of CIF as an oviposition stimulant. The genotype of swede that was preferred by both fly species in tests with plants and methanolic leaf surface extracts, also stimulated oviposition more in tests with the glucosinolate-fractions or the CIF-fractions derived from the surface extracts, respectively. Thus, glucosinolates and CIF together account for the observed preference among the genotypes and may also be responsible for their susceptibility under field conditions. In the two kale genotypes the preference for plants or surface extracts differed from the preference among the corresponding glucosinolate- and CIF-fractions, indicating that additional, as yet unknown chemical factors may also be involved.For both groups of stimulants tarsal chemoreceptors allow electrophysiological monitoring of glucosinolate- and CIF-activity in fractionated surface extracts. For D. radicum the chemosensory activity of both glucosinolate- and CIF-fractions corresponded to the respective behavioural activity in the oviposition preference tests, suggesting that preference for oviposition among genotypes can be predicted from the electrophysiological activity of their fractions. The chemosensory response of D. floralis, in particular to the CIF-fractions, was less pronounced than the response of D. radicum, indicating interspecific differences in the perception of the major oviposition stimulants. We discuss the potential application of electrophysiological techniques in support of other screening methods used in breeding for root fly resistance in Brassica crops.     

Comparison of the sensitivity of four Delia species to host and non-host plant compounds

Abstract. The behavioural facilitation hypothesis, tested in the present study, suggests that evolution of host-plant shifts by phytophagous insects is based on the preadaptation of insects to the chemistry of potentially novel plant hosts. Thus, closely-related insects should have similar sensitivities to compounds that are shared by different host plants. The chemoreception is investigated for four phytophagous flies, Delia radicum, Delia floralis, Delia antiqua and Delia platura (Diptera, Calyptratae: Anthomyiidae), belonging to the same genus but developing mainly on different plant families, with particular secondary plant compound profiles. In addition, the carrot fly, Psila rosae, an acalyptrate Diptera, is included as an unrelated species that is associated with completely different host plants. For the comparison, the known oviposition stimulants of the cabbage root fly (glucobrassicin, sinalbin, sinigrin and a thia-triaza-fluorene compound; CIF-1) present on the cabbage leaf surface were chosen. Responses from prothoracic tarsal sensilla are recorded to contact stimulation in a dose-dependent manner. Among the different flies tested, only D. radicum responds to all the compounds. By contrast, D. floralis is only sensitive to CIF-1, and not specifically on the C₅ sensillum, a finding that is in conflict with previously published results. This discrepancy is possibly an indication of the variability among flies originating from different cultures or habitats. With the exception of sinigrin at high concentration, the various compounds tested do not stimulate D. antiqua or D. platura. However, the carrot fly appears to be completely insensitive to sinigrin even at the highest tested concentration of 10⁻¹ M. The responses of the contact-chemoreceptor neurones to the selected compounds therefore provide little evidence of common sensitivities that would explain host shift in Delia species and specialization at the physiological level. The wide divergence within closely-related species and rearing cultures appears to indicate that the sensitivity and distribution of sensory receptor neurones is very variable on an evolutionary scale.     

Oviposition‐induced plant cues: do they arrest Trichogramma wasps during host location?

Plants can defend themselves against herbivorous insects before the larvae hatch from eggs and start feeding. One of these preventive defence strategies is to produce plant volatiles, in response to egg deposition, which attract egg parasitoids that subsequently kill the herbivore eggs. Here, we studied whether egg deposition by Pieris brassicae L. (Lepidoptera: Pieridae) induces Brussels sprouts plants to produce cues that attract or arrest Trichogramma brassicae Bezdeko (Hymenoptera: Trichogrammatidae). Olfactometer bioassays revealed that odours from plants with eggs did not attract or arrest parasitoids. However, contact bioassays showed that T. brassicae females were arrested on egg‐free leaf squares excised from leaves with 72 h‐old egg masses, which are highly suitable for parasitisation. We tested the hypothesis that this arresting activity is due to scales and chemicals deposited by the butterflies during oviposition and which are thus present on the leaf surface in the vicinity of the eggs. Indeed, leaf squares excised from egg‐free leaves, but contaminated with butterfly deposits, arrested the wasps when the squares were tested 1 day after contamination. However, squares from egg‐free leaves with 72 h‐old butterfly deposits had no arresting activity. Thus, we exclude that the arresting activity of the leaf area near 72 h‐old egg masses was elicited by cues from scales and other butterfly deposits. We suggest that egg deposition of P. brassicae induces a change in the leaf surface chemicals in leaves with egg masses. A systemic induction extending to an egg‐free leaf neighbouring an egg‐carrying leaf could not be detected. Our data suggest that a local, oviposition‐induced change of leaf surface chemicals arrests T. brassicae in the vicinity of host eggs.     

Behavioural plasticity in relation to egg and time limitation: the case of two fly species in the genus Anastrepha (Diptera: Tephritidae)

Reproductive opportunities in insects that deposit their eggs in discrete resource patches are frequently limited because the availability of oviposition substrates is often spatially and temporally restricted. Such environmental variability leads individuals to confront time‐ or egg‐limitation constraints. Additionally, species with different oviposition strategies (i.e. single egg layers vs clutch layers) commonly deal with different structural and ecological characteristics of larval host plants. To test the hypothesis that oviposition strategies such as laying eggs singly or in batches (clutches) are related to these constraints (i.e. egg vs time limitation), we compared the lifetime oviposition patterns of two closely related sympatric species of Anastrepha (Diptera: Tephritidae) with different oviposition strategies. We exposed five cohorts of A. obliqua and A. ludens females, over the course of their adult lifetimes, to three conditions of “habitat quality” (measured as host density per cage): unpredictable habitat quality (host density varied randomly from day to day between 1, 5, 15, 30 and 60 hosts/cage), low habitat quality (fixed density of one host/cage) and high habitat quality (fixed density of 60 hosts/cage).
Responses to host density conditions were strikingly different in the two species. (1) Frequency of host visits and oviposition events increased in A. obliqua but not in A. ludens when host densities increased. (2) Anastrepha ludens females accepted low quality hosts (i.e. fruits on which eggs had already been laid and were therefore partially covered with host marking pheromone) significantly more often than A. obliqua females did. (3) Females of A. obliqua adjusted their oviposition activity to variations in host density, whereas A. ludens females exhibited a constant oviposition pattern (i.e. did not respond to variations in host density). Based on the above, it is likely that in A. obliqua oviposition is governed by egg‐limitation and in A. ludens by time‐limitation constraints. We discuss the relationship between the oviposition strategies of each fly species and the fruiting phenology and density of their native host plants. We also address the possible influence of oogenesis modality and parasitism by braconid wasps in shaping oviposition behaviour in these insects.     

Use of habitat odour by host‐seeking insects

Locating suitable feeding or oviposition sites is essential for insect survival. Understanding how insects achieve this is crucial, not only for understanding the ecology and evolution of insect–host interactions, but also for the development of sustainable pest‐control strategies that exploit insects' host‐seeking behaviours. Volatile chemical cues are used by foraging insects to locate and recognise potential hosts but in nature these resources usually are patchily distributed, making chance encounters with host odour plumes rare over distances greater than tens of metres. The majority of studies on insect host‐seeking have focussed on short‐range orientation to easily detectable cues and it is only recently that we have begun to understand how insects overcome this challenge. Recent advances show that insects from a wide range of feeding guilds make use of ‘habitat cues’, volatile chemical cues released over a relatively large area that indicate a locale where more specific host cues are most likely to be found. Habitat cues differ from host cues in that they tend to be released in larger quantities, are more easily detectable over longer distances, and may lack specificity, yet provide an effective way for insects to maximise their chances of subsequently encountering specific host cues. This review brings together recent advances in this area, discussing key examples and similarities in strategies used by haematophagous insects, soil‐dwelling insects and insects that forage around plants. We also propose and provide evidence for a new theory that general and non‐host plant volatiles can be used by foraging herbivores to locate patches of vegetation at a distance in the absence of more specific host cues, explaining some of the many discrepancies between laboratory and field trials that attempt to make use of plant‐derived repellents for controlling insect pests.     

Short-range cues mediate parasitoid searching behavior on maize: The role of oviposition-induced plant synomones

Host searching by egg parasitoids faces a main constraint due to low detectability of cues from host eggs. Therefore egg parasitoids have developed distinctive strategies by exploiting cues that originate from non-target instars of the host and/or from plants. The scelionid Telenomus busseolae is specialized on concealed eggs of Sesamia nonagrioides and other noctuid stemborers. In this paper we show that oviposition by S. nonagrioides induces changes in the cues present on maize leaf surface, which arrest naïve females of T. busseolae. The induction appears to be systemic as the parasitoid also responds to leaves and leaf portions that are not directly affected by ovipositing females. Such oviposition-induced, short-range, plant synomones, acting in sequence with the kairomonal cues from scales left on the plant by the ovipositing host female, significantly increases parasitoid efficiency during host finding. The elicitor of plant response originates from the host female reproductive system, being contained both in the host’s ovarian eggs and in the colleterial gland secretion. Induction starts 24 h after oviposition and lasts at least till 72 h. The ecological role of this oviposition-induced plant synomone in host searching by T. busseolae is discussed.     

Tarsal chemoreceptors and oviposition behaviour of the cabbage root fly (Delia radicum) sensitive to fractions and new compounds of host-leaf surface extracts

Abstract. Contact chemoreception plays a decisive role in host selection and oviposition behaviour of the cabbage root fly, Delia radicum L. (Diptera, Anthomyiidae). Glucosinolates (mustard oil glucosides) are known to be perceived by the flies, and when sprayed on paper leaf-models induce oviposition. Recently it has become clear that other non-volatile types of compounds must also be involved in host selection.
A pair of ventro-medial C sensilla on die fifth tarsomere respond strongly to a novel compound called tentatively 'cabbage identification factor' (CIF), but not to sucrose, glucose, fructose and proline. CDF is a new non-glucosinolate oviposition stimulant. A single neurone in each sensillum is activated by this compound and the same is true for glucosinolates. In some flies a mixture of bom types of stimuli evoked an apparent mononeural spike train, whereas in odiers spikes of two separate cells were activated. The significance of this variability is not yet clear.
The new stimulant, CIF, does not evoke responses in glucosinolate receptors in the D sensilla. The involvement of the C₃ sensilla in the detection of host-specific compounds constitutes the first known function for C sensilla in D. radicum. CIF appears to be present in leaf surface extracts from the host-plant Brassica oleracea in quantities as low as 1 ng per gram leaf. In spite of this low level, it stimulates oviposition significantly better than glucobrassicin at higher concentrations, which up till now was known as the most powerful stimulant for D.radicum.     

Phylloplane location of glucosinolates in Barbarea spp. (Brassicaceae) and misleading assessment of host suitability by a specialist herbivore

Glucosinolates are plant secondary metabolites used in host plant recognition by insects specialized on Brassicaceae, such as the diamondback moth (DBM), Plutella xylostella. Their perception as oviposition cues by females would seem to require their occurrence on the leaf surface, yet previous studies have reached opposite conclusions about whether glucosinolates are actually present on the surface of crucifer leaves. DBM oviposits extensively on Barbarea vulgaris, despite its larvae not being able to survive on this plant because of its content of feeding-deterrent saponins. Glucosinolates and saponins in plant tissue and mechanically removed surface waxes from leaves of Barbarea spp. were analyzed with high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). Surface waxes from leaves of Barbarea spp. contained glucosinolates, but not feeding-deterrent saponins. Our research is the first to show that glucosinolates are present on the leaf surface of Barbarea spp., but not in other crucifers investigated, resolving some conflicting results from previous studies. Our research is also the first to quantify glucosinolates on the leaf surface of a crucifer, and to show that the concentrations of glucosinolates found on the leaf surface of Barbarea spp. are sufficient to be perceived by ovipositing DBM.     

Leaf surface chemistry of sorghum seedlings influencing expression of resistance to sorghum shoot fly, Atherigona soccata

Sorghum shoot fly, Atherigona soccata is one of the serious constraints to sorghum production, and host plant resistance is an important component for controlling this pest. We studied the expression of resistance to A. soccata in a diverse array of sorghum genotypes in relation to composition of leaf surface chemicals during the seedling stage. The sorghum genotypes IS 1054, IS 1057, IS 2146, IS 4664, IS 2312, IS 2205, SFCR 125, SFCR 151, ICSV 700, and IS 18551 exhibited antixenosis for oviposition, and suffered less deadhearts due to sorghum shoot fly, A. soccata. Compounds undecane 5- methyl, decane 4- methyl, hexane 2, 4- methyl, pentadecane 8- hexyl, and dodecane 2, 6, 11- trimethyl, present on the leaf surface of sorghum seedlings, were associated with susceptibility to shoot fly; while 4, 4- dimethyl cyclooctene was associated with resistance to shoot fly. The compounds associated with resistance/susceptibility to shoot fly, can be used as marker traits to select for resistance as well as for diversifying and increasing the levels of resistance to this pest. The role of biochemical compounds for developing sorghum varieties with resistance to shoot fly, A. soccata has been discussed.     

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

Cue hierarchy for host plant selection in Pieris rapae

During oviposition, female insects utilize a wide variety of cues to find an optimal host for their offspring. These cues may be encountered simultaneously or sequentially, and females must rely on a hierarchical categorization to rank cues for optimal ovipositional choice. During alighting, cues are often encountered at different distances, which may influence hierarchical order. Cues that are observed at longer distances and are experienced first may be more influential on oviposition choice than those experienced at shorter distances. We tested the effects of two long‐distance cues, plant size and habitat structure, and two short‐distance cues, plant nitrogen level and predator presence, on Pieris rapae L. (Lepidoptera: Pieridae) oviposition choice through a series of two‐ and four‐choice tests in both greenhouse and field settings. We found that the long‐distance cues of plant size and habitat structure had the largest impact on P. rapae oviposition in laboratory and field settings, with females preferring large plants without habitat structure. Plant nitrogen level had a smaller impact on oviposition, and the presence of predators did not affect oviposition choice. However, plant size and plant nitrogen level had a synergistic relationship, with more eggs laid on large high‐nitrogen plants compared to large low‐nitrogen or small high‐nitrogen plants, suggesting that optimal cues throughout the entire searching process may be important for P. rapae in choosing a host plant.     

Oviposition Site-Selection by Bactrocera dorsalis Is Mediated through an Innate Recognition Template Tuned to γ-Octalactone

Innate recognition templates (IRTs) in insects are developed through many years of evolution. Here we investigated olfactory cues mediating oviposition behavior in the oriental fruit fly, Bactrocera dorsalis, and their role in triggering an IRT for oviposition site recognition. Behavioral assays with electrophysiologically active compounds from a preferred host, mango, revealed that one of the volatiles tested, γ-octalactone, had a powerful effect in eliciting oviposition by gravid B. dorsalis females. Electrophysiological responses were obtained and flies clearly differentiated between treated and untreated substrates over a wide range of concentrations of γ-octalactone. It triggered an innate response in flies, overriding inputs from other modalities required for oviposition site evaluation. A complex blend of mango volatiles not containing γ-octalactone elicited low levels of oviposition, whereas γ-octalactone alone elicited more oviposition response. Naïve flies with different rearing histories showed similar responses to γ-octalactone. Taken together, these results indicate that oviposition site selection in B. dorsalis is mediated through an IRT tuned to γ-octalactone. Our study provides empirical data on a cue underpinning innate behavior and may also find use in control operations against this invasive horticultural pest.     

The role of glucosinolates and their hydrolysis products in oviposition and host-plant finding by cabbage webworm, Hellula undalis

The cabbage webworm, Hellula undalis (Fabricius) (Lepidoptera: Pyralidae), a tropical pest on crucifers (Brassicaceae), differentiated among host‐plant species for oviposition in laboratory and field tests. White mustard, Sinapis alba (L.) var. Selinda, was the preferred host‐plant, followed by Brassica juncea (L.) Czern. et. Coss var. Canadian brown mustard, and pak‐choi, Brassica campestris L. ssp. chinensis var. Joi Choi, Black Behi and Bai Tsai. Glucosinolates (GS), secondary plant compounds characteristic to the Cruciferae plant family, and their breakdown products were analyzed by using HPLC and GC‐MS‐techniques. Species differed in GS composition and concentration. Content of GS was highest in S. alba with progressively lower contents detected in B. juncea and B. chinensis. The aromatic GS, 4‐hydroxybenzyl‐GS and benzyl‐GS, were detected in S. alba. In B. juncea the alkenyl GS, allyl‐GS, dominated, whereas in varieties of B. chinensis indolyl and alkenyl GS predominated. Oviposition of H. undalis females on the non‐host‐plant Vigna unguiculata ssp. sesquipedalis (L.) Fruwirth was stimulated by application of GS extracts from the crucifer species; the extract from S. alba was preferred, followed by extracts from B. juncea and B. chinensis. Hydrolysis of GS in the plant extract from B. chinensis causes loss of the oviposition stimulatory effect of the extract. Application of the GS, allyl‐GS, and benzyl‐GS also stimulated oviposition by H. undalis. Significantly more eggs were laid on leaves treated with the aromatic GS, benzyl‐GS, than with the alkenyl GS, allyl‐GS. Host‐plant odor attracted H. undalis females but not males, in behavioral assays conducted in a Y‐tube olfactometer. Low concentrations of the GS hydrolysis product, allyl‐isothiocyanate, induced anemotaxis of females, but a high concentration of allyl‐isothiocyanate was repellent. Oviposition by H. undalis females was not stimulated by host‐plant volatiles. Females laid eggs on inserted traps and the walls of the Y‐tube regardless of presence or absence of host‐plant odor. The relevance of these results in the context of crucifer‐insect interactions is discussed.     

The role of spatial scale and intraspecific variation in secondary chemistry in host-plant location by Ceutorhynchus assimilis (Coleoptera: Curculionidae)

To understand the ecological role of secondary plant compounds in host location by phytophagous insects it is important to consider attraction at different scales in natural populations. The cabbage seed weevil, Ceutorhynchus assimilis, which lays eggs in pods of crucifers where the larvae feed on seed, is attracted to purified extracts of specific glucosinolate-derived volatiles. We considered the possibility that C. assimilis adults are attracted to and preferentially attack patches of plants and/or individual plants producing these volatiles. Using discrete natural populations of Brassica oleracea and Brassica nigra, we found that oviposition was highest in populations of B. oleracea producing high amounts of 3-butenylglucosinolate. No links were found between the other glucosinolates, 2-propenylglucosinolate, 2-hydroxy-3-butenylglucosinolate, 1-indolylmethylglucosinolate or 1-methoxy-3-indolylmethylglucosinolate, and oviposition in B. oleracea. B. nigra, which contains only 2-propenylglucosinolate, was not attacked by C. assimilis. Within populations of B. oleracea, neither oviposition nor the number of seeds eaten was related to the glucosinolate profiles of individual plants. We suggest that C. assimilis adults use 3-butenylglucosinolate-derived volatiles to locate host populations, whereas other cues determine oviposition on individual plants. The consequences of these results for natural selection of glucosinolate phenotypes are discussed.