Communication Review

Abstract With the advent of semiochemical-based control strategies used to mitigate damage of agricultural pest moths, many studies have focused on the function of male-specific putative pheromone receptors (PRs). In this investigation, we instead isolated, heterologously expressed, and functionally characterized a female-biased candidate PR, CpomOR22, from the codling moth, Cydia pomonella. Using transgenic Drosophila melanogaster for single sensillum recording (SSR) and gas-chromatographic SSR, we tested both synthetic ligands and various apple headspace extracts, identifying saturated and unsaturated aldehydes (nonanal, decanal, undecanal, dodecanal; (Z)-4-undecenal and (Z)-6-undecenal) among the most active ligands. Parallel experiments expressing CpomOR22 in Xenopus oocytes confirmed the binding of nonanal, decanal and undecanal and revealed lactones (γ-undecalactone and δ-dodecalactone) and several carboxylic acids as additional active compounds. The renowned ecological importance of aldehydes for the codling moth and the potential for newly identified ligands, such as lactones, may inform innovative control strategies based on novel semiochemicals to interfere with the female-specific chemosensory systems of this insect.

Abstract Several Convolvulaceae species harbor heritable fungal endophytes from which alkaloids are translocated to reproductive tissues of the plant host. Evidence for the distribution and ecological role of these fungal alkaloids, however, is lacking or incomplete for many host species and growth forms. Here we report on the identity of the fungal endophytes and quantities of alkaloids present in the leaves and seeds of the arborescent morning glory, Ipomoea murucoides (Convolvulaceae). Young folded leaf samples taken from the wild, harbored mycelium of one of two fungal taxa wrapped around the leaves’ glandular trichomes. Most trees harbored the swainsonine producing Ceramothyrium (Chaetothyriales) fungi while a few trees were found to harbor a Truncatella (Xylariales) species, suggesting endophyte replacement. Seeds had higher concentrations of the indolizidine alkaloid swainsonine than leaves. Additionally, seeds from trees harboring Ceramothyrium fungi exhibited less bruchid damage and had higher concentrations of swainsonine than seeds from trees harboring Truncatella fungi. Five sesquiterpenes were detected in the leaf trichomes in both Ceramothyrium and Truncatella colonized trees. The seed content of the tropane alkaloids, tropine and tropinone, did not differ significantly among the two fungal symbionts. It is likely that the host allocates the defensive chemicals from leaves to seeds, protecting them from seed predators such as bruchid beetles. Overall, our field data show that Ipomoea species provides an interesting opportunity to study vertical and horizontal fungal symbiont transmissions.

Abstract Four-eyed bark beetles of the genus Polygraphus have been involved in large bark beetle outbreaks in different parts of the world, resulting in major economic losses. A striking example is the invasive species Polygraphus proximus which is a pest on Abies sibirica in Russia. In Sweden, Polygraphus poligraphus has been involved in bark beetle outbreaks on Norway spruce, Picea abies, together with the European spruce bark beetle Ips typographus. Two related species, Polygraphus punctifrons and Polygraphus subopacus are also present in Sweden. Recently, aggregation pheromones or pheromone components have been identified for these four Polygraphus species. However, questions remain regarding the complete composition of their pheromones, particularly for P. subopacus and P. proximus, whose aggregation pheromones appear to be very similar. In an attempt to better understand the chemical communication of these species, additional studies were conducted on P. poligraphus, P. punctifrons and P. subopacus using solid phase microextraction coupled with gas chromatography and mass spectrometry (SPME–GC–MS), electroantennography (EAG) as well as SPME–GC–MS and GC–MS with electroantennographic detection (EAD). Field experiments were also conducted. In P. punctifrons, some male-specific compounds were found in addition to those previously identified. In EAG and SPME–GC–MS/EAD studies, all three Polygraphus species responded strongly to grandisol. Using a chiral column, GC–MS/EAD revealed that they were able to detect both enantiomers of grandisol. In summary, this work presents our current understanding of the aggregation pheromones in four Polygraphus species and the challenges we have met in identifying species-specific pheromone blends for some of these species.

Abstract Wireworms, the larval stage of elaterid beetles, are among the most serious soil-borne insect pests in the world. Wireworms feed on a variety of key crops, including small grains, maize, vegetables, fruits, sugar cane, sugar beets, potatoes, and sweet potatoes. Management of these pests is becoming increasingly problematic, in part due to knowledge gaps in their basic biology, which hinders development of effective crop protection strategies. In particular, little is known about the semiochemicals that mediate the reproductive behavior of these pests. Research over the past two decades has begun to fill this need, with: (1) the discovery of sex attractants for several key pest species, and (2) subsequent studies toward development of semiochemically-based pest management approaches. We used chemical and behavioral studies to identify, synthesize, and field test the sex attractant pheromone of adult Melanotus verberans, the larvae of which are important crop pests. In coupled gas chromatography-mass spectrometry analyses of extracts of ovipositors of females, five possible pheromone components were identified. Subsequent coupled gas chromatography-electroantennogram detection analyses indicated that male antennae were responsive to only two of these compounds, 13-tetradecenyl acetate and 13-tetradecenyl hexanoate. In field trials, neither compound alone was attractive, but a blend of the two was strongly attractive to conspecific male beetles, and did not attract other species. A two-year field study showed that most male beetle flight activity occurred from April through May. Because the sex pheromone of M. verberans consists of two compounds that can be readily synthesized, its development for integrated pest management should be economically feasible.

Abstract Drosophila suzukii is a pest of soft and stone fruits that is attracted to yeast volatile metabolites. Drosophila suzukii has distinct summer and winter morphs which are found in different habitats. Complex communities of yeasts likely differ between habitats and thus we hypothesized morphs differ in their attraction to yeast volatiles from different species and combinations of species. We presented D. suzukii with yeast species in isolation and in combinations, as either post-culture mixes or co-cultured, and measured activity of the flies in laboratory choice tests with a Locomotor Activity Monitor as a proxy for attraction. Candida zemplinina was more attractive to winter than summer morphs when cultured in both sterile strawberry juice (SSJ) and artificial culture media (YPD). No significant correlations were found between principle component scores derived from single yeast volatile profiles and fly activity. There was also differential attraction of morphs to certain yeast mixes, most notably post SSJ culture mixes of Hanseniaspora uvarum and C. zemplinina were consistently more attractive to winter morphs. Mixtures of Metschnikowia pulcherrima + Pichia pijperi + H. uvarum in SSJ and M. pulcherrima + P. pijperi yeasts in YPD were also preferred by winter morphs, whilst summer morphs were attracted to SSJ co-cultures of M. pulcherrima + H. uvarum. However, co-culturing yeasts did not enhance attraction compared to post-culture mixes of constituent yeasts for either morph. Differential attraction of morphs to certain yeasts suggests options for new bait formulations in integrated pest management strategies may be tailored to better target morphs.

Abstract The adaptation of monarch butterflies (Danaus plexippus) to milkweed plants and their ability to sequester toxic cardenolides is a model system for plant-herbivore coevolution. However, the physiological mechanisms underlying cardenolide sequestration and its temporal dynamics remain largely unknown. Here, we show that the polar cardenolide ouabain passes through the isolated midgut epithelium of D. plexippus in vitro and is also absorbed into the body cavity of monarch caterpillars. Remarkably, the same pattern was observed in caterpillars of the related, but non-sequestering milkweed butterfly Euploea core, and even in the non-adapted Solanaceae specialist Manduca sexta, although uptake across gut epithelia occurred at a lower rate. Furthermore, we demonstrated that cardenolides begin to cross the epithelium in the anterior part of the intestine and can be detected in body tissues as soon as one minute after ingestion. Finally, we show that not all cardenolides are translocated into butterfly tissues during metamorphosis, and that the most apolar cardenolides are removed with the last caterpillar exuviae. As a result, adult butterflies contain no cardenolides less polar than the milkweed cardenolide calactin. We conclude that uptake by the intestinal epithelium is a very rapid process and that quantitative differences in cardenolide sequestration among lepidopteran caterpillars are only partially mediated by the gut epithelium, likely involving additional mechanisms such as metabolism or excretion. In addition, the translocation of cardenolides from the caterpillar is a selective process which may be due to the limited mobility of highly apolar cardenolides.

The eastern larch beetle, Dendroctonus simplex LeConte, colonizes tamarack, Larix laricina (Du Roi) K. Koch, and has become a major tree killer in parts of its temperate North American range. To fill existing gaps in knowledge of this insect’s pheromone system, we used gas chromatography-mass spectrometry to perform quantitative studies of pheromone production by unfed beetles of both sexes, founding females excised from newly established galleries in a tamarack log, and both sexes excised post-pairing. To identify olfactory stimulants and thereby possible, additional pheromone components, we performed gas chromatography-electroantennographic detection analyses with antennae of both sexes exposed to volatiles collections from the beetles. We also exposed unfed male and female beetles to juvenile hormone III (JH III) to identify pheromone components regulated through this mechanism. Both female and male beetles exhibited antennal responses to 1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane (frontalin), 3-methylcyclohex-2-en-1-ol (seudenol), and 3-methylcyclohex-2-en-1-one (MCH) in volatiles of solitary mining females. Within approximately 24 h of joining a male, female production of seudenol and MCH largely ceased while production of frontalin continued. Unfed females produced no more than minute amounts of pheromone, and application of JH III induced production of frontalin from some females but not seudenol or MCH. Unfed males and males exposed to JH III produced almost no volatiles, whereas paired males produced minute amounts of frontalin (possibly an artifact), no seudenol or MCH, and some oxygenated monoterpenes. We found that timing of production of certain pheromone components was inconsistent with their known activity.

Sex pheromones of mealybugs are reported from more than 20 species and have generally been shown to be monoterpene alcohols esterified with short-chain carboxylic acids. Here, however, we discovered and isolated an alcohol without an acid moiety as a pheromone compound released from adult females of the papaya mealybug, Paracoccus marginatus. By means of gas chromatography – mass spectrometry and nuclear magnetic resonance spectroscopy analyses, the structure was identified to be trans-2-(2-isopropenyl-1-methylcyclobutyl)ethanol, a monoterpene with a unique cyclobutene skeleton, commonly known as fragranol. We then completely separated synthetic (±)-fragranol into each enantiomer by means of preparative high-performance liquid chromatography using a chiral resolution column, and (−)-(1S,2S)-fragranol was definitely confirmed to be the natural pheromone and to attract many males in the field when used as a pheromone trap lure. (±)-Fragranol showed attractiveness comparable to that of the pure (−)-(1S,2S)-enantiomer. This study provides not only useful information for the monitoring and management of P. marginatus but also an interesting exception underlining the great diversity of mealybug pheromone structures.

Abstract Many Drosophila species coexist by sharing their feeding and breeding sites, which may influence their oviposition choices in an interspecies social context. Whether and where to lay eggs is a crucial decision for female flies as it influences the success of their offspring, by minimizing the risk of predation, competition, or cannibalism. Significant gaps exist in our understanding of Drosophila oviposition dynamics in co-occurring species. Here we tested oviposition strategies of Drosophila melanogaster and its close relative Drosophila simulans under different conditions, to assess whether a single female would prefer to oviposit separately or together with another female, be it a conspecific or not. We find that ovipositing females, regardless whether they are conspecifics or not, prefer to oviposit at the same site. This might suggest that the flies regard the benefits of sharing oviposition sites as higher than the potential risks of competition or cannibalism. The willingness to share oviposition sites was lower when the nutritional value of the medium was increased by adding yeast, and was lost when flies were allowed to lay the eggs consecutively, instead of being tested together. The latter might be explained by our additional finding that females become attracted by the presence of other females on oviposition substrates and that this attraction is partly driven by visual cues. Ovipositing in groups might facilitate intra- and interspecific social feeding of same age offspring, as well as enrichment of microbes. However, this cooperation dynamic might change if another female’s offspring is already present, as it might be perceived as danger of competition or cannibalism.

Milkweed plants in agricultural landscapes throughout the United States and southern Canada are believed to be vitally important for the imperiled monarch butterfly (Danaus plexippus) populations. However, studies have found that these plants often assimilate a slew of pesticides from the surrounding landscape, including highly potent and widely applied neonicotinoid insecticides. This has prompted concern over the potential impacts of these compounds on monarch populations and has created a growing interest in determining the direct lethal and sublethal consequences of exposure. Fewer studies have considered how neonicotinoids may interact with milkweed defensive chemistry to indirectly influence monarch performance. Here we addressed this question by investigating whether uptake of a widely used neonicotinoid insecticide, clothianidin, could alter milkweed (Asclepias syriaca) defense responses and subsequently impact monarch growth and feeding. We found that clothianidin-treated milkweed plants grew taller, and produced monarch larvae that weighed more and consumed more leaf tissue than larvae feeding on control plants. After five days of monarch herbivory, clothianidin-treated plants had higher levels of the phytohormone, jasmonic acid, but similar levels of salicylic acid relative to control plants. Neither latex nor cardenolide production was impacted by clothianidin assimilation. Overall, these findings indicate that clothianidin exposure can improve the vitality of common milkweed plants, and may subsequently impact monarch performance.

Forensic entomology has relied on species-specificity, quantitative and qualitative variations of cuticular hydrocarbons to successfully carry out chemotaxonomic identification of insects based on species, age and gender. This work studied the effects of dichlorvos on the cuticular hydrocarbon profiles of some adult sarco-saprophagous insects of forensic importance that fed on dichlorvos-poisoned carrions for utility during death investigations. Cuticular hydrocarbons (CHCs) were extracted from adult insects of the species Chrysomya albiceps, Sarcophaga exuberans, Musca domestica, Hermetia illucens, Dermestes maculatus and Necrobia rufipes from both dichlorvos-poisoned and control pig (Sus scrofa Linnaeus) carrions and subjected to chemotaxonomic profiling using Gas Chromatography-Mass Spectrometry (GC–MS). A total of 41 CHCs were successfully identified from insects of both the dichlorvos-poisoned and control carrions ranging from C8 to C33 carbons consisting of majorly the n-alkanes, methyl branched alkanes and an alkene. There was a higher abundance of CHCs present in the insects of dichlorvos-poisoned carrions than the control group. The highest mean peak concentration and abundance of the CHCs was recorded by 2,6,10,14 -Tetramethyl Pentadecane (10.38 ± 0.53 μg/mg for dichlorvos-poisoned carrions and 8.99 ± 1.13 μg/mg for the control carrions). The visualization of the species-specific differences in CHCs compositions showed less overlapping CHCs clusters and quantitative metrics of principal component analysis plots of the insects from both carrion groups with high eigenvalues > 3 which were indications of good species level discrimination. The study showed that insects’ CHCs profiles of dichlorvos-poisoned and control carrions exhibited uniqueness cum variations in terms of abundance and chemical identity.

Cuticular hydrocarbons (CHCs) constitute an important class of chemical compounds present in the cuticular layer of insects, where their main functions are to prevent desiccation of the cuticle and as signals for intraspecific recognition. Studies concerning CHCs have shown a phenotypic flexibility of their composition, depending on environmental factors. However, the way that each of these factors influences this variation remains little explored. The aim of the present study was to evaluate the effects of environmental variations on the cuticular chemical composition of the ant Atta laevigata. Workers from four different colonies nesting in forest edge environments were collected over the course of a year, during the hot and humid and cold and dry seasons. The cuticular compounds were extracted and then analyzed by gas chromatography, revealing that the compounds of this species belonged to the classes of linear alkanes, mono, di and trimethyl alkanes, alkenes and alkadienes. Furthermore, the cuticular profile varied significantly among colonies of this species and between seasons, while intra-season variability was more significant during the hot and humid season. The observed temporal variation indicated that the numbers of compounds and the proportion of different types of CHC can vary according to the period of the year, however with a greater significant variation in colonies in the hot and humid season. These results showed that variations in environmental conditions, especially climate, can have decisive effects in the dynamics of cuticular chemistry.

Over the last 28 years, the small hive beetle, Aethina tumida (Coleoptera: Nitidulidae), a colony pest of wild and managed honey bees (Apis mellifera), and native to sub-Saharan Africa, has been recorded as an invasive and damaging pest of diverse bee species, especially managed honeybee colonies in the Americas, Europe, Asia and Australia. It poses an indirect threat to pollination services and global food security. Efforts to manage the beetle has yielded no permanent solution. However, several studies demonstrate that the small hive beetle uses various sensory cues to locate hosts including chemical and visual cues in the visible wavelength. Here, we review the chemical ecology of the beetle and discuss implications for its future management.

Abstract In modern agriculture, control of insect pests is achieved by using insecticides that can also have lethal and sublethal effects on beneficial non-target organisms. Here, we investigate acute toxicity and sublethal effects of four insecticides on the males’ sex pheromone response and the female host finding ability of the Drosophila parasitoid Leptopilina heterotoma. The nicotinic acetylcholine receptor antagonists acetamiprid, flupyradifurone and sulfoxaflor, as well as the acetylcholinesterase inhibitor dimethoate were applied topically as acetone solutions. Males treated with all four insecticides no longer preferred the female sex pheromone in a T-olfactometer. Duration of wing fanning, an element of the pheromone-mediated male courtship behavior, was also reduced by all four insecticides. The ability of females to orientate towards host-infested feeding substrate was not affected by acetamiprid in the tested dose range. However, treatment with dimethoate, flupyradifurone and sulfoxaflor resulted in the loss of the females’ preference for host odor. At the lowest doses interfering with olfactory abilities of L. heterotoma in this study (acetamiprid: 0.21 ng, dimethoate: 0.105 ng, flupyradifurone: 2.1 ng and sulfoxaflor: 0.21 ng), ≥ 90% of the wasps survive insecticide treatment. Male pheromone responses and female host finding were also disturbed in those L. heterotoma that had developed in D. melanogaster larvae reared on dimethoate-treated feeding medium at sublethal levels. Hence, doses of this insecticide sufficient to interfere with chemical orientation of L. heterotoma can be taken up by the parasitoid via the food chain.