Associative learning for danger avoidance nullifies innate positive chemotaxis to host olfactory stimuli in a parasitic wasp

Effect of fruit and host fly species on the associative learning by Fopius arisanus

Parasitoids, released in augmentative biological control programmes, which display a rapid host-location capacity, have a higher likelihood of successfully controlling target pest species. By learning to associate sensory cues to a suitable oviposition site, might parasitoids used as biological control agents, locate hosts more rapidly, and perhaps increase the efficacity of e.g. Tephritidae fruit fly management. We studied associative learning of Fopius arisanus (Hymenoptera: Braconidae) and tested its range of learning in natural and conditional hosts and host fruits, i.e. Bactrocera dorsalis, Zeugodacus cucurbitae, Ceratitis capitata and Ceratitis cosyra (Diptera: Tephritidae) and on fruits (papaya, tomato, banana). Naïve female F. arisanus were compared with experienced wasps, which had been offered infested and non-infested fruit, and been allowed to oviposit. Preferences for olfactory cues from infested fruits were thereafter assessed in a two-choice olfactometer. Naïve and trained parasitoids preference differed in general and non-responders to infested fruits were higher among naïve parasitoids. The trained wasps preferred the fruit infested in the training more than the control fruit, for all combination, except when C. cosyra infested the fruits, hence avoidance behavioural response was observed towards the odour of the infested fruit. Fopius arisanus was capable of behaviourally respond to the learned information, e.g. associative odour learning was achieved, yet limited depending on interaction level, fruit fly and fruit combination. To create F. arisanus preference of an associated odour, it might hence be needed to ensure oviposition in perceived suitable host and host fruit, for the parasitoid learning to become favourable in a biological control setup.

Multiple Mating in the Citrophilous Mealybug Pseudococcus calceolariae: Implications for Mating Disruption

The citrophilous mealybug Pseudococcus calceolariae (Maskell) (Hemiptera, Pseudococcidae) is a primary pest of various crops, including grapevines. The use of insecticides against this species is difficult in most cases because its life cycle includes an extended duration of eggs, juveniles, and adults under the bark and on the roots. Pheromone-based control strategies can present new eco-friendly opportunities to manage this species, as in the case of Planococcus ficus (Signoret) and Planococcus citri (Risso). With this aim it is critical to understand behavioral aspects that may influence pheromone-based control strategies. Herein, the capability of males to fertilize multiple females was investigated, trying to understand whether this behavior could negatively impact the efficacy of mass trapping, mating disruption, or the lure and kill technique. Results showed that a P. calceolariae male can successfully mate and fertilize up to 13 females. The copulation time in subsequent mating events and the time between copulations did not change over time but the number of matings per day significantly decreased. In a further experiment, we investigated the mate location strategy of P. calceolariae males, testing the attractiveness of different loadings of sex pheromone on males in a flight tunnel. Males constantly exposed to 16 rubber septa loaded with the sex pheromone showed a significant decrease in female detection at 1 and 30 μg loadings (0.18 and 0.74 visits per female for each visit per septum, respectively), whereas in the control about 9.2-fold more of the released males successfully detected the female in the center of the array of 16 septa without pheromone. Male location of females in the control (45%) was significantly higher than in the arrays with surrounding pheromone (5% and 20% at 1 and 30 μg loadings, respectively). Mating only occurred in the control arrays (45%). This study represents a useful first step to developing pheromone-based strategies for the control of citrophilous mealybugs.

Plasticidade da aprendizagem de Diachasmimorpha longicaudata (Hymenoptera: Braconidae) associada a voláteis de frutos e óleos essenciais

RESUMO Diachasmimorpha longicaudata (Ashmead, 1905) (Hymenoptera: Braconidae) é um dos parasitoides mais utilizados em programas de controle biológico de tefritídeos no mundo. Contudo pouco se sabe sobre a capacidade de aprendizagem e memória deste braconídeo na busca pelo hospedeiro Anastrepha fraterculus (Wiedemann, 1830) (Diptera: Tephritidae). Neste estudo, avaliou-se o tempo de residência (TR) e o parasitismo de fêmeas de D. longicaudata oriundas de larvas de A. fraterculus criadas em dieta artificial, goiaba ou em manga e posteriormente, expostas aos odores destes frutos. Foi observada a aprendizagem de fêmeas de D. longicaudata condicionadas na fase adulta aos voláteis de óleo essencial de baunilha (OEB) e de laranja (OEL) e avaliadas, com os mesmos, em testes de quimiotaxia. Também registrou-se a memória deste parasitoide exposto ao OEB. Os insetos utilizados nos experimentos foram mantidos em câmaras climatizadas (25 ± 2 °C, 70 ± 10% UR) na fotofase de 14 h (adultos) e na escotofase (imaturos). As respostas quimiotáxicas foram registradas com olfatômetro tipo “Y” e o parasitismo (condicionamento na fase imatura), em larvas mantidas em unidades com polpa de goiaba, manga ou sem polpa (controle). Fêmeas do parasitoide criadas em larvas mantidas em dieta artificial foram expostas a OEL ou OEB por 4 h e a aprendizagem e memória (OEB) avaliadas em olfatômetro, a cada 24 h e por até 72 h. Fêmeas inexperientes de D. longicaudata foram mais atraídas para os voláteis de manga e goiaba em relação ao controle. No entanto, as experientes apresentaram TR maior para os odores dos frutos nos quais se desenvolveram. Contudo, quando os odores destes frutos foram oferecidos simultaneamente, o TR foi maior para os voláteis de manga. O percentual de parasitismo de vespas inexperientes foi maior na presença das polpas e, das experientes, nas larvas que continham os odores aos quais haviam sido condicionadas. O TR de fêmeas inexperientes foi significativamente maior para a acetona do que para os odores dos óleos. Fêmeas experientes em OEB responderam mais a este odor em relação ao controle, entretanto, não houve diferença para os experientes em OEL. A memória ao odor de OEB foi mantida por até 48 h. Concluímos que fêmeas experientes reconhecem odores aos quais se desenvolvem e aos óleos aos quais receberam experiência, resultando em preferência a estes quanto ao tempo de residência. Entretanto, reconhecer fatores que interferem na comunicação entre hospedeiro-parasitoide pode possibilitar maior adequação e confiabilidade na utilização de D. longicaudata em programas de controle biológico.

Bactrocera oleae-induced olive VOCs routing mate searching in Psyttalia concolor males: impact of associative learning

Olfaction is a key sense routing foraging behaviour in parasitoids. Preferences for food, mate and host stimuli can be innate in parasitic wasps. Alternatively, learning-mediated mechanisms play a crucial role. Females of the braconid parasitoid Psyttalia concolor exploit olfactory cues arising from tephritid hosts and related microhabitats. However, little is known on the olfactory stimuli routing males searching for mates. In this study, we focused on the attractiveness of Bactrocera oleae-induced olive volatiles towards P. concolor males. Furthermore, we evaluated learning occurrence in virgin males, when trained for selected unattractive volatile organic compounds (VOCs) associated with mate rewards. (E)-β-Ocimene, α-pinene and limonene attracted virgin males in Y-tube bioassays. Unattractive VOCs evoked positive chemotaxis after associative learning training. P. concolor males exposed to VOCs during a successful or unsuccessful mating, showed short-term preference for these VOCs (<1 h). However, memory consolidation was strictly dependent on reward value. Indeed, males experiencing a successful mating showed a fast consolidation into protein dependent long-term memory, appearing after 24 h. On the other hand, males experiencing a less valuable training experience (i.e. unsuccessful courtship), did not show consolidated memory after 24 h. Overall, our findings suggest that P. concolor virgin males may exploit VOCs from the host microhabitat to boost their mate searching activity, thus their reproductive success. However, since learning is a costly process, P. concolor males retained durable memories just in presence of a valuable reward, thus avoiding maladaptive behaviours.

Factors influencing aversive learning in the oriental fruit fly, Bactrocera dorsalis

Parameters such as the intensity of conditioned and unconditioned stimuli, the inter-trial interval, and starvation time can influence learning. In this study, the parameters that govern aversive learning in the oriental fruit fly, Bactrocera dorsalis, a serious pest of fruits and vegetables, were examined. Male flies were trained to associate the attractive odorant methyl eugenol, a male lure, with a food punishment, sodium chloride solution, and the conditioned suppression of the proboscis-extension response was investigated. We found that high methyl eugenol concentrations support a stronger association. With increasing concentrations of sodium chloride solution, a steady decrease of proboscis-extension response during six training trials was observed. A high level of learning was achieved with an inter-trial interval of 1-10 min. However, extending the inter-trial interval to 15 min led to reduced learning. No effect of physiological status (starvation time) on learning performance was detected, nor was any non-associative learning effect induced by the repeat presentation of odor or punishment alone. The memory formed after six training trials could be retained for at least 3 h. Our results indicate that aversive learning by oriental fruit flies can be affected by odor, punishment concentration and inter-trial interval.

Innate and Learned Responses of the Tephritid Parasitoid Psyttalia concolor (Hymenoptera: Braconidae) to Olive Volatiles Induced by Bactrocera oleae (Diptera: Tephritidae) Infestation

Parasitic wasps can learn cues that alter their behavioral responses and increase their fitness, such as those that improve host location efficiency. Psyttalia concolor (Szépligeti) (Hymenoptera: Braconidae) is a koinobiont endoparasitoid of 14 economically important tephritid species, including the olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae). In this research, we investigated the nature of olfactory cues mediating this tritrophic interaction. First, we identified the chemical stimuli emanating from uninfested and B. oleae-infested olive fruits via solid phase microextraction and gas chromatography-mass spectrometry analyses and identified >70 volatile organic compounds (VOCs). Two of these were increased by B. oleae infestation, (E)-β-ocimene and 2-methyl-6-methylene-1,7-octadien-3-one, and four were decreased, α-pinene, β-pine ne, limonene, and β-elemene. Innate positive chemotaxis of mated P. concolor females toward these VOCs was then tested in olfactometer assays. Females were attracted only by (E)-β-ocimene, at both tested dosages, indicating an intrinsic response to this compound as a short-range attractant. Next, we tested whether mated P. concolor females could learn to respond to innately unattractive VOCs if they were first presented with a food reward. Two nonassociative controls were conducted, i.e., "odor only" and "reward only." Following training, females showed positive chemotaxis toward these VOCs in all tested combinations, with the exception of limonene, a VOC commonly produced by flowers. Control females showed no significant preferences, indicating that positive associative learning had occurred. These results clarify how learned cues can fine-tune innate responses to B. oleae-induced VOCs in this generalist parasitoid of tephritid flies.

VOCs-Mediated Location of Olive Fly Larvae by the Braconid Parasitoid Psyttalia concolor: A Multivariate Comparison among VOC Bouquets from Three Olive Cultivars

Herbivorous activity induces plant indirect defenses, as the emission of herbivorous-induced plant volatiles (HIPVs), which could be used by parasitoids for host location. Psyttalia concolor is a larval pupal endoparasitoid, attacking a number of tephritid flies including B. oleae. In this research, we investigated the olfactory cues routing host location behavior of P. concolor towards B. oleae larvae infesting three different olive cultivars. VOCs from infested and healthy fruits were identified using GC-MS analyses. In two-choice behavioral assays, P. concolor females preferred infested olive cues, which also evoked ovipositional probing by female wasps. GC-MS analysis showed qualitative and quantitative differences among volatiles emitted by infested and healthy olives. Volatile emissions were peculiar for each cultivar analyzed. Two putative HIPVs were detected in infested fruits, regardless of the cultivar, the monoterpene (E)-β-ocimene, and the sesquiterpene (E-E)-α-farnesene. Our study adds basic knowledge to the behavioral ecology of P. concolor. From an applied point of view, the field application of the above-mentioned VOCs may help to enhance effectiveness of biological control programs and parasitoid mass-rearing techniques.

Effects of Abiotic Factors on HIPV-Mediated Interactions between Plants and Parasitoids

In contrast to constitutively emitted plant volatiles (PV), herbivore-induced plant volatiles (HIPV) are specifically emitted by plants when afflicted with herbivores. HIPV can be perceived by parasitoids and predators which parasitize or prey on the respective herbivores, including parasitic hymenoptera. HIPV act as signals and facilitate host/prey detection. They comprise a blend of compounds: main constituents are terpenoids and "green leaf volatiles." Constitutive emission of PV is well known to be influenced by abiotic factors like temperature, light intensity, water, and nutrient availability. HIPV share biosynthetic pathways with constitutively emitted PV and might therefore likewise be affected by abiotic conditions. However, the effects of abiotic factors on HIPV-mediated biotic interactions have received only limited attention to date. HIPV being influenced by the plant's growing conditions could have major implications for pest management. Quantitative and qualitative changes in HIPV blends may improve or impair biocontrol. Enhanced emission of HIPV may attract a larger number of natural enemies. Reduced emission rates or altered compositions, however, may render blends imperceptible to parasitoides and predators. Predicting the outcome of these changes is highly important for food production and for ecosystems affected by global climate change.

First report of behavioural lateralisation in mosquitoes: right-biased kicking behaviour against males in females of the Asian tiger mosquito, Aedes albopictus

Lateralisation (i.e. functional and/or structural specialisations of left and right sides of the brain) of aggressive traits has been studied in a number of vertebrates, while evidence for invertebrates is scarce. Mosquito females display aggressive responses against undesired males, performing rejection kicks with the hind legs. In this research, we examined lateralisation of kicking behaviour in females of the Asian tiger mosquito, Aedes albopictus. We found a right-biased population-level lateralisation of kicking behaviour. Four repeated testing phases on mosquito females confirmed the preferential use of right legs. However, when left legs were used, the mean number of kicks per rejection event was not different to that performed with right legs. Both left and right kicking behaviour lead to successful displacement of undesired partners. This is the first report about behavioural lateralisation in mosquitoes.

Lateralisation of aggressive displays in a tephritid fly

Lateralisation (i.e. different functional and/or structural specialisations of the left and right sides of the brain) of aggression has been examined in several vertebrate species, while evidence for invertebrates is scarce. In this study, we investigated lateralisation of aggressive displays (boxing with forelegs and wing strikes) in the Mediterranean fruit fly, Ceratitis capitata. We attempted to answer the following questions: (1) do medflies show lateralisation of aggressive displays at the population-level; (2) are there sex differences in lateralisation of aggressive displays; and (3) does lateralisation of aggression enhance fighting success? Results showed left-biased population-level lateralisation of aggressive displays, with no consistent differences among sexes. In both male-male and female-female conflicts, aggressive behaviours performed with left body parts led to greater fighting success than those performed with right body parts. As we found left-biased preferential use of body parts for both wing strikes and boxing, we predicted that the left foreleg/wing is quicker in exploring/striking than the right one. We characterised wing strike and boxing using high-speed videos, calculating mean velocity of aggressive displays. For both sexes, aggressive displays that led to success were faster than unsuccessful ones. However, left wing/legs were not faster than right ones while performing aggressive acts. Further research is needed on proximate causes allowing enhanced fighting success of lateralised aggressive behaviour. This is the first report supporting the adaptive role of lateralisation of aggressive displays in insects.

The best time to have sex: mating behaviour and effect of daylight time on male sexual competitiveness in the Asian tiger mosquito, Aedes albopictus (Diptera: Culicidae)

Aedes albopictus is the most invasive mosquito worldwide and works as a vector for many important pathogens. Control tools rely to chemical treatments against larvae, indoor residual spraying and insecticide-treated bed nets. Recently, huge efforts have been carried out to propose new eco-friendly alternatives, such as evaluation of plant-borne compounds and sterile insect technique (SIT) programs. Success of SIT is dependent to the ability of sterile males to compete for mates with wild ones. Little is still known about mating behaviour of Aedes males. Most of the studies focus on comparisons of insemination ability in sterilised and wild males, while behavioural analyses of mating behaviour are lacking. Here, I quantified the courtship and mating behaviour of A. albopictus and evaluated how daylight hours affect male mating behaviour and success. A. albopictus males chased females facing them frontally, from behind, or from a lateral side. If the female allowed genital contact, copulation followed. Otherwise, females performed rejection kicks and/or flew away. Thirty-seven percent of males obtained a successful copulation (i.e. sperm transfer occurs), lasting 63 ± 4 s. Unsuccessful copulation (20 % of males) had shorter duration (18 ± 1 s). Successful copulations followed longer male courtships (39 ± 3 s), over courtships preceding unsuccessful copulation (20 ± 2 s) or male's rejection (22 ± 2 s). After copulation, the male rested 7 ± 0.4 s close to the female, then move off. In a semi-natural environment, male mating success was lower in early afternoon, over morning and late afternoon. However, little differences in courtship duration over daylight periods were found. This study adds knowledge to the reproductive behaviour of A. albopictus, which can be used to perform comparisons among courtship and mating ethograms from different mosquito species and strains, allowing monitoring and optimisation of mass rearing quality over time in SIT programs.