Back to Africa: autumn migration of the painted lady butterfly Vanessa cardui is timed to coincide with an increase in resource availability

1. The painted lady Vanessa cardui is a long‐range migratory butterfly that performs an annual multi‐generational round‐trip between Europe and Africa. Each autumn it returns to northwest (NW) Africa, presumably to track changes in resources that follow a predictable climate‐related spatio‐temporal pattern. 2. Data on the abundance of adult and immature stages in the Maghreb in 2014–2016 are used to test several hypotheses regarding the autumn migration of this species. 3. A strong seasonal migratory strategy was confirmed by the all but total absence of the species in NW Africa at the end of summer and the arrival of huge numbers migrants in October and November. Migration was timed to coincide with an increase in host plant availability but not with any increase in nectar sources. 4. Flower abundance was the main predictor of adult abundance in autumn, with Ditrichia viscosa, Verbesina encelioides, and Medicago sativa being key resources that attracted enormous numbers of butterflies to oases, ruderal habitats, and oueds. The distribution of immature stages was strongly predicted by host plant abundance (with traditional agriculture representing the most important breeding habitat) and latitude (most breeding occurred in the south of the region). Also, both adults and immature stages were more common inland than in coastal areas. 5. Changes in age structure of the adult population were also noted. The number of fresh adults slowly increased, indicating that butterflies did not return in a single wave and that the first offspring of the first returners were already emerging when some butterflies were still arriving.     

Oviposition in an eruptive moth species, Yponomeuta evonymellus, is insensitive to the population density experienced during the larval period

We studied if the population density experienced during larval development affects the reproductive schedule of Yponomeuta evonymellus L. (Lepidoptera, Yponomeutidae), a moth species characterized by outbreak population dynamics. More specifically, we predicted that reproduction would be delayed to facilitate emigration from sites with suboptimally high densities of conspecifics. We manipulated larval densities in the laboratory, as well as in those pupae collected from the extremes of natural densities. As the response, we also recorded the timing and sizes of egg clutches laid. The results did not support our initial predictions: the timing of oviposition was not dependent on larval growth conditions. This apparent lack of adaptation might be related to the loss of ‘memory’ during metamorphosis in holometabolous insects. However, oviposition schedules were also only minimally sensitive to elevated adult density. An inability to respond to high larval densities may contribute to the outbreak dynamics in this species.     

Migration to and dispersal from oilseed rape by the pollen beetle,Meligethes aeneus,in relation to wind direction

1 Host‐plant‐odour‐induced upwind anemotaxis is accepted as the main mechanism by which herbivorous insects find their host plant within an heterogenous environment, but field data supporting this hypothesis are scarce. 2 The flight directions of the pollen beetle Meligethes aeneus to and from a plot of winter oilseed rape and the direction of the wind were recorded concurrently. The beetles were sampled by eight double‐sided window traps encircling the plot. Distal sides of the traps, relative to the plot, sampled the beetles as they flew towards the plot, whereas the proximal sides of the traps sampled them as they flew away from the plot. Paired data on daily catch of beetles in distal or proximal traps and the volume of air impacting each trap were compared. 3 Correlations between daily catch of M. aeneus into distal traps and trap wind volume were negative, indicating that flights by both overwintered‐ and new‐generation insects towards the plot were upwind. 4 Correlations between daily catch of M. aeneus into proximal traps and trap wind volume varied with generation. Catch of overwintered‐generation M. aeneus was negatively correlated with trap wind volume, indicating that flight was upwind. Proximal trap catch of the new‐generation M. aeneus was weakly/modestly positively correlated with trap wind volume, indicating that flights were downwind or crosswind. 5 Understanding the effect of wind direction on flight of M. aeneus holds potential for improving the forecasting of their arrival and spatial distribution on the crop for integrated pest management.     

Olfactory orientation responses of the eucalyptus woodborer, Phoracantha semipunctata, to host plant in a wind tunnel

The eucalyptus woodborer, Phoracantha semipunctata Fabricius (Coleoptera: Cerambycidae), attacks mainly species of Eucalyptus (Myrtaceae). This study investigated walking and flight behaviour of P. semipunctata males and females exposed to an odour plume originating from a log of E. globulus placed vertically in the upwind end of a wind tunnel. In control experiments, beetles were exposed to a PVC drainpipe in the same position as the log, providing a visual stimulus without host‐tree odour. No statistical differences were found between behavioural responses of either sex when exposed to the log or PVC pipe. No beetles landed on the PVC pipe, whereas 49% of the beetles exposed to host‐tree odour plume landed on the log. Beetles aged over 24 days after emergence from the host tree were more responsive than beetles aged 20–24 days, and accounted vor 86% of the beetles that landed on the log. While walking, host‐tree odour affected the behaviour of the beetles that landed on the log as follows: upwind movement and path linearity increased, whereas turning rate, stopping frequency, mean stopping time and time to take‐off flight decreased. During flight, host‐tree odour affected the behaviour of the beetles that landed on the log as follows: increased upwind flight, turning rate, flight time, flight distance, and decreased flight speed. For beetles that never lost contact with the odour plume, flight progressed upwind with narrow zigzags, and showed higher directedness upwind, path linearity, faster flight speed and lower turning rate than for beetles that lost contact with the odour plume. After loosing contact with the plume, beetles tended to decrease their upwind progression, exhibiting a sharp turn or quick counterturns followed by crosswind or downwind excursions. This led to regaining contact with the odour plume and resumed upwind progression at higher speed provided they flew within the boundaries of the plume. The results showed that host‐tree odour affects both walking and flight behaviour of P. semipunctata beetles, inducing a more directed upwind movement and landing on the visual stimulus of a tree trunk.     

Lack of compensation by final instar larvae of the myrmecophilous lycaenid butterfly,Jalmenus evagoras,for the loss of nutrients to ants

Abstract Larvae and pupae of the Australian lycaenid butterfly, Jalmenus evagoras Donovan (Lepidoptera; Lycaenidae), are protected from parasites and predators by attendant ants. In return, the juveniles of J. evagoras secrete to the ants a solution containing substantial amounts of sugars and amino acids. Larvae of J. evagoras were reared from hatching until adult eclosion either with or without ants. Experiments were performed to examine whether fifth (final) instar larvae attempt to compensate for the nutrient loss to ants, by consuming more food, digesting food more efficiently, or extending development time. The presence or absence of ants had no effect on the feeding rate, efficiency of digestion or development time of fifth instar larvae. Larvae with ants converted a smaller proportion of ingested food into biomass, and consequently grew less than their counterparts without ants. Thus fifth instar larvae of J. evagoras do not appear to compensate for the nutrient loss to ants. Possible reasons for the failure to compensate are discussed.     

Electroantennogram, wind tunnel and field responses of male Mediterranean corn borer, Sesamia nonagrioides, to several blends of its sex pheromone components

The electrophysiological and behavioural responses of male Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae) to several synthetic blends of its sex pheromone components were investigated. Electroantennogram (EAG) recordings showed the greatest response to a 77:8:10:5 blend of the four compounds found in the female glands: (Z)-11-hexadecen-1-yl acetate, (Z)-11-hexadecen-1-ol, (Z)-11-hexadecenal and dodecyl acetate. A logistic equation was fitted to the dose-response data and differences in relative EAG responses were analysed by deriving five parameters of this equation. Flight responses of males were also evaluated in a wind tunnel where the proposed blend (77:8:10:5) also elicited the best response. Field tests confirmed the laboratory findings as the 77:8:10:5 blend captured more S. nonagrioides males than other pheromone mixtures and showed more selectivity for male S. nonagrioides over other noctuid species, Discestra trifolii Rottenberg and Lacanobia oleracea L., but not for Mythimna unipuncta Haworth, a species with a sex pheromone composition almost identical to that of S. nonagrioides.     

Responses of the biting midge Culicoides impunctatus to acetone, CO2 and 1-octen-3-ol in a wind tunnel

Responses of host-seeking female Culicoides impunctatus (Goetghebuer) (Diptera: Ceratopogonidae) to acetone, carbon dioxide and 1-octen-3-ol were measured in a wind tunnel. Carbon dioxide, presented as a filamentous plume, increased upwind flight in a dose-dependent manner, up to 0.09% concentration. A homogenous CO2 plume elicited similar upwind responses at concentrations up to 0.09%, whereas higher plume concentrations (> 0.1%) induced erratic responses with a suppression of upwind flight. Bovine equivalent concentrations of acetone (1.5 x 10(-6)g/l) and 1-octen-3-ol (1.3 x 10(-8)g/) failed to induce any significant upwind response when tested alone. In the presence of CO2, however, 1-octen-3-ol showed highly significant increases in upwind responses at concentrations of 1.3 x 10(-1) - 10(-8)g/l. Mixtures of CO2+ acetone also enhanced upwind flight at 1.5 x 10(-9)g/l. High tunnel concentrations of both 1-octen-3-ol and acetone inhibited upwind responses. These findings are discussed in relation to host finding by C. impunctatus and known mechanisms by which upwind flight is initiated and arrested at high odour concentrations.