Oviposition by Lobesia botrana is stimulated by sugars detected by contact chemoreceptors

Abstract.  The influence of glucose, fructose and sucrose on oviposition site selection by Lobesia botrana is studied by combining behavioural and electrophysiological experiments. Oviposition choice assays, using surrogate grapes treated with grape berry surface extracts of Vitis vinifera cv. Merlot at different development stages, show that L. botrana females are most stimulated by extracts of mature berries containing the highest concentrations of glucose and fructose. Choice assays reveal that the oviposition response to these sugars is dose-dependant (with a threshold of the applied solution = 10 m m and a maximum stimulation at 1  m ) and that females are more sensitive to fructose than to glucose. Tarsal contact-chemoreceptor sensilla are unresponsive to stimulation with sugars but the ovipositor sensilla contain at least one neurone most sensitive to fructose and sucrose with a threshold of approximately 0.5 m m . Corresponding to the behavioural data, glucose is significantly less stimulatory to sensilla than fructose or sucrose. It is argued that fructose may be of special importance for herbivorous insects exploiting fruit as an oviposition site.     

Variation in trichome density and resistance against a specialist insect herbivore in natural populations of Arabidopsis thaliana

Abstract.  1. The annual herb Arabidopsis thaliana is a prime model organism of plant molecular genetics, and is currently used to explore the molecular basis of resistance to herbivores. However, both the magnitude and the causes of variation in resistance among natural populations of A. thaliana are poorly known. The hypotheses (a) that resistance to a specialist herbivore, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is positively related to the density of leaf trichomes, and (b) that oviposition preference by female moths is positively correlated to larval performance on different populations and maternal lines of the host plant were examined.
2. Variation in leaf trichome density and resistance to P. xylostella within and among six natural populations of A. thaliana in Sweden was quantified . Resistance was quantified by examining the number of eggs laid on plants exposed to ovipositing female moths and by monitoring larval development on plants of different origin.
3. Trichome density varied significantly among populations; for 4-week-old plants (exposed to ovipositing moths), it also varied significantly among maternal families within populations. The rate of oviposition varied significantly both among populations and among families within populations. This variation could partly be explained by a negative relationship between trichome density and egg number, and a positive relationship between plant size and egg number. Time to pupation, pupal mass, and time to adult eclosion did not vary among populations or maternal lines of the host plant, and offspring performance was not related to P. xylostella oviposition preference . The results indicate that A. thaliana populations may respond to selection for increased resistance to P. xylostella , and suggest that trichome production contributes to resistance against this specialist herbivore.     

<Emphasis Type="Italic">Vespula germanica</Emphasis> Foragers Do Not Scent-Mark Carbohydrate Food Sites

We tested whether foragers of Vespula germanica leave a chemical attractant at a carbohydrate food site. Foragers were allowed to make 50 or 100 visits to a feeder filled with a rich, unscented fructose solution. We then gave them a choice between this and an identical but unvisited feeder. There was no difference between the number of individuals that chose the visited feeder and those that chose the unvisited one. Instead, wasps showed a tendency to choose a feeder based on its relative position on the feeding stand. We conclude that foragers of V. germanica do not scent-mark food sites. Instead, in the absence of food odor, they rely on local visual cues to relocate carbohydrate food sources.     

Antagonistic effects of floral scent in an insect–plant interaction

In southwestern USA, the jimsonweed Datura wrightii and the nocturnal moth Manduca sexta form a pollinator–plant and herbivore–plant association. Because the floral scent is probably important in mediating this interaction, we investigated the floral volatiles that might attract M. sexta for feeding and oviposition. We found that flower volatiles increase oviposition and include small amounts of both enantiomers of linalool, a common component of the scent of hawkmoth-pollinated flowers. Because (+)-linalool is processed in a female-specific glomerulus in the primary olfactory centre of M. sexta, we hypothesized that the enantiomers of linalool differentially modulate feeding and oviposition. Using a synthetic mixture that mimics the D. wrightii floral scent, we found that the presence of linalool was not necessary to evoke feeding and that mixtures containing (+)- and/or (−)-linalool were equally effective in mediating this behaviour. By contrast, females oviposited more on plants emitting (+)-linalool (alone or in mixtures) over control plants, while plants emitting (−)-linalool (alone or in mixtures) were less preferred than control plants. Together with our previous investigations, these results show that linalool has differential effects in feeding and oviposition through two neural pathways: one that is sexually isomorphic and non-enantioselective, and another that is female-specific and enantioselective.     

VOLATILE CONSTITUENTS OF CHAROPHYTES AS OVIPOSITION DETERRENTS OF CULEX PIPIENS PALLENS (DIPTERA: CULICIDAE)1

Four populations of charophytes (including three species), Chara inconnexa T. F. Allen (populations 1 and 2), C. vulgaris L., and Nitellopsis obtusa (Desv.) J. Groves, were studied for effective chemicals as oviposition deterrents of Culex pipiens pallens. The charophyte volatile organic compounds (VOCs) were retained in Tenax GR, subsequently desorbed using a thermal desorption cold trap injector (TCT), and analyzed by gas chromatography/mass spectrometry (GC/MS) to elucidate that charophytes have repellent properties. C. inconnexa (1) and C. inconnexa (2) exhibited strong repellent activities, and C. vulgaris showed some repellent activity against C. pipiens pallens with terpenes and benzothiazole playing major roles, while N. obtusa lacked those compounds and did not have an effect. These results suggest that charophytes have potential application as pesticides, but there are interspecific differences. In addition, benzene hydrocarbons were among the volatiles in Chara but not in N. obtusa, implying that some charophytes could be used to absorb these compounds.     

On the respiratory mechanism during underwater oviposition in a damselfly Calopteryx cornelia Selys

Calopteryx cornelia females oviposit almost exclusively underwater in forest streams. Field observation showed that the duration of uninterrupted submerged oviposition ranged between 20 and 120 min and the number of eggs laid was linearly related to the time spent underwater. By holding a damselfly under water in a small jar, we measured the maximum 'submergence potential', which was defined as the time elapsed between placing the insect underwater and asphyxiation. A series of experiments showed that there was no gender difference in the submergence potential. This was about 120 min if a damselfly was allowed to change its position while under water. The submergence potential was shorter if the damselflies were kept motionless, if air bubbles trapped on the wing surfaces were removed by coating with Vaseline or if the water was hypoxic. By contrast, submergence potential was longer if a part of the wings were kept above the water surface, or if the water was agitated using a magnetic stirrer. These results suggest that ovipositing C. cornelia females depend for oxygen on the physical-gill action of the thin air layer trapped on the body and wing surfaces. Respiration capacity under water is not likely to be a limiting factor for ovipositing females during the production of a single clutch.     

The Predatory Mite Phytoseiulus persimilis Adjusts Patch-leaving to Own and Progeny Prey Needs

Integration of optimal foraging and optimal oviposition theories suggests that predator females should adjust patch leaving to own and progeny prey needs to maximize current and future reproductive success. We tested this hypothesis in the predatory mite Phytoseiulus persimilis and its patchily distributed prey, the two-spotted spider mite Tetranychus urticae. In three separate experiments we assessed (1) the minimum number of prey needed to complete juvenile development, (2) the minimum number of prey needed to produce an egg, and (3) the ratio between eggs laid and spider mites left when a gravid P. persimilis female leaves a patch. Experiments (1) and (2) were the pre-requirements to assess the fitness costs associated with staying or leaving a prey patch. Immature P. persimilis needed at least 7 and on average 14±3.6 (SD) T. urticae eggs to reach adulthood. Gravid females needed at least 5 and on average 8.5±3.1 (SD) T. urticae eggs to produce an egg. Most females left the initial patch before spider mite extinction, leaving prey for progeny to develop to adulthood. Females placed in a low density patch left 5.6±6.1 (SD) eggs per egg laid, whereas those placed in a high density patch left 15.8±13.7 (SD) eggs per egg laid. The three experiments in concert suggest that gravid P. persimilis females are able to balance the trade off between optimal foraging and optimal oviposition and adjust patch-leaving to own and progeny prey needs.     

Preimaginal conditioning does not affect oviposition preference in the diamondback moth

Abstract.  1. In holometabolous insects, learning has been demonstrated in both larval and adult stages. Whether learning can be retained through metamorphosis from larva via pupa to adult has long been a subject of debate. The present study is designed to distinguish between preimaginal and imaginal conditioning in the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae) using oviposition preference tests on females exposed to various types of learning experiences during immature and adult stages.
2. Cohorts of test insects were reared from egg to pupa on an artificial diet, or on one of two host plants, Chinese cabbage, Brassica campestris L. ssp. pekinensis , and common cabbage, Brassica oleracea L. var. capitata . The ensuing females reared on the three kinds of food showed similar oviposition preference between the two plants. A brief experience of the less preferred host, common cabbage, by adults slightly increased their preference for this plant.
3. Cohorts of test insects were reared from egg to pupa on an artificial diet with or without the addition of a neem-based oviposition deterrent (Neemix^® 4.5). Larval feeding experience did not alter oviposition response to the deterrent. However, emergence conditioning and early adult learning, achieved through experience of a residue of the deterrent carried over from the larval food on pupal cuticle and cocoons, altered oviposition preference significantly.
4. The combined results revealed no evidence of preimaginal conditioning in this insect but a strong effect of emergence conditioning and early adult learning on oviposition preference.     

A fecundity cost of (walking) mobility in an insect

Evolutionary theory predicts trade‐offs between fecundity and mobility, but there is substantial lack of empirical evidence if and how basic mobility relates to fitness costs. In a field experiment, we investigated fecundity costs of mobility in a non‐migratory, wing‐monomorphic grasshopper, Stenobothrus lineatus, and at the same time tested for possible effects of reproductive state (egg‐load) on the mobility. For 10 days, body weight and activity radius of 60 females were recorded daily and oviposition events were inferred from abrupt weight losses. We found a strong and significant relationship between the individual mobility and the time between egg pods laid (interpod period). Individual egg‐laying was reduced by a rate of 0.36 eggs per day with each meter increase in mean daily activity radius. The trade‐off was not biased by the size of the females, that is, constitution did not positively influence both offspring number and mobility. Egg‐load had no significant influence on the individual distances travelled. We could demonstrate that mobility – as induced and selected for by foraging, thermoregulation, predator escape, shelter seeking, and reproduction – can be directly paid off by fecundity. This direct consequence of mobility on individual fitness was detected for the first time in a walking insect.