René L. Veenendaal

Trade-off between diapause and other life-history traits in the spider mite Tetranychus urticae

1. Diapause is generally believed to entail costs that manifest themselves as decreased survival, rate of development, and/or reproduction after diapause completion. The present study is a first step in investigating such diapause costs in the spider mite Tetranychus urticae.

Photoperiodic induction of diapause in the large white butterfly, Pieris brassicae: Evidence for hourglass time measurement

Abstract Diapause incidence in pupae of the large white butterfly, Pieris brassicae , showed an increase from 0 to 100% when the larvae experienced an increasing number of long-night cycles during their sensitive period, while being kept in continuous darkness during the remainder of their development. About 5 long nights sufficed to induce diapause in 50% of the insects; 7 long-night cycles were required to raise diapause incidence to 100%. When the summation of cycles with nightlengths of 16 and 40 h was compared, the inductive “strength” of nights of 16 and 40 h long appeared to be about the same. This means that nights of 40 h long are not measured twice by the photoperiodic clock, which would have been expected if the clock were a circadian oscillator or a complex of circadian oscillators. These experiments show that the photoperiodic clock in P. brassicae is not a self-sustained oscillator, but more likely an hourglass. In resonance experiments with a constant photophase of 8 h and a variable dark phase a circadian influence on photoperiodic induction was found at 22.5°C, but not at 19°C. Measurement of the critical nightlength was equally sharp in resonance experiments at both temperatures, however. The above results are best explained by the “hourglass timer-oscillator counter model” or “resonance model” for photoperiodic induction of diapause, originally developed for the spider mite Tetranychus urticae .

The pharmacology of Microbracon venom

1. Wasps of the genus Microbracon generally paralyse larvae of Lepidoptera. The wasp larva is exophagous on the paralysed host. The venom acts slowly and causes a flaccid paralysis. The venom of M. hebetor is extremely active in Lepidoptera, less active in honeybee workers, much less active in locusts and mealworms and probably inactive in non-insects. 2. In Lepidoptera and in locusts the venom presynaptically blocks the excitatory, but not the inhibitory, neuromuscular transmission. M. hebetor venom contains two high mol. wt toxins. A-MTX and B-MTX, with essentially the same action on neuromuscular transmission as the crude venom. Differences in dose-response curves of the two toxins, as well as differences in sensitivity to the two toxins between insect species are described. In wax moth larvae both A-MTX and B-MTX cause a slow and transient paralysis with a maximal effect after about 20 hr. The rate of recovery is dose-dependently decreased by the toxins.

Paralysis caused by the venom of the wasp, Microbracon gelechiae.

Abstract In Galleria mellonella a good correspondence between the speed of paralysis caused by the venom of the wasp, Microbracon gelechiae , and of the decay of neurally evoked excitatory muscle potentials is found. Inhibitory postsynaptic potentials, heart beat, and movement of the gut are unaffected. The venom affects the frequency of spontaneous miniature excitatory postsynaptic potentials. This indicates a presynaptic effect. Electron micrographs of the synaptic region in muscle fibres do not show any difference between normal and paralysed animals. The active principle of the venom probably has a molecular weight of about 60,000. The above facts show that the venom of M. gelechiae is nearly identical to the venom of M. hebetor . However, the activity spectrums of both venoms differ quantitatively.

Sleeping with the enemy—predator-induced diapause in a mite

Diapause in arthropods is a physiological state of dormancy that is generally thought to promote survival during harsh seasons and dispersal, but it may also serve to avoid predation in space and time. Here, we show that predation-related odours induce diapause in female adult spider mites. We argue that this response allows them to move into an area where they are free of enemies, yet forced to survive without food. Spider mites are specialised leaf feeders, but—in late summer—they experience severe predation on leaves. Hence, they face a dilemma: to stay on the leaf and risk being eaten or to move away from the leaf and risk death from starvation and thirst. Female two-spotted spider mites solve this dilemma by dramatically changing their physiology when exposed to predation-associated cues. This allows them to disperse away from leaves and to survive in winter refuges in the bark of trees or in the soil. We conclude that the mere presence of predation-associated cues causes some herbivorous mites to seek refuge, thereby retarding the growth rate of the population as a whole: a trait-mediated indirect effect that may have consequences for the stability of predator–prey systems and for ecosystem structure.

Diapause incidence in the two-spotted spider mite increases due to predator presence, not due to selective predation.

We recently reported evidence for increased diapause incidence in the spider mite Tetranychus urticae in presence of the predatory mite Typhlodromus pyri. This effect may arise from (1) selective predation on non-diapause spider mites, (2) predator-induced diapause in spider mites, or (3) both. Using a different strain of T. urticae, we first recovered increased diapause incidence in association with predators. Then, we tested for selective feeding in two-choice experiments with equal numbers of non-diapause and diapause spider mites. We found that the predatory mite had a significant preference for the latter. This indicates that increased diapause incidence in association with predatory mites is not due to selective predation. Therefore, predator-mediated physiological induction of diapause seems a more likely explanation. The cues leading to induction appear to relate to the predators, not their effects, since predation simulated by spider-mite removal or puncturing did not significantly affect diapause incidence. Why spider mites benefit from this response, remains an open question.

Predation risk affects diapause induction in the spider mite Tetranychus urticae

Whenever diapause induction triggers movement into another microhabitat or the development of protective morphological structures, this may also alter predation risk. If the risk of being eaten is lower in the diapause phase, then there may be selection favouring diapause induction in response to predators or their cues. In this article, we studied the effect of the predatory mite Typhlodromus pyri on diapause induction in the spider mite Tetranychus urticae. We used a Greek strain because under long-night photoperiods and low temperature only part of the population enters diapause, thereby leaving room for the impact of another factor. In spider mite groups under predation, the percentage diapause induction increased whenever night-lengths were such that diapause was induced (13–16 h of night). Given this diapause induction in response to predation risk, the question arises whether entering diapause helps spider mites to escape from predation and contribute more offspring to the spring generation next year.

Effects of photoperiod, temperature, food and relative humidity on the induction of diapause in the predatory miteAmblyseius potentillae

Aspects of the induction of diapause were studied in a Dutch strain of the phytoseiid miteAmblyseius potentillae. The photoperiodic response curve was of the long-day type, with a sharply defined critical daylength of 14.5 h. Critical daylength varied only little at temperatures between 15.0 and 22.5°C.All post-embryonic and possibly even late-embryonic stages of development were found to be sensitive to photoperiod; sensitivity appeared to be maximal during the protonymphal stage.It is shown that β-carotene is necessary for some early step in the physiological mechanism of photoperiodic induction, and not (or not exclusively) for the expression of the diapause response.Two points of sensitivity to light could be demonstrated in the nights ofl∶d 13∶11 andl∶d 12∶12 long-night regimes, using 1-h night interruptions. These results are similar to those obtained in lightbreak experiments with spider mites and insects. However, no effect was found with light interruptions applied during the dark phase of anl∶d 10∶14 long-night regime.In resonance experiments with a constant photophase (12 h) and a variable scotophase, a weak rhythmic response was found at 22.5°C; at 19.0°C this effect was completely absent.The relative humidity experienced by the mites during diapause induction as well as during diapause development influenced the rate of diapause completion under long days (l∶d 16∶8). Diapause duration appeared to be shortest when the mites experienced low relative humidity (35±5%) during diapause induction and high relative humidity (75±5%) during diapause termination, and longest under the reverse conditions.

Effects of photoperiod and temperature on the termination of diapause in the univoltine seed wasp Eurytoma plotnikovi

Abstract Mummified pistachios containing fully grown diapause larvae of Eurytoma plotnikovi Nikolskaya (Hym., Eurytomidae) were collected in early August and late September in coastal northern Greece and subjected to various photoperiod and temperature treatments, then maintained at 19 or 26°C and a long‐day (LD 16:8 h), a changing, or a short‐day (LD 10:14 h) photoperiod until pupation.