Stuart E. R. Bailey

Circannual and circadian rhythms in the snailHelix aspersa Müller and the photoperiodic control of annual activity and reproduction

Summary1.Individual adultHelix aspersa show a circadian rhythm of locomotor activity when kept in constant darkness, temperature, and high humidity.2.When kept for 14 months at temperatures with little annual variation from a mean of 17.4 °C, but with photoperiod varying in step with the seasons, snails moved and fed least during the short days from October to April. This is the time of hibernation in the field. Mating and egglaying ceased in September. All activities were resumed when days lengthened in January, before hibernation ends in the field.3.When transferred from the field to a photoperiod regime reversed by six months, snails re-phased their normal annual cycle of locomotor and reproductive activity to become most active in the long days.4.In an unvarying photoperiod of 12 h, snails exhibited a circannual cycle of movement, feeding, and reproductive activities which was faster than the normal annual cycle.

Slug preferences for winter wheat cultivars and common agricultural weeds

1. Slugs are a serious pest of many crops, including winter wheat, but current methods of control are unreliable. As part of research study into the possibility that alternative food sources may reduce slug damage to winter wheat crops, the relative palatabilities to field slugs (Deroceras reticulatum) of different wheat cultivars and common agricultural weeds were examined. 2. Individual slugs were given one of 12 winter wheat cultivars for 72h, either as seeds, germinating seeds or young leaves. Individual slugs differed considerably in the amounts of wheat eaten, but there was no clear correlation between slug weight and amount consumed. The slugs did not exhibit any preferences for cultivars at any stage of development. 3. When individual slugs were offered a choice of the leaves of one of 12 common weeds together with wheat leaves, they manifested a hierarchy of preferences for the weeds, some of which were preferred to the wheat leaves. There was some evidence to suggest that when slugs were exposed to less preferred weeds, they consumed less overall, but they ate more wheat than slugs given palatable weed species. 4. The results indicate that the choice of wheat cultivar would not affect levels of slug damage. Weeds could potentially act as a readily available source of alternative food for slugs in the field as part of an integrated pest management programme for slugs. However, because weeds vary considerably in their palatability to slugs, the degree of protection afforded to a wheat crop would depend on the palatability of the weed species present.

The potential for common weeds to reduce slug damage to winter wheat: laboratory and field studies

1. Slugs are serious pests of winter wheat crops in temperate climates, but current methods of chemical control are often unreliable. This paper investigates the potential for common agricultural broad-leaved weeds to act as an alternative food source for slugs, thereby reducing damage to the crop, as part of an integrated approach to pest slug management in wheat crops. 2. An experiment carried out in the field examined the relative effectiveness of metaldehyde pellets and three weed species in reducing damage to wheat seeds and seedlings. Treatments were carried out in open-topped arenas, each containing eight adult field slugs Deroceras reticulatum. The presence of weeds that were palatable to slugs did limit damage to the crop but, over a 72-h period, metaldehyde provided the most effective level of control. 3. A laboratory experiment was carried out to study the feeding behaviour of the slugs in more detail. A single slug was placed in an arena containing food items attached to electronic probes that could detect bites by a slug. The presence of dandelion Taraxacum officinale leaves, a palatable species, reduced the number of wheat seeds damaged, but chickweed Stellaria media leaves, which are less palatable to slugs, had no effect. Most slugs ate the first food item encountered. When dandelion was eaten first, significantly fewer wheat seeds were damaged, and slugs subsequently took fewer bites on seeds than when either a seed or chickweed was eaten first. Slugs were more likely to ignore wheat seeds after a meal on dandelion. 4. Metaldehyde pellets tend to degrade a few days after application. It is suggested that weeds could provide an on-going degree of protection to the crop after the pellets have degraded and until the wheat plants have developed beyond the vulnerable stages. However, the importance of the palatability of the weeds to slugs, and a high weed density to ensure an early encounter with a weed plant during a foraging session, are highlighted by the laboratory study.

Inverse temperature acclimation of heart rate in hibernating land snails.

SummaryThe heart rates of quiescent land snailsHelix lucorum andH. aspersa were recorded by impedance pneumography over several days. When snails acclimated to warm, humid, long days were transferred in late autumn to cool, dry, short days, in order to permit hibernation inverse rotational acclimation occurred, so that heart rates at low temperatures were lowered. However, temperature dependence increased so that heart rates at higher temperatures showed less difference. When hibernatingH. lucorum were brought into warm conditions and allowed to emerge from hibernation, their heart rates at low temperatures were raised. WarmacclimatedH. lucorum showed lower rates and higher temperature dependence before hibernation than after emergence: this may assist their entry into hibernation.H. lucorum showed a higher temperature dependence thanH. aspersa whether warm- or cold-acclimated: this may reflect the lower summer temperatures experienced by this population ofH. lucorum and the obligate nature of their winter dormancy.

Electrophysiological responses to metaldehyde in neurones of the feeding circuitry of the snail Lymnaea stagnalis

Abstract The molluscicide, metaldehyde, induced bursting activity and paroxysmal depolarizing shifts in identified motoneurons in the feeding system of Lymnaea stagnalis . This was accompanied by a reduction in after-spike hyperpolarization and action potential broadening. The cerebral giant cells, a pair of interneurons in the feeding circuitry, also showed increased firing activity and marked spike broadening in response to metaldehyde; action potential duration increased from 10 to 20 msec to up to 800 msec. Rhythmic feeding motor output, generated in the isolated central nervous system, was disrupted following application of metaldehyde; activity in central pattern-generating interneurons and in motoneurons appeared to become uncoupled. The effects of metaldehyde on neurons in the feeding system may partly explain the suppression of feeding behavior seen in whole animals fed on metaldehyde baits.