Microclimates

The importance of microclimate in insect physiological ecology was reviewed by Willmer (1982) and is receiving increasing attention. Soil and underground environments provide refuge from the temperature extremes which occur at the surface (see Chapter 6), while vegetation ensures that terrestrial habitats are varied and complex for small animals like insects. Transpiration accentuates the microclimate around a leaf, and lower leaf surfaces develop a boundary layer of relatively still, cool, and humid air. Both the steepness of the gradients and the rapid changes possible are emphasized by Willmer (1986), who discusses the effects of factors such as leaf size, shape, thickness, reflectance (includes colour and hairiness), and position on the plant on adjacent temperatures. Humidities within the leaf boundary layer have been less easy to measure, and Gaede (1992) used a model developed by Ferro and Southwick (1984), verified by measurements with hygroscopic KCl crystals, to estimate water vapour activities in the microhabitat of a predatory mite, Phytoseiulus persimilis: av at the leaf surface frequently exceeded 0.90, providing opportunities for active vapour absorption to replace water deficits. The undersur-face of leaves is also a favoured oviposition site for herbivorous insects which live permanently on their food plants and fully exploit the heterogeneity of these environments in time and space.

Plant microclimates affect the physiology of insects, hence their behaviour, and translate into effects on ecology and distribution. This is nicely illustrated by the study of Willmer et al. (1996) on the distribution patterns of raspberry beetles Byturus tomentosus (Byturidae) on their host plant in Scotland. Plant chemistry is much less important than microclimatic effects in determining seasonal and diurnal distribution patterns of raspberry beetles. On 'dry' days with av of 0.5-0.7, the av was 0.83-0.96 within 5 mm of the leaf undersurface, where stomata are more numerous and there is an unstirred layer of humid air (Willmer 1986). Recently eclosed beetles climb the raspberry canes from soil emergence sites, but their rate of water loss is high and they remain in the humid microclimate of new leaftips. Mature adults (5 mg) spread over the plant, preferring insolated sites which may enable their body temperature to reach the threshold of 15°C required for flight. Raspberry flowers serve as feeding, mating and oviposition sites, and the larvae feed on the developing fruit before dropping to the ground to pupate, so all stages except the adults are in highly protected microenvironments. Even so, physiological constraints are important for adult beetles and determine the level of floral infestation which impacts on growers. Microclimatic effects on soft-bodied larvae and more vulnerable adults such as aphids are likely to be profound (Willmer et al. 1996).

Sometimes insects modify the leaf microenvironment. Nymphs of eugenia psyllids Trioza eugeniae develop in pit-shaped galls, and moderate densities of nymphs increase leaf curling, although competition at high densities then cancels out potential microclimatic benefits (Luft et al. 2001). Nettle leaf rolls made by Pleuroptya ruralis larvae (Lepidoptera, Pyralidae) maintain the av above 0.95 (Willmer 1980). Gall-producing and leaf-mining insects create microenvironments within plant tissues (Connor and Taverner 1997), while many insects manufacture microenvironments from other materials. (Benefits also include protection from parasites, predators, and mechanical injury.) Spittle bug (Homoptera, Cercopidae) nymphs live in a mass of foam which protects them from enemies and desiccation. Chinese mantids Tenodera aridifolia sinensis (Mantodea, Mantidae) deposit their eggs in exposed locations, but within a specialized case which prevents desiccation of the developing embryos over a six-month period, although a favourable hygric environment is necessary at the time of hatching (Birchard 1991). The barrier function of an ootheca, puparium, or cocoon is useful for immobile eggs or pupae which gain water by metabolism only. In addition, the larval case of the clothes moth Tinea pellionella (Lepidoptera, Tineidae) passively absorbs water in high humidities (Chauvin et al. 1979), as does the hygroscopic pupal cocoon of the leek moth Acrolepiopsis assectella (Plutellidae) (Nowbahari and Thibout 1990). The benefits of manufactured microclimates—and group effects—are seen to an extreme in social insects, many of which have complex nest architectures that permit precise control of both temperature and humidity.

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