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668 production of summer migrants, whereas the shorter days of late summer and early fall induce development of sexuparae and oviparae. For some species there is a critical day length for induction of oviparous forms. In Megoura viciae (Figure 21.15), for example, which does not alternate host plants (i.e., it has no migrant form, and the oviparae are produced directly from fundatrigeniae), the critical day length is 14 hours 55 minutes at 15°C. At greater day lengths continuous production of viviparous, parthenogenetic females occurs; when the day length is below this critical value oviparae are produced.

In some species production of males also is induced by short days, though temperature and maternal age exert a strong influence. For example, in the pea aphid Acyrthosiphon pisum male offspring are not produced by young females or by females reared under long-day conditions. Old females reared at short day lengths and temperatures from 13°C to 20°C produce a large proportion of males. Outside this temperature range the proportion of males declines.

3.2.2. Reproductive Ability and Capacity

The effects of photoperiod on reproductive processes are almost all indirect, that is, result from other photoperiodically induced phenomena, especially adult diapause (see below). By its effect on the nature of development, as in aphids, photoperiod may indirectly modify the fecundity of a species. Beck (1980) noted one example of an apparently direct effect of photoperiod on fecundity. In Plutella xylostella, the diamondback moth, egg production in individuals reared under long-day photoperiods averaged 74 eggs/moth, whereas egg production under short-day conditions was only half this value.

3.2.3. Diapause

Beck (1980, p. 119) described diapause as "agenetically determined state of suppressed development, the expression of which may be controlled by environmental factors." It is a physiological state in which insects can survive cyclic, usually long, periods of adverse conditions, unsuited to growth and reproduction, including high summer or low winter temperatures, drought, and absence of food. In other words, it includes both hibernation (overwintering) and estivation (summer dormancy). Insects enter diapause usually some time in advance of the adverse conditions and terminate diapause after the conditions have ended. In other words, natural selection has favored the development of a safety margin against prematurely unseasonal conditions. Furthermore, the factor that leads to the induction of diapause (most often photoperiod) is not in itself an adverse condition. Thus, diapause differs markedly from quiescence, which is a temporary form of dormancy, usually induced directly by the arrival of adverse conditions.

Occurrence and Nature. Diapause may occur at any stage of the life history, egg, larva, pupa, or adult, though this stage is usually species-specific. Only rarely does diapause occur at more than one stage in the life history of a species. Such is typically the case in species that require 2 or more years in which to complete their development. For example, in the cockroach Ectobius lapponicus, which has a 2-year life history, the first winter is passed as a diapausing egg, the second as a quiescent second- or third-instar larva or as a diapausing fourth-instar larva.

Anticipation of the arrival of adverse conditions means that the environmental stimuli that induce diapause must exert their influence at an earlier stage in development. Thus, egg diapause is the result of stimuli that affect the parental generation. These stimuli act on

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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