Fid

704 sites, resting places, and, occasionally, food. Such competition may itself have a regulatory effect, as a proportion of the population will have to be satisfied with less than optimal conditions. Thus, if oviposition sites are marginally suitable, few or no progeny may result. In less than adequate overwintering sites, insects may die if the weather is severe. If insects cannot find proper resting places, their chances of discovery by predators or parasitoids are increased, as are their chances of dying because of unfavorable weather conditions. As noted earlier (Section 2.1) food is seldom limiting, though under unusual circumstances it may become so.

In addition to the general regulating mechanisms just mentioned, some insects regulate population density in more specific ways. Migration (Chapter 22, Section 5.2) is a means by which a species may reduce its population density. In such species, it is crowding that induces the necessary physiological and behavioral changes that put an insect into migratory condition. Crowding may also lead to reduction in fecundity. For example, in Schistocerca gregaria gregarious females lay fewer eggs than solitary females because (1) their ovaries contain fewer ovarioles, (2) a smaller proportion of the ovarioles produce oocytes in each ovarian cycle [as a result of (1) and (2), each egg pod contains fewer eggs], and (3) they have fewer ovarian cycles (Kennedy, 1961). Likewise, in the migratory grasshopper, Melanoplus sanguinipes, mating frequency, which is a function of population density, is inversely related to number of eggs produced and longevity (Pickford and Gillott, 1972).

A few species employ a very obvious means of reducing crowding, namely, cannibalism of either the same or a different life stage. In larval Zygoptera, for example, cannibalism of earlier instars is common under crowded conditions. For species whose larvae inhabit small and/or temporary ponds with limited food resources, cannibalism may be important in ensuring that at least a proportion of the population reaches the adult stage. Another consequence is that stragglers are eliminated, which results in greater synchrony of adult emergence (see also Chapter 22, Section 2.3). In the confused flour beetle, Tribolium con-fusum, and some other beetle species, all of whose life stages are spent in grain or its products, egg cannibalism occurs. Adults eat any eggs they find, and, therefore, the higher the adult population density, the greater the number of eggs consumed.

In some species of insects that inhabit a homogeneous environment, population density is regulated by making the environment less suitable for growth. For example, T. confusum larvae and adults "condition" the flour in which they live. As a result, a smaller proportion of the larvae survive to maturity, and the duration of the larval stage is increased. The nature of this conditioning is not known.

In some species, regulation of population density is achieved by having individuals that dominate others so that the reproductive capacity of the latter is either reduced or totally suppressed. This is seen most clearly in social Hymenoptera where one individual, the queen, dominates the other members of the colony, which are mostly female. In more primitive species, dominance is achieved initially by physical aggression, though in time the subordinates recognize the queen by scent and consequently avoid her. In highly social forms, such as the honey bee, dominance is asserted entirely through the release of pheromones.

In many species, dominance has taken on another form, namely, territoriality, the defense of a particular area. The size of the area defended (territory) may vary, but not below a minimum value, so that a maximum population density is attained. Territoriality is shown by insects in a number of orders, both primitive and advanced, and is typically associated with some aspect of reproduction. Most often males establish territories and defend them against other males, usually by chasing and fighting but occasionally by nonaggressive means such as chirping in some Orthoptera. Females enter males' territories for

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.

Get My Free Ebook


Post a comment