Migration

Johnson (1969, p. 8) described migration as "essentially a transference of adults of a new generation from one breeding habitat to others." Implicit in this statement is the idea that migration takes place because the current habitat is already, or will soon become, unsuitable. In many species migration is preceded by highly synchronized adult emergence and begins shortly after the molt to the adult. In other words, it occurs in adults that are sexually immature or in reproductive diapause. In a sense, therefore, migration forms part of a species' development just as do mating and oviposition. The apparent incompatibility between reproductive development and migration led to the concept of the oogenesis-flight syndrome as a general phenomenon (Johnson, 1969). However, this viewpoint no longer seems tenable, given the many demonstrations of interreproductive migration (i.e., the migration of mature females that had already laid a proportion of their eggs) (Gatehouse and Zhang, 1995).

Given its central role in development and reproduction, it is not surprising that juvenile hormone (JH) also plays a major part in the initiation of migratory behavior (McNeil etal., 1995; Dingle, 2001). In most insects JH stimulates egg development (Chapter 19,

FIGURE 22.7. Interactions leading to population outbreaks in the spruce budworm (Choristoneurafumiferana). Terms in parentheses relate to populations under non-outbreak conditions. [From P. W. Price, Insect Ecology, 2nd ed. Copyright © 1984 by John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.]

FIGURE 22.7. Interactions leading to population outbreaks in the spruce budworm (Choristoneurafumiferana). Terms in parentheses relate to populations under non-outbreak conditions. [From P. W. Price, Insect Ecology, 2nd ed. Copyright © 1984 by John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.]

Section 3.1.3). However, early experiments in which JH or its mimics were applied to insects indicated that the hormone also promotes migratory flight, which is seemingly at odds with the oogenesis-flight syndrome. Careful analyses have shown that the JH level is key: low levels affect neither activity; intermediate concentrations trigger migratory behavior; and high levels induce egg development (Rankin, 1991). Note, however, that in Lepidoptera, which produce a suite of JH homologues, it may be a specific blend of JH forms that induces migratory flight (McNeil et al., 1995). The level of JH may also affect longevity in relation to migratory ability. For example, in the monarch butterfly (see below) the southbound migratory adults live from August/September to March of the following year. During this time, they are in reproductive diapause and their JH level is low. By contrast, adults of the summer generation live only about 2 months, reproduce, and have high JH levels (Herman and Tatar, 2001). In species with migratory and non-migratory populations the effects of JH may be expressed as morphological differences (polyphenism), especially with respect to the flight system. Thus, high JH levels can be correlated with short-winged or wingless non-migratory populations with high fecundity, whereas lower levels are found in fully winged forms capable of migration but with lower fecundity. Interestingly, the differing levels of JH seem not to be the result of altered corpus allatum activity, rather changes in the level of the degrading enzyme JH-esterase (Dingle, 2001).

The form of migration varies widely among species. Some of the variables are the proportion of the population that migrates, whether migration occurs in every generation or only in certain generations, the distance traveled, and the nature of the migratory movements

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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