April May June July August Sept

FIGURE 1 Heterogeny in grape phylloxera. In October males and females of the bisexual generation emerge, mate, and lay eggs on leaves. Their offspring form blister galls on grape leaves. In April the all-female leaf gallers lay eggs on leaves or drop to roots to lay eggs. In June all-female root gallers lay eggs on roots. Their offspring form nodular galls on grape roots. In late August a winged generation of females crawls out of the soil to lay their eggs (of the bisexual generation) under bark.

FIGURE 1 Heterogeny in grape phylloxera. In October males and females of the bisexual generation emerge, mate, and lay eggs on leaves. Their offspring form blister galls on grape leaves. In April the all-female leaf gallers lay eggs on leaves or drop to roots to lay eggs. In June all-female root gallers lay eggs on roots. Their offspring form nodular galls on grape roots. In late August a winged generation of females crawls out of the soil to lay their eggs (of the bisexual generation) under bark.

A number of gallmaking insect species exhibit heterogeny, alternating generations that include both sexes with generations including only females. Alternate generations frequently make very different galls on different parts of a plant, as does the homopteran grape phylloxera, Daktulosphaira vitifoliae, of which one generation induces galls on the leaves and stems while another generation induces galls on the roots of grapes. (Fig. 1). Some alternating generations of gallmaking insects form galls on different host plants, as is found in the cynipid gall wasp, Andricus quercuscalicis, with the unisexual generation forming galls on acorns of Quercus robur (English oak) and the bisexual generation forming galls on staminate flowers of Q cerris (Turkey oak).

Other Insects in the Gall Community

Although a plant gall would appear to offer a place for an insect to escape predators and parasites, it is also a sedentary structure where natural enemies can predictably locate insect larvae. Predators of hidden insect larvae, such as woodpeckers, regularly prey upon galls. Many parasitoids, insects which receive their nutrients from a single host insect, eventually killing the host, have adapted to the predictability of plant galls.

In addition to gallmakers and parasitoids, the gall community also contains inquilines, which are insects which live in the gall and consume the plant gall tissue. Some inquilines deliberately kill the gallmaker, probably to prevent lignification of the gall tissues. However, many galls have both gallmaker and inquiline emerging as adults unless one or both have succumbed to parasitoids. The presence of inquilines, and the chemical stimulants they secrete, can sometimes alter the shape of the final gall, especially among cynipid gall wasps.

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