Illinois Natural History Survey - University of Illinois

Complex Life-cycle Puzzles

One of the common sights in spring and summer on tree leaves are strangely shaped deformations known as galls. Gall formation by plants happens in response to stimuli caused by a variety of organisms, such as insects, mites, and fungi. Insects can cause galls simply by depositing an egg in leaf tissue, or feeding by larvae or adults may be necessary before the gall begins to develop. Many common galls on trees are caused by aphids, small insects that feed on plants by inserting their tubelike mouthparts into a plant and sucking the plant sap for food. Gall-forming aphids, as do many other gall-forming insects, have complex life cycles as shown by the following examples.

Poplars are host to a group of aphids that cause galls by their feeding (see photo showing three different galls). Visit a large cottonwood tree in the spring or early summer and you will probably be able to find two or three different types of galls. An aphid, hatched from an egg in early spring, begins

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Galls on poplar leaf petioles.

to feed at a specific point on the leaf or petiole. In response to this feeding the plant grows a hollow, variously shaped gall enclosing the aphid. The aphid matures and produces live offspring until the gall is filled with her daughters, which in turn produce granddaughters that have wings. When the gall matures, openings develop through which these winged aphids escape. These escaping winged adults fly in search of a specific host on which their offspring will be deposited. These so-called secondary or summer hosts are most often herbaceous plants upon which migrating aphids land and then move to the lower stem, where they produce nymphs that follow the root into the soil and begin a colony underground. In the autumn the underground colonies produce a generation of winged aphids that leave the summer host in search of poplars. Here they produce males and oviparous females that mate, and deposit overwintering eggs. Species in this group of aphids often choose cultivated plants on which to spend their summer, and when large aphid populations develop they become pests. Lettuce and sugar beets are two crops in which aphids from poplars are common pests.

Another insect species, Kaltenbachiella ulmifusca, forms galls on the leaves of elm trees and was originally named from specimens collected in southern Illinois. Its galls, shown in the accompanying photo, begin forming as soon as the leaves begin to grow. Studies have shown that the gall maker produces anywhere from 3 to 8 nymphs that develop wings and leave as soon as the gall develops an opening. The secondary hosts of this species are in the mint family and successful transfers have been made to bugle weed (Lycopus virginicus), a common wetland plant. In autumn, winged aphids leave bugle weed in search of elms, where they give birth to both males and oviparous females. These sexual forms do not feed but undergo several molts, getting progressively smaller, and gather in cracks in the bark where mating occurs. Each mated female produces a single egg that fills her body and she dies with the eggs still inside her.

Complex life cycles are living puzzles to those studying aphids. Most often the aphids that develop in galls look very different from their daughters that develop on roots; these daughters have lost most of their color and other diagnostic features. Experimentally proving that the root aphids found on a 

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Galls of K. ulmifusca on elm leaves.

herbaceous plant originated from a particular gall is difficult and is usually done by releasing aphids from a gall into an arena of potential host plants. These plants are kept for several months, then dug up and the roots examined for aphids. Ideally these root colonies will produce winged offspring that can be taken back to the host on which the gall developed, and the aphid hatching from the overwintering egg will produce an identical gall. Solving these puzzles is part of the continuing study of aphid biology and taxonomy at INHS.

David Voegtlin, Center for Biodiversity



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