Aphids have adapted to the changing seasons and attacks by predators, parasitoids and diseases, by local migrations between summer and winter hosts. For example, soybean aphid will feed on soybeans in the spring and summer and spend the winter on Buckthorn.During the summer in Indiana, soybean plants are plentiful for aphids to find. Predators such as lady beetles and diseases may make buckthorn an undesirable location. Soybean aphids leave the deteriorating neighborhood of the buckthorn plants for the lush fields of soybeans and live the good life during the summer. The massive areas of soybean foliage provide a good habitat for aphids to rapidly expand populations.
However, all good things must come to and end and that applies to soybean aphid as well. Soybeans are annuals and fall harvest will wreck the home for the aphids. The soon to be homeless aphids must find a suitable buckthorn plant to spend the winter. Compared to soybeans, the buckthorns are fewer and further between. Millions of aphids may leave a soybean field but only a small percentage will land on a suitable plant for overwintering. Most of the aphids will fail to find a suitable plant and die. Those few aphids that do find buckthorn will carry on.
Aphids have developed reproductive strategies that are adapted to these conditions. The winged females that fly from buckthorn to soybean can lay eggs and reproduce without mating. These “parthenogenic” females can produce over a dozen generations or one every 5-7 days. This allows aphid populations to expand many fold on the abundant summer host. In the fall, the aphids on soybean will produce winged females that will fly back to buckthorn and lay eggs on the buckthorn. Some aphids will produce males that fly to the buckthorn to mate with the daughters of the alate females. The aphids have a couple of generations on buckthorn that require males and females to mate, before the parthenogenic females depart in the spring for the soybean fields.
This lifecycle and mating behavior allows aphids to expand populations to very large sizes during the summer. The success rate for finding an overwintering plant is low so the large summer populations are necessary for the survival of the species. Mating is important for exchange and reassortment of genes. That part of the life cycle is preserved on the overwintering host. The life cycle is complicated, but it works. Aphids are very successful insects.
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