Cabbage Root Aphid/Poplar Petiolegall Aphid, Vol. 5, No.2

Cabbage Root Aphid/Poplar Petiolegall Aphid

Pemphigus populitransversus

Order: Hemiptera
Family: Aphididae

Ever pulled up a turnip and noticed a white, cottony material on the roots?  Look more closely next time and you may see the tiny, pale yellow aphids that produce this material.  These are cabbage root aphids, which, in another life are also known as poplar petiolegall aphids.  Granted, they are not very photogenic, but, much like Dr. Jekyl and Mr. Hyde, these aphids lead dual lives and have an unusual life history.
 
Unlike most aphids, but as their name suggest, cabbage root aphids live on roots and not on leaves.  Even though they live in the soil, they do not burrow through soil to reach the roots; they simply crawl through cracks and crevices in the soil.  Like many other aphids, these are only females, which reproduce parthenogenetically (without mating) throughout the winter, increasing in numbers with each generation.  Then in the spring things really get strange.
 
In early spring the nymphs develop into dark-colored winged females, which move to the soil surface and fly away in search of a cottonwood tree, Populus deltoides.  If they succeed in finding a cottonwood tree, these migrant females seek shelter in bark crevices, where they give birth to male and female aphids, which then mate and produce a single egg.  The aphids that hatch from these eggs then crawl to the twig tips, where they begin feeding on emerging leaves and cause the formation of distinctively-shaped galls on the leaf petioles.  These swollen, hollow galls, which are about ¾ to 1 inch in diameter, provide food and shelter for the nymphs that are produced inside by the founding female.  Eventually, the aphids inside the galls develop wings and the galls develop a horizontal slit, which opens to allow the aphids to escape. These winged aphids fly away in late summer and early fall to search for a cabbage, turnip or some other brassica host, including several species of brassica weeds.  If they are successful in finding a suitable host, they crawl through cracks in the soil to reach the roots and begin producing the fall/winter generations.  Now that is a complicated life history!
 
Control: Although heavy infestations can cause stunted growth of cabbage and other brassica crops, and the white cottony material is sometimes viewed as a contaminant on turnip roots, cabbage root aphids are not normally a serious problem and treatment is not usually required.  One of the best control options for commercial plantings is to delay fall planting late enough to avoid the flight of female aphids moving from cottonwood.  This insect is not a pest on spring plantings because no founding females are flying then. 
 
If you encounter cottonwood trees this summer, check them out to see if they have petiole galls.  You can even cut some of the galls open to see the aphids developing inside.