Aphids appearing in commercial vegetable fields
Conditions this growing season have favored aphids, so check your vegetable crops for this pest.
Aphids early in the season fly into vegetable fields where they settle and make aphid colonies (Photos 1-2). These insects have sucking mouthparts that allow them to take up juices from the plant. The resulting symptoms are the inward curling of leaves and distorted growth. They also leave behind on the leaves their sugary excrement on which fungus grows, turning it black. Another sign of their activity is a white dusting on the plants. Aphids molt as they grow and leave their old skins behind, which shrivel and turn into white dust (Photo 1).
Photo 1. Spirea aphids on celery. The white specs are the skins
cast by growing aphids. Photo credit: Zsofia Szendrei, MSU
Photo 2. Cabbage aphids. Photo credit: A.B. Bryant
Many aphids transmit plant viruses, and once the virus enters the plant, there is no protection against it. Aphids will “taste” many plants as the winged adults move from field to field in the spring and summer. During these short “probing events,” the plant virus is transmitted to the plant if the aphid is already carrying it. Stylet oils can reduce the incidence of aphid-spread viruses, but they won’t provide complete protection when aphids are abundant and many of them carry viruses.
Aphids have many biological control agents (Photos 3-7) such as lady beetles, lacewings, syrphid flies, parasitic wasps, etc., that naturally occur in Michigan and if these are encouraged to stay in the field and protected from broad-spectrum insecticides, they can keep aphid populations low for a long time. These biological controls can delay aphid breakouts and provide continued aphid suppression after a selective insecticide application.
Photo 3. Lady beetle egg mass on celery leaf. Photo credit: Zsofia Szendrei, MSU
Photo 4. Aphid mummy (black circle). After a parasitic wasp lays an egg (or eggs) inside the aphid, it turns into this tan, mummified aphid shell. Meanwhile, the parasitoid larva is developing inside, consuming the aphid. Photo credit: Zsofia Szendrei, MSU
Photo 5. Parasitoid wasps lay eggs into many vegetable insect
pests where their offspring develop while killing the pest.
Photo credit: A.B. Bryant
Photo 6. Lady beetles eat aphids. The lady beetle on the bottom left is the multi-colored Asian lady beetle, and the one on the upper right is the seven-spotted lady beetle. Both are commonly found in Michigan vegetable fields. Photo credit: A.B. Bryant
Photo 7. Lacewing larvae are good aphid predators and are
commonly found in Michigan vegetable fields. Photo credit: A.B. Bryant
Some selective insecticides that kill only the aphids, but will not hurt biological control agents, are Fulfill and Beleaf, which are registered for use on many vegetable crops. Both of these products work by paralyzing the aphid’s mouthparts, therefore aphids don’t die right after an application is made – they starve to death over a period of days. See the Michigan State University Extension E312 bulletin “Insect, Disease and Nematode Control for Commercial Vegetables” for insecticide choices.
Always read, understand and follow the label directions. Mention or exclusion of specific products does not represent an endorsement or condemnation of any product by Michigan State University.
Dr. Szendrei’s work is funded in part by MSU‘s AgBioResearch.