Bean Aphid

Bean aphid is present in the Imperial Valley, but it is not a common pest of sugarbeets there.

Bean aphid is a dark olive green to black aphid. It is most easily confused with the cowpea aphid. Bean aphid has a dull, matte appearance while cowpea aphid is shiny. The cauda of the bean aphid has more hairs than that of the cowpea aphid and thus appears bushy. Except for the presence of wings, the winged form of the bean aphid is similar in appearance to the wingless one.

Injury from bean aphid occurs from virus transmission and from direct feeding on sugarbeet leaves. Bean aphids transmit Beet yellows virus, Beet western yellows virus, and Beet mosaic virus. Although bean aphids do not vector viruses as efficiently as green peach aphid, generally bean aphid occurs at higher densities, which tends to negate the differences in virus transmission efficiency.

Infestations of bean aphid generally begin on young leaves in the center of the crown. As the number of individuals increases, older leaves are colonized. The aphid is found mainly on the underside of leaves and only rarely on upper surfaces. Infested leaves curl under and inward and become severely distorted. The leaf margin and eventually the entire leaf become necrotic. Heavy populations may kill foliage, even in large mature plants. Bean aphid produces large amounts of honeydew, and infested leaves are usually covered with sooty mold. If the aphids are killed, either by insecticides or natural enemies, leaves resprout from the crown and new foliage begins growing.

The principal way of reducing virus transmission by the bean aphid is adherence to the beet-free restrictions and planting dates established by grower and processor agreement. These planting date restrictions are established to avoid planting during major aphid flights and to prevent the virus source (i.e., infected sugarbeet plants), from bridging the time between old and new plantings. Planting date restrictions and beet-free periods vary considerably from location to location; contact your farm advisor, processor, or the California Beet Growers Association for the latest restrictions in your area. Strict adherence to these restrictions is absolutely necessary in order to reduce the amount of virus.

A second, and equally important, factor in reducing virus spread is good field sanitation. Infected keeper beets that produce new vegetative growth after harvest act as sources of virus inoculum for new plantings. Following harvest, thoroughly disc fields and chop remaining beets into small pieces. Watch fields closely and redisc if new growth appears. Take special care where keeper beets resprout in other crops, such as cereals or alfalfa. In such cases, herbicides may be required to control the new growth in order to reduce virus inoculum. These measures help control the incidence and spread of viruses transmitted by bean aphid but do little in controlling the aphid itself.

Bean aphids are attacked by a variety of common aphid predators and parasites. Lady beetles, green lacewing larvae, and syrphid fly larvae are frequently found associated with bean aphid colonies. Note the presence of these predators and their impact on aphid populations during routine monitoring. If these predators are present and aphid numbers are declining, delay chemical intervention.

Bean aphid is attacked by a very prolific parasitic wasp, Lysiphlebus testaceipes. Parasitized aphids become bloated and their bodies turn tan in color. This parasite can control extensive populations of bean aphid in a few days, and if parasite activity is evident, chemical treatments should be delayed or canceled. Bean aphid is also attacked by a fungus disease that leaves the aphid body flattened so it appears to be glued to the leaf. Like the parasite, this fungus disease may control the aphid population within a matter of days. This disease is most prevalent in spring during rainy periods. In most cases, a combination of these biological control agents work in concert to reduce aphid numbers.

Infestations of black bean aphid and infection with beet yellow virus are most damaging to the plant near the time of seedling emergence. For April to May plantings, the first 6 to 8 weeks after emergence is the most critical time to protect sugarbeet from black bean aphid and beet yellow virus. Monitor sugarbeet plants and evaluate the aphid population and the extent of direct feeding damage to plants. Determine damage levels caused by direct feeding from the following table:

The following treatment guidelines are provisional, but helpful in making treatment decisions. These guidelines are based on: (1) plant age (from seedling emergence) at the time of infestation; (2) severity of infestation and associated injury; and (3) the length of time the plants remain infested. Based on the above damaged levels and age of the plant, treatment guidelines are as follows: