Green peach aphid (Myzus persicae Sulzer)
Potato aphid (Macrosiphum euphorbiae Thomas)
Note: Over 30 species of aphids can be found in potato fields; many are considered non-colonizing.
Pest description and crop damage Aphids are soft-bodied insects with a pair of abdominal cornicles that exude droplets of a defensive fluid. Green peach aphid and potato aphids are the two most common aphid species in potato. Potato aphid is more common in the spring and fall, while green peach aphid peaks during July and early August. Large populations of aphids can cause yield reductions through direct feeding, but their principal economic impact is due to their ability to transmit plant viruses such as Potato leaf roll virus (PLRV) and Potato virus Y (PVY).
PLRV (Polerovirus: Luteoviridae) causes tuber symptoms in some varieties called tuber net necrosis. This internal tuber discoloration is not acceptable in the marketplace. Foliar symptoms occur in younger leaves where leaf margins become necrotic, turning brown and purplish, and curl inwards. PLRV is transmitted in a persistent manner: an aphid that has acquired the virus can transmit it to uninfected plants for the rest of its life. Persistent viruses require a latent period within the aphid before they can be transmitted. The low tolerance in the marketplace for net necrosis, and the high vectoring capacity of green peach aphid, means there is a very low treatment threshold for this pest in most potato crops destined for storage and processing.
Potato virus Y (PVY) (Potyvirudae) is a serious disease in potato production in the PNW. New strains of PVY that cause internal brown lesions in tubers are becoming common. This damage is unacceptable in the marketplace. PVY is transmitted by many different aphid species in a non-persistent manner, which means that aphids probing an infected potato plant acquire the virus from an infected plant, then, when moving to a non-infected plant, inoculate the plant mechanically with the virus. There is no latent period. PVY is transmitted very quickly-in a far shorter time than most insecticides can act to prevent aphid feeding. This, and the fact that most PVY transmission is by non-colonizing aphids (those that do not live in the potato field), means that PVY control with insecticides is poor at best.
Biology and life history Winged green peach aphids arrive on potatoes in the spring from weeds and various crops where they overwintered as nymphs and adults, and from peaches and related trees where they overwintered as eggs. Potato aphids overwinter as nymphs, adults or eggs; eggs are laid on roses and other plants. Both species feed on many crops, weeds, and native plants. Throughout the growing season, aphids produce live young, all of which are female and can be either winged or wingless. In the fall, winged males are produced which fly to overwintering hosts and mate with egg-laying females produced on that host. Winged aphids also occur when their food source is overcrowded. Both species of aphids found on PNW potatoes undergo multiple overlapping generations per year.
Scouting and thresholds Fields should be checked for aphids at least weekly starting shortly after emergence. When plants are upright, the most effective scouting method is beating sheets or beating trays. Field scouts also use half gallon ice cream buckets. In some varieties, the vines become very long and lay down on the soil and become intertwined, making beating sheets or buckets difficult to use. In this case, a leaf sampling scheme might be useful. Entomologists generally report aphid numbers in terms of aphids per plant, so it is best to sample for aphids in a way that numbers can easily be converted to numbers per plant. There are no well-established treatment thresholds for aphids on potato in the PNW. Varieties that are susceptible to net necrosis when infected by PLRV require a low treatment threshold, while other varieties can tolerate much higher numbers before treatment is warranted. Seed potato fields also require a low treatment threshold.
Potatoes can harbor large numbers of generalist predators that feed on aphids. These include the hemipteran bugs Orius pirate bugs, Geocoris big-eyed bugs, and Nabis damsel bugs. Other common aphid predators include lady beetles and their larvae, lacewings, and flower fly larvae. Aphid-specific parasitoid wasps can also be common in potato fields that are not heavily treated with insecticides.
The most important source of virus in a potato field is infected potato plants. Therefore, purchasing certified seed with low or no virus infection is the best first step in controlling aphid-related damage to potatoes. Controlling other sources of virus inoculum such as cull piles and volunteer potatoes is also important.
Management-chemical control: HOME USE
- azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
- Beauvaria bassiana-Some formulations are OMRI-listed for organic use.
- insecticidal soap-Some formulations OMRI-listed for organic use.
- kaolin-As a spray to foliage and stems it acts as a repellent to some insect pests. Some formulations are OMRI-listed for organic use.
- plant essential oils (clove, garlic, rosemary etc.)-Some formulations are OMRI-listed for organic use.
- pyrethrins (often as a mix with other ingredients)-Some formulations are OMRI-listed for organic use.
- spinosad-Some formulations are OMRI-listed for organic use.
Note: Pyrethroid insecticide (Group 3 in Tables 1-2) applications make aphid management more difficult and can lead to outbreaks of spider mites.
Management-chemical control: COMMERCIAL USE
When used to control other pests, pyrethroid (Group 3 in Tables 1-2) applications make aphid management more difficult. Pyrethroids eliminate aphid predators and parasitoids which naturally control pests in the field. Data show that early applications of pyrethroids cause flare-ups of not only aphids, but other pests such as mites and thrips. When possible, it is best to use aphid insecticides with narrow spectra of control. Avoiding package mixes (Table 2) and other products that kill beneficials.