Use of Predators to Control Insect Pests in Potato
(prepared by Dr. Gary L. Hein, UNL Extension Entomologist)
The predators sometimes can be used for controlling insect pests in a potato field. Oftentimes, insect management tactics are going to change depending on a grower's market, seed versus table versus chipping potatoes. When looking at predatory insects, there may be methods that fit into any one of these market types, but certainly table stock is where these methods are going to be most applicable. Certainly, they are less applicable to seed production because, with respect to viruses. It's tougher to make them work since the threshold is essentially present or absent. With that in mind, the following is a general exposure to some possibilities. This issue has come partially due to the chemical resistance developed by the Colorado potato beetle (CPB).
Lady beetles, or lady bugs, are recognized as beneficial insects in most areas. There are several species of lady beetles in this region. The convergent ladybeetle is one that is seen most often. In the last few years, a recent introduction into Nebraska, is becoming more and more prevalent. C-7, or the 7-spotted lady beetle, is much bigger than the typical lady beetle usually seen here. Lady beetles will primarily feed on aphids, but they will also feed on CPB eggs and perhaps mites. Lady beetles lay their eggs on leaves. Lady beetle larvae are very voracious predators much like the lacewing larvae and will feed on aphids and other small insects very well.
The green lacewing is a very common predator, primarily feeding on aphids but it will feed on other small larvae. The green lacewing lays its egg on stalks from which the eggs are suspended off the leaf. Once the larvae hatches from that egg, it crawls down. The purpose of the stalks is to protect the egg from its brothers and sisters. They are voracious, will eat the first thing that they contact and will eat other eggs. They have sickle-shaped mouth parts on the front of the head. They poke these into an aphid and inject digestive juices. The contents are then sucked out of the attached insect. They will take on anything smaller than themselves and perhaps some things that are larger.
The two-spotted stink bug is noted as a very good CPB larvae predator. They feed on CPB eggs and perhaps other insects.
The big eyed bugs have piercing, sucking mouth parts, that stick in and suck the contents out of its prey. They will feed on small larvae, aphids, some other fairly small insects, and CPB eggs. Adults are 3/15" long.
Nabids are very common in potato and alfalfa fields. They have piercing, sucking mouthparts. They are a little bit bigger than big-eyed bugs and will be about 3/8" long. Nabids will eat larger insects and larvae which include CPB and cutworm larvae. They also feed on CPB eggs and perhaps aphids.
The minute pirate bug is another voracious predator. Its size is about 1/8". It is probably one of the more effective predators. These insects have piercing-sucking mouth parts. They can have a painful bite and are very annoying when present in high numbers.
Spiders can be an important predator. A number of species may be found in potatoes. Spiders are general predators and will feed on just about any insect that they can catch.
The adults of Syrphid or Hoover flies will lay their eggs near aphid colonies, and the larvae will feed on the aphids. The adults have a distinctive flying pattern. They keep hovering in the air and move from spot to spot. The maggot is usually greenish. When the maggots get into an aphid colony, they can clean them up fairly rapidly. Unfortunately with this particular predator, it isn't common until aphid populations are fairly high. Colonies have to be fairly well established before there are significant numbers.
We need to be aware of the presence of these natural predators and take advantage of them when possible. In most situations, when an insect population is approaching an economic threshold level or a significant level, the best thing is not to wait for the predators to act because they are usually slow at controlling a population. A lot of times in agricultural crops, because the thresholds are relatively low, one can't wait long enough for them. But there are exceptions, there are situations where you can wait for these predators to be of some benefit. In many situations, background predator populations can keep secondary pests in check. One thing that can be done to increase the effectiveness of predators is to minimize the use of insecticides. Use insecticides only when necessary. This requires monitoring pest populations. Unnecessary applications may result in flaring secondary pests. Flaring aphids results from an insecticide application that wipes out all of the natural enemies. Even when there is 99% or greater control of aphids, there are still a number of them out there and they will return. They will return because their reproductive time is short, each individual can produce a large number of aphids, and the natural enemy complex has been completely destroyed. Aphids will come back without environmental or predatory restrictions. As a result, the aphid populations will flare, increase very dramatically.
There are a number of other factors that can be used to try to assist predators. In the future, transgenic varieties will increase the importance of predators to control pest insects. For CPB, a Bt-insecticide can be used. These insecticides will affect the early instar CPB, but Bt will not adversely affect the predators which will be retained to control aphids and other pests.