Biological control

From BugwoodWiki

Authors: Dave J. Shetlar, Ohio State University and Frank Hale, University of Tennessee

Biological control is the use of parasites, predators and pathogens (diseases) to control pests. We have to realize that in the urban landscape and nursery, there are a multitude of beneficial insects and mites that can prey on pests. In many cases, these naturally-occurring beneficials will do a good job of controlling the pests if we do not disturb the system too much. As stated above, we usually disrupt this system by overusing pesticides that kill the beneficials better than the pests. On the other hand, there are occasions where we can actually increase these biological controls. The classical way to implement biological controls is through introductions, conservation and augmentation.

1. Introductions of exotic parasites, predators or diseases are made when foreign pests become established. This is an attempt to establish some of the checks and balances found where these pests are naturally controlled. Occasionally, foreign biological controls are found that may better control native pests. Many of these organismas are commercially available.

1. Conservation is the use of other control tactics, usually pesticides, so they have the least adverse affect on predators and parasites. It can also be the providing of habitat or food needed by the biological control organisms to improve their survival. In the urban landscape or nursery, we can use targeted sprays on those specific plants where pests are getting the upper hand. We can also plant flowers that provide nectar and pollen to feed the adults of many of the parasitic insects.
2. Augmentation is usually the rearing and release of biological control agents. Many of the organismas used in this way are commercially available. Unfortunately, this technique is usually expensive and we must use those biological controls that fit into the definition of a “good” biological control.

What is meant by a “good” biological control? Not all predators, parasites and pathogens are useful in their ability to be used in pest management. Useful ones have the following characteristics:

1. High Reproductive Potential — they must be able to keep up with the high reproduction of the pests.
2. Good Mobility — they must be able to search out the pests or come into contact with the pests.
3. Host-specific — they should not be generalists that may adversely affect other, sometimes beneficial, organisms.
4. Persistent— they should have the ability to exist when pest populations become low and remain from season to season.
5. Easily Reared or Encouraged — this will allow them to be inexpensive and competitive with other controls.
6. Tolerant of Other Controls — to fit into a true IPM system, they need to be tolerant of cultural and chemical controls.

To illustrate these concepts, let us look at a praying mantis versus a lady beetle. The praying mantis has one generation per year; eats anything in sight (including each other and other beneficials); usually ignores small insects such as aphids, mites and scales; often does not survive the summer to lay another egg case; and is sensitive to any pesticide. Therefore, praying mantids do not qualify as a useful biological control. On the other hand, lady beetles have many generations per year, they only eat a narrow range of pests (usually they are aphid, mite or scale specialists), usually overwinter well and can often withstand some of the softer pesticides, especially soaps and oils. Therefore, lady beetles easily qualify as a useful biological control.

Unfortunately, we often think that we have to actively introduce predators and parasites in our urban landscapes. Because most of these animals already exist, we merely have to be able to recognize them and avoid using cover sprays of pesticides.

Predators you should learn about are:

1. Lady Beetles are commonly sold as adults and are useful control agents if properly handled. The adults need to be fed honey (resembling aphid honeydew) in a cage (to suppress a strong urge to fly away) before release in the garden. Larvae are often mistaken for pests because they look like leaf beetle larvae or some other pests (e.g., the “mealybug destroyer” lady beetle larva looks like a mealybug).
2. Green Lacewings are not to be confused with the lace bug pest. The larvae feed on aphids, scales and mites. Eggs are purchased and sprinkled where small pests are noted to be active. The larvae must search for the pests because they do not have wings.
3. Ground and Rove Beetles are active predators present in most soil/turf habitats. Both the adults and larvae feed on a wide variety of pests, but are highly intolerant of pesticides.
4. Syrphid Flies (=Hover Flies) are common yellow and black flies that have voracious larvae (maggots) that eat aphids.

Parasites are insects (often called parasitoids) with larvae that feed on the inside or outside of their host, usually killing or sterilizing it. Some common parasites you should learn about are:

1. Trichogramma Wasps (=Egg Parasite Wasps) are microscopic (usually less than 0.02 inch long) and lay their eggs in the eggs of other insects. They are usually very host-specific and generally limited to butterfly or moth (caterpillar) pests.
2. Ichneumonid and Braconid Wasps are small wasps that commonly attack caterpillars and aphids. The larvae usually emerge from the dying host and spin small-white or yellow cocoons.
3. Tachiniid Flies are generally medium-to-large flies that lay eggs on caterpillars or various leaf-feeding beetles. The eggs hatch into maggots, which feed on and eventually kill the host insect.

Pathogens can cause diseases that kill insects. They are usually bacteria, virus, fungi and protozoa. Insect pathogens are fairly ideal in that they are host-specific. They are also non-infective to vertebrates. Examples are:

1. Bacteria have been the easiest of the pathogens to use because they can often be reared “in vitro” (in artificial culture) and form spores fairly resistant to adverse environments. Examples are:
   1. Bacillus thuringiensis (Bt) - has several strains that produce toxins lethal to various insect groups (and are thus technically a chemical control). The most common types are:
      1. Bt ‘kurstaki’ — that affects only young caterpillars.
      2. Bt ‘israelensis’ — that affects aquatic fly larvae such as mosquitos and black flies.
      3. Bt ‘tenebrionis’ — that affects some leaf-feeding beetles.
   2. Bacillus popilliae (= white grub milky disease) — has one strain available that kills Japanese beetle grubs.
2. Fungi have been identified, but are difficult to use because the spores are easily dried out or need high moisture and/or water to germinate. An example is:
   1. Beauveria spp. have been identified infecting a wide variety of insects including bugs and beetles. A commercial strain, Naturalis - OTM, is available for indoor and outdoor use on a variety of ornamental plants.
3. Viruses are common pathogens of insects, but are one of the most difficult to use because they require living insects to grow. Recent development of insect tissue culture has allowed for rearing of some of the virus strains, but the only commercial product is Nuclearpolyhedrosis Virus (NPV) — for gypsy moth control under the trade name of Gypcheck™.
4. Entomopathogenic Nematodes are a group of tiny parasitic roundworms that carry a bacterium lethal to insects. Once the nematode gains entry into an insect, it regurgitates the bacterium, which paralyzes and kills the insect. The nematode then feeds on the reproducing bacteria. The most commonly mentioned species are:
   1. Steinernema carpocapsae, which has several strains good at attacking insects that live in the upper soil or on the soil surface. Biosafe™, Vector TLTM and Scanmask™ are commercial preparations.
   2. Heterorhabditis spp. are better at attacking insects that live deeper in the soil. This group can also bore through the insect cuticle.