Editor's Note: As we have written in past issues of MBCN, there are three general groups of natural enemies of pest insects and mites: predators, parasitic insects, and insect pathogens (some would include entomogenous nematodes as a separate fourth group). Although we have highlighted specific natural enemies, or specific groups, in our monthly Know Your Friends column, we haven't yet given a broader overview of the major groups. This is the second of a series of feature articles that addresses this topic. Further information on these groups can be found in North Central Regional Extension Publication 481, Biological Control of Insects and Mites.
This month's article will focus on the beetles, which are classified as the Order Coleoptera. The beetles constitute the single largest group of animals on earth.
Approximately one quarter of all animal species known to science, and 40% of all insects, are beetles. There are almost 30,000 species known in the United States and Canada; roughly ten times that number in the world. With such great diversity, we would expect that the beetles would have a great number of different life styles. There are plant feeders, scavengers, fungus feeders, parasites of vertebrate animals, predators, and true parasitoids of other insects. Beetles occur in virtually all habitable terrestrial and freshwater environments. Within individual families of beetles there is generally some uniformity of life history. For example, the tiger beetles are all predaceous and the leaf beetles are mostly all leaf feeders. The life histories within some families are a bit more variable. For example, although the majority of members of the lady beetle family are predators, a few species (such as the pestiferous Mexican bean beetle) are plant feeders, and others are fungus feeders.
Beetles undergo complete metamorphosis, meaning that the life stages consist of egg, larva (several instars), pupa, and winged adult. The length of the life cycles varies from species to species; certain wood boring beetles and scarab beetles may take 2-5 years or more, whereas other beetles may have several generations each year. Some species have similar feeding habits as both larvae and adults, and may even live in the same environment; the aphidophagous (aphid-feeding) lady beetles are one example. In other groups, the larvae and adults do completely different things. For example, blister beetle larvae are parasitic on soil-dwelling insects but the adult beetles feed on plants.
There are approximately 40 families of beetles that have members that prey on or parasitize other insects. Some of these families are primarily aquatic and others have few if any species that are important in pest management. Two families, the lady beetles and ground beetles, are particularly important in agricultural pest management. Several other families also have members that are important in either natural control or biological control of pests of agriculture or forestry.
Lady beetles--Family Coccinellidae. Lady beetles (more frequently, but less accurately, referred to as ladybugs; also called ladybird beetles) are well known for their importance in biological control, even by the home gardener. The first great success in biological control, in the 1880's, was the importation from Australia and New Zealand of the vedalia beetle into California for control of the cottony cushion scale on citrus. Lady beetles have been prominent in the history of all three approaches to biological control. Although they have already frequented the pages of MBCN (including five Know Your Friends columns; including an overview of lady beetles), there is much more that can be said about them. For example, what about the spots? The color pattern can be diagnostic to determine the species of the lady beetle, as the number and pattern of spots can be quite consistent. The twospotted, sevenspotted, tenspotted, and thirteenspotted lady beetles are all distinct species. Some lady beetles have no spots, such as the tiny black spider mite predators of the genus Stethorus. Some, such as in the genus Chilocorus, have the spots reversed, red on a black background. Some species are more variable, such as the introduced multicolored Asian lady beetle, which may be unspotted, or have a variable number of spots.
The common species of lady beetles--those that are larger, orange or red, often with black spots--are most frequently specialized predators of aphids. One frequent question relates to the number of aphids that can be consumed by a lady beetle. Not surprisingly, this varies somewhat with species of both predator and prey, and also with environmental conditions such as temperature. But on average, fourth instar larvae (the last and largest larval stage) consume about 50 aphids per day, a number very similar to that consumed by the adult beetles. The species of lady beetles that are aphid predators will occasionally take other types of insects and mites as food. However, the adults generally lay their eggs only where there are large numbers of preferred prey (aphids) available for their offspring. Therefore, many of these lady beetle species are better at reducing large numbers of aphids that may already be causing plant damage, rather than keeping small aphid populations from getting larger. Adult lady beetles may require food in addition to prey, and many feed on flower pollen, nectar, or honeydew.
In addition to the aphid predators, there are other species that specialize on other types of prey. For example, Cryptolaemus is a mealybug predator; members of the genus Chilocorus are scale predators; and Stethorus species are predators of spider mites.
Most lady beetles overwinter in the adult stage, and many form overwintering aggregations. Adults become active in spring when new plant growth has started and aphid colonies have begun to build. Eggs are laid near the prey. Many of the aphidophagous species lay clusters of 10-50 yellow to orange, oblong eggs. Females of aphidophagous species are capable of producing 500-1000 eggs each, over a period of one to two months. The eggs hatch in a few days, and the larvae begin feeding and continue to grow through the four instars. When the last larval stage is fully grown, pupation occurs, usually on a plant part, such as foliage or stem, where the larva was when it finished feeding. The pupal stage lasts about a week. The adults mate fairly soon after they emerge, and females begin laying eggs within about one week. On average, lady beetles require about a month to complete their life cycle. In warm tropical climates, lady beetles are active year-round. In the upper Midwest, there are generally 2-3 generations per year.
Ground beetles--Family Carabidae. Ground beetles are probably the second largest group of beetles important in biological control. Ground beetles were introduced to readers in an earlier Know Your Friends column, so I will only briefly summarize, and include some additional information here. Ground beetles belong to the family Carabidae, which is one of the largest beetle families, with approximately 40,000 species worldwide. They are generally small to large (1/4 - 1 inch or more long), and brown or black in color. As the name suggests, they are usually found associated with the soil. Most are nocturnal, and during the daytime they can be found under plant debris, stones, logs, and in other hidden places. At night they come out to feed. During their foraging many will climb onto plants to feed on prey such as aphids and caterpillars. Others feed primarily in, or on the surface of soil and take a variety of types of prey that are encountered there. The larvae are also predators, but most spend their larval lives in the soil or other protected locations. The adults of many species will also feed on plant material, including pollen, fungi, and decaying plants; a few species are considered to be damaging to agricultural crops.
Many ground beetles have a single generation per year, but some complete two or three generations annually, and others may live 2-5 years. Usually it is the adult stage that overwinters. In the spring, females begin laying eggs, usually depositing them singly in the upper surface area of the soil. After the eggs hatch, the larvae grow through three instars before pupation, which occurs in cells excavated in the soil. The pupation cells are frequently several inches below the soil surface. Adults emerge in late summer or fall, but egg laying does not begin until the following spring.
The nocturnal habits of most species makes them difficult to study. Although we know that many species occur in agricultural and horticultural settings, and we assume that they are having a beneficial impact on pest populations, there are relatively few studies that have confirmed this. However, where studies have been conducted, it appears that ground beetles are indeed important predators. Here are a few examples.
Several studies have indicated that ground beetles are important predators of fully-grown codling moth larvae in apple orchards. When codling moth larvae are full-grown, they often wander on the soil surface and eventually pupate under leaf litter or within the soil. During this wandering phase, they are subject to predation by a number of species of ground beetles. One study has indicated that more ground beetles occur in those orchards with a ground cover.
Studies have also been conducted on carabids as predators of pests in corn. In one study, several ground beetles were found to be important predators of the armyworm. Laboratory studies demonstrated a willingness for nine species of beetles to prey on armyworm larvae; mean daily prey consumption rates were as high as 10 second or fourth instar larvae per beetle. In field studies, predators were removed using pitfall traps and exclusion cages. In these studies, the number of armyworm-damaged plants was 3-4 times higher in the absence of predators, and the mean damage rating was also significantly higher where predators were absent. The entire predator complex also included spiders and rove beetles.
Bembidion quadrimaculatum is a ground beetle that has been studied because of its importance as a predator of root maggot eggs and larvae. In a Michigan study, Bembidion was found to eat as many as 25 onion maggot eggs per day; when beetles encountered an egg cluster, they would stay with that cluster until all the eggs were consumed. Predation was more efficient when the eggs were on the soil surface than when they were buried 1/2 inch deep. In caged field studies, Bembidion was able to reduce onion maggot infestations by over 50%. However, the beetles are highly susceptible to common onion insecticides, and even some herbicides and fungicides, so reduced pesticide usage would be important to get full benefits from this species of predator.
Although ground beetles are considered to be generalist predators, and are frequently abundant in fields, they may not always provide the amount of control desired. For example, in one Canadian study, ground beetles, although abundant in potato fields, were not found to be particularly important at reducing aphid populations.
In the next article in this series, I will discuss the importance of some of the smaller families of predatory beetles.
- Dan Mahr, University of Wisconsin - Madison
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