All year-round, but especially during the summer, it is important to recognize, work with and encourage beneficial insects. Beneficial insects are those that provide natural, or biological, control for insect pests that injure, stress and can even kill desirable plant species, including fruits, vegetables, grasses and ornamental woody plants. Beneficial insects include active predator species as well as parasites.
As many of these insects undergo complete metamorphosis, hatching from eggs, feeding as larvae, entering a growth period in a pupa and then emerging as adults, they appear very differently throughout their life cycles. In order to protect and encourage populations of beneficial insects, gardeners must learn and know how to identify their presence at various life stages. Gardeners should aim to avoid inhibiting beneficial insects with broad-spectrum garden pest-management techniques including applications of chemical insecticides that can be harmful. It is advantageous to instead encourage beneficial insect populations throughout the growing season, so they may effectively prevent or limit pest problems in the yard and garden.
Commonly known and generally abundant beneficial insects found in southwest Colorado yards and gardens include various species of lady beetles, green lacewings, syrphid flies (hover flies), minute pirate bugs, spiders and tachinid flies. Each of these helps to control insect populations that can be harmful to yard plants and garden crops.
The adult form of various lady beetle species is familiar to most everyone. Adult lady beetles generally have oval-shaped bodies and are bright red or orange colored, punctuated with black spots. Common species of lady beetle include the convergent lady beetle, multicolored Asian lady beetle, sevenspotted lady beetle, twospotted lady beetle and a smaller, black lady beetle known specifically for its appetite for plant-damaging spider mites.
As lady beetles appear very differently throughout their life cycle, it is key for gardeners to recognize them in the egg, larval and pupa stages in addition to their adult form. Lady beetles lay yellow-orange spindle-shaped eggs in groups of 10-50 where prey species are abundant. Upon emerging, lady beetle larvae take to preying on various soft-bodied insect species including aphids, spider mites and soft scales. While some adult lady beetles also actively prey on insects, lady beetle larvae are very active predators, so it is important to spot them and encourage them. The larvae are generally dark in color and shaped somewhat like an alligator; they are flecked with yellow orange coloring on their side. After eating their fill, the larvae enter the pupa stage of development and emerge finally as adults after about a week. Different species of lady beetle are active predators at various life stages, but all are most active when temperatures are warm. While most species are voracious eaters as larvae, some species of lady beetle are also active predators at the adult stage. Many lady beetle species are commercially available and can be introduced into the garden.
Like lady beetle species, the green lacewing also undergoes complete metamorphosis. These unique insects are recognizable as adults as they have a pale green body and large, translucent, intricately veined wings. Adult lacewings primarily feed on nectar and pollen, although some also consume small soft-bodied insects. Similarly to most lady beetle species, green lacewings in the larval stage are the most ravenous predators. These brown and white, similarly alligator-shaped larvae emerge from unique eggs individually laid on the end of long silks after a period of four to 10 days. Unlike lady beetle larvae, green lacewing larvae have visible hooked jaws — an indicator of their hunting aptitude that allows them to consume large numbers of caterpillars, beetles, aphids and other damaging insects. Gardeners can also purchase lacewing eggs and sometimes adults from commercial suppliers of biological pest controls.
Syrphid flies are another beneficial insect commonly found in yards and gardens. Although they mimic the coloration of bees and wasps, they are innocuous and the adult form feeds on pollen and nectar from flowers. Unlike bees and wasps, syrphid flies have only one set of wings, a hairless body and a unique hovering flight pattern. Again, it is the larval stage of this insect that actively consumes detrimental soft-bodied insects. These larvae are generalist eaters and, although difficult to spot, their presence can be confirmed by a telltale tar-colored smear on leaves — all that remains of insects that have fallen prey to these aggressive predators. Unlike other beneficial insects, syrphid flies actively control unfavorable insects early in the season, when cool temperatures inhibit other beneficials.
Another type of fly, the tachinid fly, also helps to control populations of detrimental insects, including caterpillars and beetles. Unlike syrphid flies, however, tachinid flies are parasitic, laying eggs on their insect hosts. Upon hatching from the eggs, small maggots enter the host, consume and eventually kill it. Various species of parasitic flies are available for purchase commercially.
In addition to the insect predators already discussed, a small beneficial insect called the minute pirate bug, as well as many species of spider, actively hunt harmful insect prey species in the garden. Minute pirate bugs are difficult to see, though they serve to control thrips, spider mites and other small insect populations. Meanwhile, active hunting spiders such as wolf spiders, crab spiders and other jumping spiders help to naturally control populations of larger insect species including aphids, beetles and caterpillars.
It is important for gardeners to recognize beneficial insects, both so they can encourage them and so they can avoid damaging their populations. Although chemical insecticides may be an effective option for controlling damaging insects, they may also harm beneficials. In order to avoid damaging populations of beneficial insects, gardeners should consider utilizing control methods that work in tandem with beneficial insects to manage unfavorable insect populations. Some management tools to consider are horticultural oils, horticultural soaps, Bt insecticide and some chemical insecticides known to injure only target insect species.
More information about management tools that can be used without injuring beneficial insect populations can be found in CSU Extension Fact Sheets Nos. 5.556, 5.569 and 5.547.
Beneficial insect populations can also be encouraged if adequate supplemental food sources are provided for adults, encouraging them to reproduce. Flowering plants that increase beneficial insect activity in the garden include alyssum, fennel, yarrow, dill, lavender and coriander, among others.
For more information about attracting beneficial insects to your yard or garden, please reference CSU Extension Fact Sheet 5.550.
Although this article reviews a number of beneficial insects common in southwest Colorado gardens, there are many more species of beneficial insects found in the area. For more information, photographs of and resources about beneficial insects in the Pagosa Springs area, please visit the Colorado State University website at www.ext.colsotate.edu and download Fact Sheet No. 5.550, Beneficial Insects and Other Arthropods, written by W.S. Cranshaw.