Good and Bad Bugs
Every garden is teeming with insect life, both beneficial and harmful. In a balanced environment, insect populations are held in check by limited available food and water sources and by predators. In a cultivated environment, however, gardeners introduce a host of plants to an area that are quite attractive to harmful insects that seek out those plants for food. In order to control these pests, gardeners have typically used chemical treatments and insecticides. Unfortunately, insecticides do not target solely the harmful pests. Beneficial insects such as butterflies and honeybees are also affected by these chemicals, reducing their populations as well. Because the natural food source (i.e. the cultivated plants) remains, and beneficial, predatory insect populations have been reduced by insecticides, however, the gardener has to continually apply insecticides to keep the harmful insect population in check.
It is good news that more and more gardeners are steering clear of synthetic chemicals, choosing instead to grow their gardens organically. In so doing, they are turning to a variety of natural pest-control products such as garlic and soap sprays, Diatomaceous earth, and neem, as well as plants that deter bugs like marigolds and certain herbs. Another form of insect control is the cultivation and introduction of even more bugs into your garden.
What?! More Bugs?
Beneficial insects like ladybugs, lacewings, predatory wasps and mantis all help protect your garden by feeding upon the insects that do your plants harm. Ladybugs, for example, feed voraciously on the soft bodies of aphids. Aphids have specially-designed mouths which allow them to suck nutrients out of the stems and leaves of plants. As aphids feed, they secrete “honeydew”, which is attractive to sugar ants and even some predatory insects like hoverflies. Aphids reproduce by the hundreds and it takes little time for an entire population of aphids to destroy vegetation by injuring the plant repeatedly and stealing its nutrients. Aphid bites are also a vector for disease such as the mosaic virus, which is deadly for certain plants like cucumbers and squash. Ladybugs, however, make quick work of aphids, disposing of nearly one every minute by piercing their soft bodies and sucking them bone-dry.
Beneficial Insect Spotlight: The Syrphid Fly
The Syrphid Fly is also called the “Flower fly” because adult syrphids can often be seen feeding upon the nectar of flowers. Syrphids are also commonly referred to as hoverflies, aptly named for their ability to hover effortlessly above surfaces like a dragonfly. They also have extraordinary dexterity, allowing them to fly backwards and side-to-side with as much ease. At first glance, hoverflies resemble wasps and bees because of their colored bands but these insects are entirely harmless as they can’t bite or sting (1). They can easily be identified from bees and wasps because hoverfly have two wings (not four), which are held out to the side when at rest (bees and wasps fold their wings behind them)(2).
Syrphids are considered beneficial insects for two reasons: adult flies are important pollinators, while larvae are heavy feeders on aphids. Adult syrphids lay eggs in or near aphid populations, and larvae appear within three days. (3) Hoverfly develop in the larvae stage from seven to fourteen days, during which they consume up to thirty or more aphids per day. During its larvae stage alone, one hoverfly can consume over four hundred aphids! (4) Then they enter the pupa stage for 1-2 weeks before emerging as adults when they feed on nectar and honeydew. In addition, these insects can reproduce up to seven times a year, providing much protection from aphid populations throughout the growing season. (5)
The Toxomerus Hoverfly (Toxomerus marginatus) is a species that rivals ladybugs with its ability to eat over an aphid an hour. (6). It is similar to the American Hoverfly, but it can be identified by a centered black dot near the abdomen. (7)
If you want to attract the Syrphid family to your garden, plant cornflower, alyssum, calendula, dill, marigold, cosmos, zinnia, lavender, lemon balm, yarrow, penstemon and parsley. The flowers of these plants are important nectar sources for the adult hoverfly. (8)
Below are images of the Toxomerus Hoverfly, seen feeding on a newly emerged cornflower in our wildflower bed: