Who Am I? A Quick Guide to the Unseen Arthropod Heroes in Your Crops

Believe it or not, in a well-managed field or hort crop, you will often find more species of beneficial insects and mites than pests. Natural enemies of plant pests can have a significant impact on the population levels of those on which they dine. Insect and mite "heroes" may be generalist feeders - consuming a wide variety of prey items - or they may be quite specialized and picky in their choice of acceptable prey.

Excluding beneficial pathogens and entomopathogenic nematodes, natural enemies of insects and mites are typically divided into to main groups: predators and parasites. Predators overcome, capture and consume their prey items. They are often (but not always) larger than their prey. A parasitoid lives in or on the body of a single host individual, eventually killing that individual over the course of its own development. Most parasitic insects are wasps or flies. Aphids, leafminers, scales and caterpillars are often attacked by parasitic insects.

We are all familiar with common and highly visible natural enemies including the showy ladybird beetles that we see munching away on aphids, mites, scales and mealybugs. But there are others that you may not be aware of. Here's a quick list of some common beneficials you might see in fruit, vegetable, and field crops, along with a few comments to help get you started on where to find them.

Predators

Ground beetles: Very small to large (most 1 to 30 mm) dark-coloured ground dwellers often found hiding during the day under stones and debris, with legs designed to run. Many are unable to fly. Feed on soft-bodied insects including caterpillars and the larvae of other beetles, as well as snails and earthworms.

Rove beetles: Small slender beetles, less than 2 cm, with short wing covers or elytra that do not cover the entire length of the abdomen. Rove beetles will often raise their abdomen and will run away when disturbed. Both larvae and adults will feed on small soft-bodied larvae, pupae and maggots, as well as aphids and mites. They are usually found on the soil surface or under plant debris, but adults are good fliers.(Photo by: W.A. Attwater – PDC, University of Guelph)

Ladybird beetles: Rounded or oval in shape (1-10 mm) and often brightly coloured, many species spots or bands on the elytra. The larvae are elongate and overed with tubercles or spines ("alligator-like"). Some are very tiny, such as Stethorus spp., small (0.1 mm) black beetles which feed almost exclusively on mites.

Syrphids or Hoverflies: Small to large flies (to 25 mm), many of which are brightly coloured and mistaken for bees or wasps. Adults often observed hovering near flowers. The larvae are predators of aphids, scale and other insects. Larvae are wrinkled and slug-like in appearance, with no obvious head capsule. Larvae are brown or green with whitish areas or bands. Look for them in aphid colonies!

Flower bugs: Small (2-4 mm) oval-shaped insects, often dark with light, patchy wing covers. Several Orius spp. (example: minute pirate bug), are readily found in diverse crop habitats, including field, fruit and vegetables, flowers, herbaceous plants and deciduous trees. The adults and larvae feed on aphids, mites, thrips.

Damsel bugs: Long (up to 1 cm) slender insects with drab grey-brown colouration. Both adults and nymphs are predators of other insects and mites that they stalk in diverse crop habitats.

Lacewings: Adults have large eyes and large delicate-looking, lace-like wings held roof-like over the body. Larvae resemble tiny alligators and possess sickly-shaped mandibles which are used to pierce prey and extract body fluids. They are voracious feeders of aphids and other soft-bodied insects.

Predatory mites: Like phytophagous or plant-feeding mites, predatory mites are very small / microscopic, oval with little or no differentiation of the two body regions. The larval stage has 3 pairs of legs while nymphs and adults have 4 pairs of legs. Tear-drop-shaped, these relatively fast-moving mites may take on the colour of their prey as they suck out their juices! Make sure to take them into account when scouting for pest mites.

Parasitic wasps and flies:There are many species of parasitic insects within the orders hymenoptera (the insect order including ants, bees, sawflies, wasps and allies) and diptera (flies) that attack agricultural pests. You are not likely to see the adult females actually laying their egg(s) into or on the host pest, but you may see their efforts at work if you look closely! Eggs masses that appear black are symptomatic of attack by parasitic wasps. Aphids are commonly attacked by parasitic wasps: when scouting, look for papery or crusty, brown, swollen aphids (infested aphids are called "mummies"). The generation time of most parasites is quite short when the weather is warm, and once mummies are observed on your plants, the aphid population is likely to be reduced substantially within a week or two. Keep in mind that parasitic insects tend to be quite susceptible to pesticides.

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