What's that Bug? How to Identify the Insects in Your Garden

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Tomato hornworm

Summer bug season is here, the time when organic gardeners should make a habit of patrolling the garden to see what’s happening with six-legged guests. The same unwanted ones return almost every year, and getting pest insects under control early, while their numbers are small, is always a sound defence.

How do you tell one insect from another? Start with location, because insects are drawn to specific food plants or habitats that provide them with what they need. Leaf-eating larvae usually hatch from eggs laid on carefully selected plants, and the caterpillars or little larvae never leave the host plant until they change into adults. Even insects that can fly return to their preferred host plants minutes after you shoo them away.

What is the insect doing?

My favourite reference book on the subject, Garden Insects of North America, by Whitney Cranshaw, sorts insects by how they interact with plants, such as leaf chewers, fruit feeders, or root feeders. This forces the gardener to look closely at multiple factors – the plant, the pest, and the nature of the damage being done. Damage patterns can yield important clues. For example, the holes made in leaves by slugs often have smooth edges, beetle larvae often scrape out ragged patches, and leaf-cutter bees harvest precise little circles of tender green leaf.

“Small
Small cabbage white butterfly caterpillars

Also consider whether the insect is moving or still. Not moving is an option, as with scale insects, and many sucking insects get themselves installed in feeding position and stay put. Blobs of frass (insect excrement) don’t move, either, and may look like pests to worried gardeners. Leaf or stem lesions caused by edema, also spelled oedema, looks like tiny pests, but are leaf ruptures that occur when plants try to relieve themselves of excessive water. Cabbage, peppers, tomatoes and many other vegetables can develop edema.

Insect behaviour

Next question: Is the insect causing damage, or merely passing though? Winged insects can come and go as they please, and adults are more mobile than young larvae, which seldom have a way to move far from where they were born.

Are the insects alone or in a group? Where you find one aphid, you always find plenty more. The same is true for many other insects that hatch from clusters of eggs. For example, newly hatched potato beetle larvae usually feed together, probably as a defence against predators. Insects that have a short mating season also stick together. Japanese beetles are a formidable pest in much of North America, but they are so obsessed with food and sex that you can knock dozens of them into a pail of soapy water on a cool morning because of their grouping behaviour.

“Japanese
Japanese beetles

Look for field marks

Bird watchers use field marks like the colours or a bird’s beak, legs and eyes to identify birds, and the same approach is a good way to get to know insects. A camera is a great help here, because you can enlarge your bug images to see fascinating details. Regardless of what kind of camera you use, tiny insects are hard to photograph clearly, and the best advice I can give is to take multiple shots from different angles, in bright but diffuse light, and hope for the best.

Making a sure identification is fun! I thought the little green caterpillar I found on rasping on a green tomato was a baby hornworm, and the picture (at the top of the page) confirmed it by revealing the tiny red “horn” sticking out of the little guy’s tail. Now I know to keep a closer watch on my plants, in case wasps don’t do a good job of harvesting the newly hatched hornworms.

Insects that have been trapped in sticky traps or yellow pails of soapy water are often badly damaged, but still interesting to study using a magnifying glass. Not to be morbid, but the extended legs and splayed-out underwings of dead insects sometimes reveal more than you can see in live ones.

Insect life stages

Insect life cycles are highly variable and often downright weird, and except in the case of true bugs, the young insect may look nothing like the adult. The photos below are of a young nymph and two adults of the four-lined plant bug, a mirid bug that makes small circular feeding marks in new leaves of mint, sage and many other herbs.

“Four-lined
Four-lined plant bug nymph (left) and adults (right)

In this instance you can see the resemblance between the young and mature stages, but without the help of known science, few of us would make the connection between carrot maggots and their rust fly mothers. We might figure out the link between cabbage white butterflies and their caterpillars because the butterflies are so hard to miss as they flit among your broccoli, dropping eggs at every stop.

Check in at The Big Bug Hunt

Now you are ready to get a positive identification through The Big Bug Hunt insect guides, which usually show several images of insects likely to be encountered in the garden. If you are still stumped, search engines can take you to databases hosted by major universities when given accurate information such as the insect’s scientific name, or a description of what the insect looks like and what it is doing. For example, the search phrase “metallic copper bug” will send you down some rabbit holes, but “metallic copper bug eating rose” takes you to Japanese beetles. Good luck with these and the many other bad bugs of summer.

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