Gardeners are no strangers to insect pests. While typically a mild nuisance, insect damage can weaken plants and lead them to be more susceptible to disease. There are even times when insect feeding alone can damage a plant sufficiently to kill it, so noticing when insect feeding is occurring and the different types of insect feeding damage are important skills for gardeners to keep in their tool belt.

The Nibblers

We all know these. Nibblers cause the most obvious type of feeding damage – the holes and leaves munched away. Insects that commonly cause this type of damage are grasshoppers (order Odonata), caterpillars (order Lepidoptera), immature sawflies (order Hymenoptera), and others with mandibles (mouthparts) made for chewing. Usually, the most economic way to deal with these pests is to simply pick them off of your plants when you observe them.

Although many types of Lepidopteran pests simply chew through leaves, some remove leaves (and needles!) to form casings needed for pupation and metamorphosis, as is the case with these bagworms (likely Thyridopteryx ephemeraeformis). Photo credit: Nick Goltz

The Borers and Miners

This subgroup of the nibblers are tougher to deal with. While they have similar chewing mouthparts, they are the usually found within their plant hosts. Borers are usually beetles that chew through woody plants (order Coleoptera), though sometimes caterpillars (order Lepidoptera) chew through herbaceous plants (such as the squash vine borer, Melittia cucurbitae). Leafminers may also be Lepidopterans, though most are immature flies (order Diptera). They are best managed by using a systemic insecticide – one that is taken up by the plant and distributed throughout. As with all insecticides, be sure to apply following label instructions and not while pollinators are visiting the plant.

Beetles have bored through this wood. Some species burrow deeply into the plant’s vascular tissue while others burrow along the bark, forming tunnels called “galleries”. Both types of damage can be seen on this log in the Sonoma forest. Photo credit: Nick Goltz

The Piercing-suckers

These insect pests have a modified mouthpart called a stylet, which works like a straw. Piercing-sucking pests use their stylets to suck plant “juices” from soft tissue, stunting growth and causing leaf distortion, spotting, and reduced vigor. Common insects that cause this type of damage are aphids and whiteflies (both are order Hemiptera). Insects in this group are more likely to transmit viruses than those in most other orders.  

Aphids (order Hemiptera, family Aphididae) are the bane of many a gardener! They reproduce quickly and often target young, supple tissue like new leaves and flower buds. Above is a photograph of aphids feeding on my roses earlier this year. Photo credit: Nick Goltz

The Gall-makers

Some insects, such as thrips (order Thysanoptera) can cause some similar disfigurement damage as those mentioned above, but may also cause the formation of galls, a type of unusual growth on plant tissue caused by insect feeding and/or the production of unusual plant growth hormones by the insects. The larvae of some wasps (order Hymenoptera) can cause the production of really interesting galls. There are non-insect pests, such as mites (class Arachnida), and certain types of fungi and bacteria that can also cause galling. Most of the time, the production of these galls do not seriously injure the plant and are only an aesthetic issue, but be sure to keep an eye out for any reduced vigor associated with these galls.

Plant galls take many different shapes, sizes, and forms! Often, an insect will lay her egg on/in a leaf, and the feeding young larva will cause the gall to form around it, providing necessary nutrients and protection from predators. Some insects only lay one egg on a leaf. This was obviously not the case in the above photo. Photo credit: Nick Goltz

…and (Nearly) Everyone Else

It’s important to remember that most of the insects we encounter in the garden are harmless or beneficial – pollinating our plants, eating pests and keeping the insect community diverse and healthy. Be sure to only apply insecticides as a last resort and only when pollinators and other beneficial critters aren’t present. The best time of day to apply insecticides (to minimize sun injury and contact with pollinators) is in the evening when plants are dry unless otherwise specified on your product label. Not sure what insect is visiting your garden? Contact the folks at the UConn Home & Garden Education Center by emailing ladybug@uconn.edu for advice and identification services.

Nick Goltz, DPM