Controlling Common Insect Pests in Your Garden

Spring means new life and new beginnings, not only for plants but also for the insects that have evolved to eat them. Dealing with insect pests is best done when populations are small and before infestations get out of hand. As we here at the William T. Kemper Home Gardening Center look ahead to a busy growing season, we want to educate the gardening public about some of the most common spring and summer insect pests encountered in the St. Louis region and how best to manage them.

Rose Slugs

Identifying Rose Slugs

Rose slugs have a very misleading common name since they are not true slugs but rather the larvae of a flying insect known as a sawfly (which itself is a misnomer because sawflies are actually more closely related to bees, wasps, and ants than true flies). Nomenclature aside, these small green larvae are voracious consumers of rose foliage, and they leave a very distinct feeding pattern. When they are young, the larvae feed primarily on a single layer of the leaf, creating light brown, skeletonized patches. As they grow larger, the larvae are able to chew through the whole leaf, creating holes. These two types of damage are often seen on a plant at the same time.

Rose slugs and their damage. Photos by Daria McKelvey.

Removing Rose Slugs

Begin scouting for rose slugs and rose slug damage in mid-spring. Simple cultural control measures such as removing and discarding leaves with the feeding larvae or knocking the larvae off the plant with a forceful spray of water are most effective when populations are low.

Continue monitoring for rose slug damage throughout the growing season. Chemical insecticides such as insecticidal soaps and neem oil can be effective but should only be used when cultural control measures fail. Insecticides can do more harm than good by also killing beneficial insects such as parasitic wasps that help keep the rose slug population in check.

Boxwood Leafminers

Identifying Boxwood Leafminers

Boxwood leafminers are fly larvae that feed on the spongey interiors of boxwood leaves, forming distinctive yellow blisters. This damage should not be confused with winter injury, which typically manifests as uniformly beige or light brown leaves at the ends of the branches. In May, the larvae pupate and emerge as orange-red-colored adult flies that somewhat resemble small mosquitos. Swarms of the flies can be seen above boxwoods at this time as they mate and prepare to lay eggs.

From top left, clockwise: Boxwood leafminer larvae, boxwood leafminer adults caught in a spider web, boxwood leafminer damage, and adult boxwood leafminer on hand. Photos by Daria McKelvey.

Removing Boxwood Leafminers

English boxwoods are more resistant to this pest, but any boxwood can be affected. Pruning in May before the adults emerge and begin to swarm can help to reduce their population. Be sure to remove the trimmings from around the shrubs. Chemical insecticides can be used, but timing is crucial to ensure the best results with the least damage to non-target species.

Boxwood leafminer adults in flight. Video by Daria McKelvey.

Lace Bugs

Identifying Lace Bugs

There are many species of lace bugs, and most have a specific host plant they prefer to feed on. Gardeners with azaleas may be most familiar with azalea lace bugs. These small, cream-to-brown-colored insects use their piercing-sucking mouthparts to feed on the underside of azalea leaves, causing white stippling and possible defoliation. While any member of the Rhododendron genus can be affected by this pest, evergreen varieties are the most vulnerable because the eggs can overwinter inside the foliage. Defoliation is also a more serious threat to the overall health of evergreen compared to deciduous rhododendrons and azaleas.

Lace bugs and lace bug damage on linden and azalea leaves. Photos by Daria McKelvey.

Removing Lace Bugs

Plants in full sun seem to have greater infestations of lace bugs, so try to choose partly shaded locations in the garden. Also be sure to keep your rhododendrons and azaleas well-watered during periods of drought to reduce stress. Familiarize yourself with what the damage from these pests looks like as well as with the appearance of the nymphs and adults. Start monitoring for lace bugs in spring and continue through the growing season. A forceful spray of water from a hose directed at the underside of the foliage may dislodge and kill some of the pests. Chemical insecticides such as horticultural oils may be necessary to control lace bugs on evergreen azaleas.

Cucumber Beetles

Identifying Cucumber Beetles

Despite their name, cucumber beetles will feed on any cucurbit (member of the Cucurbitaceae family) including squash, pumpkins, and melons. They have also been known to feed on other types of plants such as roses and zinnias. The adult beetles will overwinter in soil, fallen leaves, and other organic debris. In early summer, they emerge to feed on young plants. They lay their eggs in the soil, and the larvae feed on plant roots for several weeks before emerging as adults and continuing to feed on leaves, stems, flowers, and fruits. While their feeding damage can be quite severe, the bacterial wilt disease that these beetles can transmit to cucurbits is their most serious threat.

Cucumber beetles and their damage. Photos by Daria McKelvey.

Removing Cucumber Beetles

Row covers can be used while plants are young to keep cucumber beetles away. Make sure to remove these barriers when plants flower so that they can be pollinated by bees and other insects. Delaying when cucurbits are planted out into the garden can also reduce the chances of bacterial wilt disease from cucumber beetle feeding on young plants, but this may not work for varieties that require longer growing periods. Chemical insecticides may be warranted if the cucumber beetle population is high, but treatments should be applied early in the season before overwintering adults have a chance to lay eggs.

Justine Kandra

Kristina DeYong
Public Information Coordinator

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