Douglas A. Spilker, Ph.D., is a consulting ornamental plant pathologist and entomologist, garden writer and lecturer. Dr. Doug can be reached at

This article applies to:



10 Most (Un)Wanted Pests and What to Do about Them
by Douglas A. Spilker, Ph.D.       #Disease   #Pests

They don’t have their photos hanging on the post office walls, but these garden pests are notorious. Here are the ‘Most Wanted’ of the Midwest garden, their rap sheets and how to bring them to justice.

Japanese beetles

Problems, problems, problems — all landscapes have them. They may be insects, diseases or weed problems, yet they are problems nonetheless. By listening to gardeners and experts alike, this is my top 10 list (in alphabetical order) of frustrating and (un)wanted pests in our area.



Fire blight



Rose rosette disease

1. Aphids are tiny pear-shaped insects found clustering on young foliage. Although aphids rarely kill plants, feeding causes cupping or curling of leaves. As aphids suck plant sap, they excrete a sticky substance called “honeydew” that coates leaves and other surfaces. Unsightly sooty mold grows on these excretions. Durable plants can be sprayed with a forceful stream of water to wash off the aphids. Pruning aphid-infested young growth can also be effective. On occasion, insecticide applications may be needed. 

2. Bagworms are especially a threat to evergreens. The most notable sign of the pest is the silken bag hanging like a little ornament. Infested trees may be partially defoliated and weakened by these caterpillars before they are even noticed. On small trees and shrubs, hand-pick and destroy the bags. For larger trees, sprays should be timed to egg hatch, while caterpillars are still small, about early to mid-June. 

3. Fire blight is a bacterial disease causing blossom blight, severe dieback and even death to especially crabapple, flowering pear and mountain ash. Infected shoots die quickly, taking on a scorched look with a “shepherd’s crook” symptom. There is no cure for fire blight. Prune out blighted limbs 1 foot or more back from the affected plant tissue. If dormant pruning is not possible, disinfect pruners between cuts with a water-bleach solution. When selecting new trees, avoid highly susceptible cultivars. 

4. Geranium (tobacco) budworms damage petunias and geraniums by chewing holes in leaves and attacking flower buds, reducing blooming. Tiny budworms may be light green, brown or pink. Damaged buds have tiny holes with sawdust-like matter. In small flower beds or containers, pick off budworms by hand. They tend to feed at dusk and leave black droppings. Rototilling beds before planting may destroy the overwintering pupae. In larger plantings, consider applications of insecticides labeled for caterpillar control, including Bacillus thuringiensis (Bt) products. 

5. Japanese beetles are destructive plant pests as both adults and larvae (grubs). Adults feed on several hundred species of plants, but especially roses. They skeletonize leaves or chew irregular holes in leaves and petals. Japanese beetle larvae and other white grubs that develop in the soil can totally kill a lawn. Although neem sprays have shown some repellency of adult beetles, weekly sprays of broad spectrum insecticides have proven the most effective for residual control. Nothing will keep Japanese beetles from feeding on flowers that open between spray applications. 

6. Moles make ridges or mounds of heaved-up soil that uproot garden plants and disrupt grass roots leading to lawn browning. Stomach contents of moles show that earthworms, grubs and ground beetles are preferred foods. The strategy of controlling lawn grubs to reduce mole activity is often unsuccessful due to the mole’s appetite for earthworms. Many home remedies abound, but trapping is the most effective method of mole control. To ensure safe and humane deployment, follow trap instructions. Some turf managers have been successful using baits, especially ones containing bromethalin. Always read and follow label directions.

7. Rose rosette disease first appears as rapidly developing red-colored canes with an over-abundance of thorns and tiny deformed leaves. Plants commonly die within one or two years. The cause of this disease is still under investigation, so there is no cure for infected plants. Controlling insects and mites may help reduce potential spread. Symptomatic plants should be dug and discarded as soon as the disease is noticed.

Slugs and snails

Two-spotted spider mites

Yellow nutsedge(nutgrass)

8. Slugs and snails are becoming more bothersome in landscapes with the use of more mulch and green covers. Irregular holes in leaves and “slime trails” signal their presence. They are most active at night, seeking hiding places during the day. Eliminate or alter daytime hiding places, such as moist areas around rocks, boards and dense ground covers. Second, remove or kill the slugs in these areas by either routine handpicking, or by using traps and baits. Baits containing iron phosphate are safer for use around pets and children.

9. Two-spotted spider mites cause damage by sucking plant juices. Initial mite feeding causes stippling (light foliar dots) which develops into total leaf bronzing. Hot, dry weather favors rapid mite development, with high populations generating webs. Most homeowner chemicals are not very effective, so consider using insecticidal soap, with thorough coverage of both upper and lower leaf surfaces. As suggested for aphids, hosing durable plants with a forceful spray of water may wash off mites and knock down webs. 

10. Yellow nutsedge (nutgrass) is a fast-growing grass-like weed that, if left unchecked, can quickly spread to large areas of turf and flower beds. It has a triangular stem, yellow-green foliage and grows faster than the turf around it. Nutsedge is difficult to eradicate because it forms nutlets (tubers) in the soil that grow into new plants. Unless the infested area is small, hand removal of plants is not effective since the nutlets will remain. Maintaining a dense, vigorous turf and improving drainage in low-lying areas will discourage nutsedge. Specific herbicides labeled for sedges (especially those containing halosulfuron-methyl) are needed to achieve satisfactory control. 

These are my top 10 pests, but I am sure each of you has your own frustrating archenemies!

The Most Important ‘Chemical’ in the Garden… Water!

Proper watering or irrigation is vital — especially to woody ornamental survival. Since it takes a month or so for newly planted ornamentals to establish fresh roots, it is critical to keep the root ball moist with light frequent watering. Clay soils are very hard to manage, and waterlogging is not uncommon. Both overwatering during hot weather and severe drought stress can equally cause root death. Once a plant is weakened by drought stress or waterlogging, it is susceptible to attacks by insects and root rots that normally pose little threat to plant health. 

From State-by-State Gardening July/August 2012. Photography by Douglas A. Spiker, Ph.D.


Posted: 10/31/12   RSS | Print


Share this story on:
Facebook       Twitter            

Other People Are Reading