Hero Image

Springtime and pesky pests

You generally do not need a calendar to tell you when spring has arrived. The days are longer and warmer. Trees burst with brilliant blossoms and luscious new growth. Birds sing their tunes in the hopes of attracting a mate. Tender new growth emerges from tiny vegetable seeds carefully sown last month.

Spring is also when the bane of the garden season hits us in full force - bugs. A portion of a tree reveals a wriggling clump of fuzzy caterpillars instead of leaves. A pulsating mass of tiny pests cloak the new growth on roses, elderberries or peach trees. Seedlings just beginning to develop disappear overnight.

Who are these marauders and how does one deal with them?

Some of the major villains in the spring garden include aphids, caterpillars (particularly tent caterpillars), snails and slugs, thrips, and scale. A few of these garden pests are easy to recognize and manage while others require more diligent monitoring and rigorous control measures.

Potato Aphids. Photo: UC Integrated Pest Management Program
Potato Aphids. Photo: UC Integrated Pest Management Program

One of the most prevalent and recognizable spring pests is the aphid. These small, pear-shaped, often green soft-bodied insects suck the juices out of a wide variety of landscape plants and agricultural crops.

Frequently seen in masses covering new growth on plants, they can weaken a plant, cause leaves to yellow, curl or drop early or distort plant stems or fruit. As they feed, they produce honeydew that attracts ants that in turn protect the aphids from various predators. Any honeydew the ants don't consume can end up supporting the growth of black sooty mold.

The best method of control is prevention. Aphids love rapidly growing supple new growth on plants that get a lot of nitrogen fertilizer, so don't use any more than needed.

Aphids are the favored food of many natural enemies, including lady beetles, soldier beetles and lacewings, so encouraging beneficial bugs into your garden can often keep aphid populations in check. A blast of water is usually enough to knock them off the plant, but if you need something more, try using an insecticidal soap, or a botanical insecticide containing neem oil.

Tent caterpillar

The tent caterpillar is also easy to recognize, sporting rather furry-looking reddish brown with blue spots and tufts of orange to white hairs. Generally seen in huge numbers within their silken webs, they feed on deciduous trees and shrubs including ash, birch, fruit and nut trees, madrone, oak, poplar, redbud, toyon and willow. Their damage can be slight to serious on individual trees depending on the level of infestation. Inspect plants regularly, and if you see these voracious pests, prune out the infested branches and dispose of them.

Tent Caterpillar. Photo: UC Integrated Pest Management Program
Tent Caterpillar. Photo: UC Integrated Pest Management Program
Tent Caterpillar. Photo: UC Integrated Pest Management Program
Tent Caterpillar. Photo: UC Integrated Pest Management Program

Snails and slugs

You may not see snails and slugs, but you can sure see their damage. These slimy mollusks emerge from hiding at night and chew holes in leaves and flowers of many garden plants and fruit.

Slugs and snails move by gliding along on their muscular "foot" that constantly secretes mucus that forms their trademark silvery trail. They are hermaphrodites - each snail is both male and female, hence their ability to reproduce so rapidly.

Check your plants early in the morning and you'll likely find them moving back to their daytime hiding places - ivy, weedy areas, debris, boards or other dark, moist, protected locations. An integrated management approach that includes reducing moisture and hiding places, trapping, barriers and handpicking is the most effective.

Baits can be helpful, but be careful when using around children and pets. Products based on iron phosphate (available under many trade names including Sluggo and Escar-Go) are not toxic to plants or animals and break down to provide some additional iron for the soil.

Leaf showing scars caused by thrips. Photo: UC Integrated Pest Management Program
Leaf showing scars caused by thrips. Photo: UC Integrated Pest Management Program


Thrips are generally bad news for the gardener. Not visible to the naked eye, they often feed within buds and furled leaves or other enclosed parts of the plant. These extremely small insects cause unsightly, cosmetic damage to a wide range of plants. Western flower thrips, Frankliniella occidentalis, one of the most common thrips, favors many herbaceous ornamentals (impatiens, petunia), vegetables (cucurbits, pepper), fruits (grape, strawberry) and some shrubs and trees (rose, stone fruit).

Managing thrips can be a challenge and preventing an infestation is the best option. Along with good cultural practices, providing habitats that encourage beneficial predators such as green lacewings and minute pirate bugs can help keep populations in check.

Where appropriate, use barriers such as row covers to exclude them. In small gardens, a spray of water can knock them off the plant. Limit heavy applications of nitrogen fertilizers as these actually promote higher populations of the pests (along with aphids).

If preventive measures aren't adequate, prune out damaged or infested plant material and discard. Healthy, vigorous plants should outgrow the damage.


Scale looks like bumpy, crusty bark or fine ash on branches of trees and shrubs. They are another insect that sucks sap from the plant, weakening it and causing dieback of twigs and stems. They also produce sticky honeydew ants love and that supports black sooty mold.

Black scale. Photo: UC Integrated Pest Management Program
Black scale. Photo: UC Integrated Pest Management Program
Oak leaf with sooty mold. Photo: UC Integrated Pest Management Program
Oak leaf with sooty mold. Photo: UC Integrated Pest Management Program

Your best bet is to prevent infection through good garden sanitation, monitoring susceptible plants and treating any visible signs immediately. Prune away and destroy infected canes. You can apply a horticultural oil spray after pruning in the winter that will smother the pest; repeat spraying may be necessary.

For more information and assistance with springtime garden pests

For more information on any of these springtime pests or garden problems, contact the UC Marin Master Gardener Help Desk, 1682 Novato Blvd., Suite 150B. It is open 9 a.m. to noon and 1 to 4 p.m. weekdays. You can also ask your questions by phone or e-mail, at 473-4204 or helpdesk@marinmg.org.

You can also find more detailed information on how to manage pests on the UC Integrated Pest Management website:

Original article by Nanette Londeree for the Marin Independent Journal
Edited for the Leaflet by Lisa MacCubbin