Insects, Mites, and Slugs                Mammals                        Plant Diseases         
Tent Caterpillars Cats and Squirrels Powdery Mildew
Aphids Deer Black Spot on Roses
Spider Mites Mice and Rats Dogwood Anthracnose
Cabbage Maggots Moles  
Slugs and Snails    
Cutworms    
Leaf Miners    
Crane Flies    
Mosquitoes    
Pest-Resistant Plants
   

Tent Caterpillars

Tents can be pruned out in the early morning or evening, when the weather is cool and the caterpillars are inside the tents. Pruned tents can be immersed in a bucket of soapy water or sealed in a plastic bag. Egg masses, inch-long gray or brown frothy material that resembles Styrofoam when hardened, can be hand stripped or pruned out of plants in winter.

Bear in mind that most trees can withstand up to 25% loss of leaves without permanent damage, and some will recover from nearly 100% defoliation in a single year. The trees will leaf out again in summer after an infestation. Several tents in a tree should not be a problem unless aesthetic standards absolutely prohibit it. Tent caterpillar infestations come in cycles, and one or two bad years are usually followed by much lower populations in succeeding years, even if nothing is done to control them.

Bacillus thuringiesis (B.t.) bacteria are effective as a stomach poison for caterpillars if applied to leaves while caterpillars are young and feeding. B.t. is selectively toxic to caterpillars but will also kill non-pest caterpillar species, so should not be applied indiscriminately. Caterpillars that have ingested B.t. may appear healthy for several days, but will stop feeding and become inactive. Other insects are not affected. While B.t. is a better option than broad-spectrum insecticides, physical controls should be tried first.

For more information see our Managing Tent Caterpillars fact sheet (42kb PDF file).

Aphids

Complete eradication of aphids in a landscape is neither realistic nor necessary. However, by selecting resistant plants, ensuring plant health, and choosing less-toxic controls, damage from aphids can be minimized. Bear in mind that aphids usually do not kill plants, but can cause defoliation, stunted or distorted growth, or release unacceptable amounts of honeydew beneath infested trees. Vegetables are also susceptible to damage.

If possible, avoid or consider replacing plants such as birch that have ongoing, substantial aphid infestations. Healthier plants are better able to resist pests, so ensure that your plants are receiving appropriate light, water, and nutrients. Excess nitrogen can stress plants as well as encourage the wispy new growth that is so attractive to aphids, so choose a slow release fertilizer containing nutrients levels appropriate for your plants and soil.

Physical control methods include knocking aphids off of plants with a strong water spray, pruning out particularly infested plant parts, and setting traps. These controls can be supplemented with the release of aphid predators such as ladybugs, lacewings, or parasitic wasps and flies into the landscape. These predators may not remain where they are released, but are likely to be helpful, especially if they are released on a staggered schedule. You might consider including some plants in the Umbelliferae or carrot family (such as angelica, sweet cicely, and dill) in your landscape, as they are especially attractive to these aphid predators. Finally, a less-toxic chemical control is insecticidal soap. It kills aphids on contact but does not provide any lasting preventive effect so applications will probably have to be repeated. 

For more information on aphid control, please see our fact sheet Aphids – Safe and Successful Control (41kb PDF file).

Spider Mites

Damage from spider mites becomes more noticeable when plants are stressed, especially due to drought, so be sure to provide adequate water. Low soil organic matter can also cause stress, especially in combination with poor watering. Add compost and fertilize appropriately. Spider mites thrive in hot, dry conditions, and populations usually peak and then decline in summer.

A strong water spray on undersides of affected leaves is an effective physical control. Be sure to spray at least three times, either three days in a row or every other day. A less-toxic chemical control is insecticidal soap. Repeat applications may be necessary, and in cases where webbing is present, a pressure spray that penetrates the webbing is more effective. It is especially important not to use broad spectrum insecticides to control spider mites because their natural predators include other mite species.

Some plants are so susceptible to mites that replacement may be the most practical long-term strategy. Especially susceptible plants include skimmia, spruce, yew, bamboo, rock cotoneaster (Cotoneaster horizontalis), and hawthorn. 

For more information, see the ProIPM fact sheet Mites on Landscape Plants.

Cabbage Maggots

Cabbage maggots are about 1/4 to 1/3 inch long and look like white, legless worms with  a blunt end. The eggs are laid in the soil at the base of plants, and the maggots tunnel into plant roots and stems, leaving behind brown tunnels that are sometimes slimy. These tunnels cause plants to wilt, and also make them more susceptible to bacterial and fungal diseases. Plants in the cabbage family, including broccoli, cabbage, cauliflower, and collards, are especially susceptible, though cress, celery, and beets have also been affected. 

Collars or mats can be placed on top of soil at the base of plants to keep flies from laying eggs there. Collars can be made from inverted wax paper cups by cutting a hole in the bottom, cutting a radial slit, then fitting the cup around the plant stem, with the rim set into the soil. 5- to 10-inch disks can be made from carpet padding or tar paper by cutting a slit along the radius and placing them at the base of the plant stem. Nylon netting, fine screening, or similar coverings placed over transplants or a newly seeded area will also keep flies from laying eggs. Make sure that the coverings are sealed firmly to the ground. 

Sowing and planting times can be adjusted to avoid times when maggot populations are typically highest – May, early June, and late summer. Intercropping cabbage and related plants with plants from other families will reduce populations, as will planting cabbage and related plants in different parts of the garden each year. Cabbage maggots may be discouraged by a strongly alkaline environment, and wood ashes or powdered limestone can be applied around your plants to raise the pH of the soil.

Slugs and Snails

Many people simply spread slug bait, but this approach is neither the safest nor the most effective. A long-term solution employs a mix of tactics. First be sure the damage is really from slugs or snails. Look for the telltale slime left behind, or go out at night and catch the critters red-handed. While you’re at it, use tongs to pick up any slugs or snails you find and dispose of them.

Reduce slug habitat near the garden by keeping things somewhat tidy: mow the grass, pull weeds, and remove any items slugs might hide under such as empty pots, boards, bricks, etc. Don’t plant things slugs like to eat near heavy groundcovers. Protect individual plants or groups of plants that are susceptible with slug traps baited with beer. Plants in pots or raised beds can be protected by a barrier of copper strip or copper tape at least four inches wide.

If slug bait is needed, use the newer iron-phosphate baits that are less toxic to pets. Avoid the so-called “slug and bug” baits that contain a mixture of metaldehyde and carbaryl.

For more information, see our fact sheet Protecting Your Plants from Slugs (90kb PDF file).

Cutworms

Cutworms are plump, rather drab brownish or grayish caterpillars about 1 or 2 inches long. They are the larvae of so-called “miller moths.” There are more than 650 species of cutworms in Washington state alone. Cutworms are seldom seen in daylight, but you will find them if you dig through the soil or mulch at the base of plants. When disturbed, they curl up into a “C” shape.

Plants damaged by cutworms may look as if their stems have been cut through by a lawn mower. Damage is especially serious on seedlings or other small plants that cannot recover from it. 

Seedlings can be protected from cutworms by surrounding them with cages made from hardware cloth or collars made from a tin can with both ends removed. Tilling the soil helps expose larvae to predation by birds. Products formulated with Bacillus thuringiensis (B.t.) can be applied to the leaves of plants and will be effective if eaten by the cutworms. Chemical controls are not recommended.

For more information, see the ProIPM fact sheet Cutworms and Armyworms.

Leafminers

Leaf miners are the larvae of a small fly. They hatch from tiny white eggs laid on the underside of leaves, then burrow in between the top and bottom surfaces of the leaves and eat the leaves from inside. The result is unsightly, especially on edible plants such as spinach, chard, and beets. Although leaf miners also attack some ornamental plants, it is the leafy greens where they really do the most damage.

The solution is simple and straightforward. Cover the plants with agricultural row cover (e.g. Reemay) when they are small seedlings. Cover the edges of the cloth with soil so that the adult flies cannot get through. Leave enough slack so that the plants can grow upwards to their full height. The cover lets in both light and water, so it can remain in place until harvest. In fact, it acts like a greenhouse and accelerates growth. This approach is nearly 100% effective. If you don’t want to cover your plants you can scrape off the eggs as soon as you see them and remove any damaged leaves.

Crane Flies

The European Crane Fly (Tipula paludosa) is the only significant insect pest of lawns in the Pacific Northwest, and it rarely does enough damage to require treatment. Adults, which look like large clumsy mosquitoes, are seen in August to October and are harmless. Larvae are present in the soil from about October through June and may damage grass roots from January through April, resulting in brown patches. Often, however, brown patches in the lawn are caused by other things, such as dog urine damage, herbicide damage (e.g. spraying dandelions with a non-selective herbicide like RoundUp or vinegar), scalping due to mowing on lumpy soil surface, fertilizer burn from uneven distribution, poor drainage, and poor soil conditions.

To see if you have crane flies, dig up a measured square of sod (either a one-foot by one-foot square, or several smaller ones) in early spring. Break apart the roots, thatch, and soil over a piece of light-colored paper or cardboard, and count the blunt-ended, grayish-brown larvae. Divide by the number of square feet in your sample. Determine your need to treat based on the number of larvae you find per square foot:

  • 0-25/sq ft: Do nothing. Fertilize appropriately and maintain healthy lawn.  (For information on lawn care, see our Lawn Care fact sheet [38kb PDF file].) 
  • 25-50/sq ft: If lawn is vigorous and healthy, do nothing.
  • >50/sq ft: Treatment may be needed depending on your personal standards. Consider replacing problem areas with alternative groundcovers. Aerate, thatch, overseed, and top dress with compost as needed. Aerating will actually kill quite a few crane flies while it punches holes to allow water, nutrients, and air to penetrate.

We do not recommend insecticides for crane fly treatment because they are rarely needed and they endanger birds and other crane fly predators. An organic control that may help is beneficial nematodes. Ask your organic garden center or landscaper whether they are appropriate in your situation. 

For more information, see our Crane Fly fact sheet (336kb PDF file).

Mosquitoes

West Nile Virus (WNV) is a reason to protect yourself and your family from mosquito bites but not a reason to panic. There are some effective steps you can take both to reduce the number of mosquitoes and to reduce your chances of being bitten. We do not, however, recommend that individuals attempt to kill adult mosquitoes by spraying insecticides.

The single most important thing you can do is to eliminate breeding sites on your property. Mosquitoes lay eggs and spend the first part of their life cycle in still or standing water. The entire process can take as little as a few days in warm weather, but it can continue year round in temperate climates. Mosquitoes can breed in clogged roof gutters, containers left outside, hot tub or pool covers that collect water, birdbaths, old tires, ponds, wading pools, rain barrels, watering cans, etc. Eliminate these sources or be sure the water is changed or agitated frequently. Birdbaths should be rinsed and refilled once a week. Fountains are not a problem if they are running. Ponds should either have fish in them or they should be treated with a Bacillus thuringiensis (B.t.) product such as Mosquito Dunks.

Dress appropriately for outdoor activities, especially at dawn or dusk, with light-weight clothing that covers arms and legs. When needed, use a mosquito repellent. If you wish to avoid repellents with DEET, consider newer products containing lemon eucalyptus (e.g. Repel or Off! Botanicals) or soybean and geranium oils (Bite Blocker products). These have proven to be more effective than other alternative repellents.

To reduce mosquitoes at outdoor parties, consider burning citronella candles.

For more information, see our fact sheet Mosquito Megabites (68kb PDF file).

Pest-Resistant Plants

The most effective way to prevent pest problems on your plants is to choose plants that are naturally resistant, plant them in the right place, and ensure their vigor by supplying appropriate water and nutrients and building healthy soil. Be aware of the environmental factors that will affect the plants in your landscape: know what type of soil you have and how it drains, and look at how light patterns change both daily and seasonally. Use this knowledge when selecting plants to make sure that the environmental conditions of the site match the environmental requirements of the plant.

Plants that are frequently grown in the Northwest and are especially prone to pest problems include birch, Colorado spruce, dwarf Alberta spruce, crabapple, skimmia, subalpine fir, hybrid tea roses, native and eastern dogwood, and flowering cherry cultivars Prunus ‘Autumnalis’ and Prunus ‘Whitcombii.’ Pest resistant varieties or good substitutes are available for all of these plants. Native plants, once established, require little maintenance and benefit local wildlife. 

Here are some websites with lists of appropriate plants for the maritime Northwest:

Available for download from the Saving Water Partnership:

  • Choosing the Right Plants for a Beautiful, Trouble-Free Garden
  • The Plant List

For fruiting apple, pear, plum, and cherry trees, please see our Fruitful Northwest Orchard fact sheet (80kb PDF file).

Cats and Squirrels

Cats and squirrels are cute, but they can do a lot of damage in the garden. Cats seem to think that your carefully turned and planted seedbed is just the best cat box ever and they leave a little present just to thank you. Squirrels bury their nuts in your garden and then tear the place up looking for them. Squirrels also like to eat some bulbs and flowers.

Chemical repellents are available and may be helpful in some situations. However, the most effect deterrent is a physical barrier. Chicken wire, screening, or agricultural netting laid over beds freshly planted with either seeds or transplants will discourage either kind of four-legged forager until the plants are large enough to withstand most of their mischief. Cut chicken wire or other mesh material into smaller pieces to protect groups of bulbs. Be extremely careful working with chicken wire. The edges are sharp and potentially rusty, and the springy material is difficult to control. Wear heavy gloves and long-sleeved clothing. It may be helpful to weight the chicken wire down with stones to keep it in place.

Deer

Deer can be very destructive to gardens and young trees, especially in rural or suburban areas. While many lists of supposedly deer-resistant plants have been published, the fact is that when they are really hungry, deer will eat most anything, including many of the plants on those lists and perhaps even the lists themselves.

A wide range of home-remedy solutions can be found in various publications: deodorant soap hung from trees, loud noises (e.g. radio in a barrel), flashing lights, motion-triggered sprinkler systems, dogs, etc. Most will work temporarily, and results are unpredictable. Some experimentation may be worthwhile. There are also deer repellent chemicals available, but you have to decide how practical they would be, especially in a large landscape.

The only surefire deterrent is a properly designed fence. Deer can jump 12 feet in the air, but an 8-foot high fence is usually enough. Shorter fences (4 to 6 feet high) can also work if they tilt out towards the deer, obscure the view of where the deer would land if they jumped, or are electrified.

For more information, see the ProIPM fact sheet Deer Damage Control.

Mice and Rats

Having mice or rats in your home isn’t fun, and it isn’t safe either: they do carry diseases. Avoid the temptation to just go to the store and buy poison. It may not solve the problem, can endanger children and pets, and the rodents may die inside the walls of your home.

If you want to hire a professional to do the job, go right ahead. Make sure, however, that they will suggest or perform needed repairs to prevent recurrence of the problem and that they will use primarily trapping and only use poison when and where absolutely necessary.

If you are attempting a do-it-yourself solution, determine first whether you are dealing with rats or mice. Size is the indicator, but baby rats can be confused with adult mice. The difference is that if you have rats, you should see some larger droppings 1/2” long or longer. The reason this is important is that mouse traps will be ineffective against adult rats.

The most important thing is to find out how they got in and block the openings. Use droppings and smudge marks to locate their runways. Both rats and mice can squeeze through very tiny spaces, so don’t neglect small openings. Block openings as appropriate with wood, sheet metal, heavy-duty screening, or caulk. At the same time, address any sanitation issues such as garbage cans, compost bins, recycling bins, or food practices that may attract rodents. Now it’s time to set traps.

Use a lot of traps. Bait them with hot dog, bacon, liver, marshmallows, or peanut butter mixed with oats. Place them where you think the rodents are running, but not where children or pets may get caught. Placing traps in pairs may increase success. Triggers should be oriented towards the wall. Traps can also be attached to wood framing or pipes if you have evidence they are pathways. Deploy traps baited but unset for several days to reduce wariness. If you experience a lull in trapping success, discontinue for a few days, then restart as above with the traps in new locations. (Use heavy gloves or tongs to remove dead animals from traps or discard trap and animal together. If you plan to reuse traps, wash them well with detergent and oil metal parts.) If your trapping proves unsuccessful, contact a professional to finish the job.

For more information, see our fact sheet I Smell A Rat: Solving Rodent Problems (44kb PDF file).

Moles

Moles damage lawns by digging tunnels underneath and depositing piles of soil at the entrances. They are less likely to be destructive to other kinds of landscaping unless they happen to tunnel right under a favorite plant.

Unfortunately, there is no easy solution for problem moles. Most of the home-remedies that people use are either ineffective (Ex-Lax in tunnels) or downright dangerous (drain cleaner, gasoline) or illegal (diazinon). Flooding tunnels with water is effective but probably temporary. Stomping tunnels may send the critters elsewhere and is certainly therapeutic, but may also be a temporary measure. Lethal trapping is probably the most effective, if barbaric, technique and not something that everyone will be comfortable with. However, you can trap a lot of moles and have enough leftover to still be a problem.

A sensible integrated management plan might include reducing lawn area and substituting some less susceptible perennial and shrub plantings, stomping and/or flooding when needed, and perhaps trying one of the castor-oil based spray products now available. Lethal trapping remains an option for those willing to use it.

For more information see this Department of Fish and Wildlife website.

Powdery Mildew

Powdery mildew is a fungal disease that affects many garden plants, including roses, rhododendron, and a variety of fruits, vegetables, and flowering plants. The symptoms include a fuzzy, powdery, whitish coating on leaves, stems, and flowers. There may be yellow to purplish areas on leaves, especially on rhododendrons.

Resistant varieties are available for some plants, especially roses and some perennials. Consult catalogues or appropriate lists for your area. Also look at our fact sheet Preventing Plant Diseases: Leaves (300kb PDF file).

Make sure plants get adequate sunlight and air circulation. Remove diseased leaves and destroy. For roses, remove all leaves up to 1 to 1-1/2 feet from the ground to prevent splashing of spores from the ground. 

Never apply chemicals without first having a diagnosis of the problem. The least-toxic chemical controls for powdery mildew include sulfur, fungicidal soaps, neem, and products based on baking soda or potassium bicarbonate. Many rose growers have had success with a homemade mixture of baking soda, dishwashing liquid, and vegetable oil in water.

Black Spot on Roses

Black spot is a fungal disease that causes circular black spots about 1/16 to 1/2 inches in diameter on leaves. Leaves may turn yellow and drop off.

Cultural/physical controls for black spot are the same as for powdery mildew. Plant resistant varieties, increase sun and air circulation, prune properly, remove leaves from base of plant up to 1-1/2 feet. Remove any diseased material and destroy, do not compost. Avoid overhead watering.

Chemical controls for black spot are similar to those for powdery mildew, but the disease is usually more difficult to control: sulfur, fungicidal soaps, neem, and products based on baking soda or potassium bicarbonate.

For more information on foliar plant diseases, see our fact sheet Preventing Plant Diseases: Leaves (300kb PDF file).

Dogwood Anthracnose

Anthracnose is the general name for a group of diseases that affect a variety of plants. On western dogwood (Cornus nuttallii) and eastern dogwood (Cornus florida), a fungus called Discula destructiva causes leaf spots and eventually blotching with a dark gray-green or purplish edge, usually beginning in a wedge shape at the mid vein and spreading towards the tip of the leaf. The tree can lose leaves but may retain some gray crumpled leaves throughout the winter. Twigs and branches can also be affected. 

Dogwood anthracnose is best prevented by planting resistant dogwoods or related species instead of the susceptible dogwoods. Examples of resistant species or cultivars include Cornus kousa var chinensis, Cornus kousa ‘Milky Way,’ Cornus kousa ‘Steeple,’ and Cornus florida ‘Spring Grove’ or ‘Sunset.’ In addition, hybrids of Cornus kousa and Cornus florida C. ‘Celestial,’ C. ‘Stardust,’ and C. ‘Stellar Pink’ have shown resistance in the eastern United States. Note that resistant cultivars may still exhibit some symptoms in severe seasons but should be better able to cope. Another option is Cornus mas (Cornelian cherry dogwood), which has small yellow flowers but not the form of the larger dogwoods.

Some damage can be tolerated on existing trees. Avoid letting trees go into drought stress. On smaller trees, pruning can be an effective control strategy. Prune out and destroy infected twigs, leaves, buds, or blossoms. Rake and destroy fallen diseased leaves. If chemical seems treatment is necessary to save important trees, consult a local tree expert for proper diagnosis and appropriate chemical choice and timing.

For more information, see the ProIPM fact sheet Dogwood Anthracnose.

Leave a Reply