Pests

If you have an ailing tree, here are some questions you can ask yourself to begin diagnosing the problem: is the entire canopy of the tree affected? If the answer is yes, you can reasonably guess that something is wrong below the soil. A lack of nutrients (refer to the UC Pest Note on Mineral Deficiencies) will likely cause the leaves to either die (necrosis) or lose color (chlorosis). Too much or too little water will also cause foliage problems (See UC Pest Note on Poor Water Management, Poor Drainage).

If only parts of the tree are affected, it is likely your problem is above ground. Is there a pattern to the distress? You can rule out or suspect sunscald by determining which side of the tree faces the harshest sun (UC Pest Note on Sunburn).

What kind of tree is it? Is there new growth? If there is, that’s a great sign that a single event rather than an ongoing problem distressed your tree. The UC IPM website will direct you to species-specific pests and disorders to begin diagnosing your tree's ailments.

Almonds are harvested when the shell is cracked and brown. Freeze nuts for 1-2 weeks to kill resident worms, store nuts in plastic bags to prevent re-infestation, and spray the tree with fixed copper during or after leaf fall but before rains start to reduce damage from shot hole fungus.

Walnuts are fully mature when the green hull begins to break away from the shell. Harvest by polling or shaking the tree. Remove the green hulls, then freeze nuts in the shell to kill any resident worms. Store in plastic.

On outdoor and sometimes indoor plants, ants protect and care for honeydew-producing insects such as aphids, soft scales, whiteflies, and mealybugs, increasing damage from these pests.

Ants are among the most prevalent pests in households. UC IPM offers steps to follow in an Ant Emergency.

Ant management requires diligent efforts and the combined use of mechanical, cultural, sanitation, and often chemical control methods. It is unrealistic and impractical to attempt to totally eliminate ants from an outdoor area. Focus your management efforts on excluding ants from buildings or valuable plants and eliminating their food and water sources. Reducing outdoor sources of ants near buildings will reduce the likelihood of ants coming indoors.

Ants on Trees and Shrubs

When numerous ants are found on plants, they are probably attracted to the sweet honeydew deposited on the plants by honeydew-producing insects such as aphids or soft scales. Ants may also be attracted up into trees or shrubs by floral nectar or ripening or rotten sweet fruit. These ants can be kept out by banding tree trunks with sticky substances such as Tanglefoot. Trim branches to keep them from touching structures or plants so that ants are forced to try to climb up the trunk to reach the foliage.

When using Tanglefoot on young or sensitive trees, protect them from possible injury by wrapping the trunk with a collar of heavy paper, duct tape, or fabric tree wrap and coating this with the sticky material. Check the coating every one or two weeks and stir it with a stick to prevent the material from getting clogged with debris and dead ants, which will allow ants to cross. Ant stakes with bait can also be used around trees.

For more information about what ant baits and insecticides to use, please consult the UC Pest Note on Ants.

Bean aphid, melon (cotton) aphid, potato aphid, and cabbage aphid, all by Jack Kelly Clark

You can encourage beneficial insects in your garden by including a variety of plants that flower at different times throughout the season. Avoid chemicals that kill good and bad indiscriminately. If ants are protecting the aphids in a plant, control the ants. Decide what population you will tolerate in your garden, and have patience because it can take time to establish a healthy ecosystem.

More information: Aphid management guidelines in English| Spanish

For information: Armillaria Root Rot

These parasites suck the living sap from shrubs and trees. Armored scale insects are in the crawler stage in early summer (June). Armored scale has a hard stage that is very resistant to sprays. Control them during the crawler stage when they are soft and vulnerable. Spray with a horticultural (not dormant) oil, once a month for three months. See the UC Pest Note on Scales for important information about spraying.

The brown patch that looks like a turtle's back is called Carapace Spot. It is corky and usually cracked into angular divisions. It is caused by rubbing or brushing of tender young fruit on leaves or stems in the wind, but the fruit is usually undamaged under the spot. Just cut out the spot. More pictures of avocado problems can be found on UC Pest Note On Causes of Avocado Fruit Damage.

The Bagrada bug is a small (1/4”) stink bug that is most commonly found on vegetables in the Brassica family including cabbage, cauliflower, broccoli, kale, turnip, and mustard greens. Home gardeners should carefully inspect their plants and shipping containers prior to planting. A good time to inspect is right after watering when pests hiding in the space between the potting mix and the sides of the container may be flushed out and more easily detected. Plant seedlings late this month when they are big and robust. If you find nymphs on the plants, use insecticidal soap. See UC Pest Note on Bagrada Bug for further information.

Birds can cause extensive damage to tree fruit crops. Unlike squirrels, birds are more likely to peck at one piece of fruit until it’s gone. If they are doing too much damage, netting over a tree can keep them away from the fruit. If you use visual repellents (such as Mylar streamers or noisemakers) to frighten them, be sure to vary the method so that the birds don’t become immune to the effects. Read the UC Pest Note on Birds on Tree Fruits and Vines for more information.

Bitter pit is a physiological disorder that affects many varieties of apples. The condition develops after fruit has been picked. It is caused by low levels of calcium in fruit tissues which leads to small brown, sunken lesions that become dark and corky. Highly susceptible cultivars include Red Delicious, Granny Smith, Golden Delicious, Jonathan and Gravenstein. The UC Bitter Pit Pest Note recommends cultural practices for control.

For even more information, the UC Postharvest Technology Center website has grower information, including using calcium sprays starting in June.

They feed and reproduce on a variety of plants and are particularly damaging to fruit. You can cut cosmetic damage off fruit and still eat the rest of the fruit. To keep out stink bugs, cover vegetable plants with row covers. You can pick the bugs off plants and squish them or knock them off into soapy water. They are attracted to light and can get into homes where if vacuumed up, they can stink up your vacuum bag.

If your apricots or peaches had brown flesh last year, especially in the part surrounding the pit, they were probably infected with brown rot. It's a common fungal disease of stone fruit. You can spray with a copper spray at pink bud stage. A more important means of control is to remove affected fruit as soon as you notice it. The UC Pest Note on Brown Rot has more information.

The grey-green cabbage aphid is often found on cool season vegetables. They prefer to feed on the youngest leaves and flowering parts and are often seen on cabbages, broccoli, and Brussels sprouts.

Hose them off plants or prune out infestations. Grow flowers in your vegetable garden to attract beneficial insects, which are their natural enemies.

See UC Pest Note on Cabbage Aphid to identify and manage this insect.

More information: Camellias Petal Blight

Have you ever seen weirdly shaped lemons or oranges that appear to have 'fingers'? This condition is caused by the Citrus Bud Mite. 

It attacks newly forming flowers and fruits. The mite is only visible with a magnifying glass and has a elongated yellow body with four legs that appear to come out of its head. The mites feed inside the buds, killing them or causing a rosette-like growth of the subsequent foliage and distortion of flowers and fruit. The problem is usually limited to just a few fruit on the tree.

Previously recommended oil sprays have not been proved effective. This is one of those pests that is best left alone. The oddly-shaped fruit is edible.

For more information see UC Pest Note on Citrus Bud Mite.

Leaf drop from citrus trees is normal. Washington Navel oranges may lose over 3,000 leaves a day during peak leaf drop in the spring. Valencia oranges may lose about 1500 a day. Problems that can cause excessive leaf drop beyond these numbers are lack of water and a heavy infestation of spider mites. The tree's leaves will have brown spots if affected by the mites. You can wash them off with a strong water spray. Bud and small fruit drop is also normal. For further information see UC Pest Note on Diseases and Disorders of Citrus Leaves and Twigs.

Sooty mold on citrus may be a byproduct of sucking insects such as aphid, mealy bug, soft scale or whitefly. Ants will protect these pests against predators in exchange for the honeydew that the pests produce. The sooty mold grows on the honeydew. Try washing off the sucking insects with a strong water stream. The next step is control of the ants. Ants may be managed by applying a sticky compound around the trunk and trimming limbs touching buildings or other access points. Baits at the base of the tree also help. For more information about specific controls, see the UC Pest Note on Sooty Mold.

Pick up fallen fruit daily to prevent attracting critters or diseases. If your fruit is being eaten at night then rats are the likely culprit, if it's during the day it may be squirrels. Holes, rather than bites, are made by birds. In addition to harvesting regularly, ripening fruit can be protected with a netting fine enough to exclude birds and small animals

Codling moth larvae bore into the center of fruit to feed. Note the excrement, including the frass pushed out of the entry hole, Jack Kelly Clark, UC.

Trees should be monitored every week for signs of infestation. Infested fruit should be removed and discarded to break the codling moth life cycle. Sanitation is an important non-chemical step in controlling this pest. Make sure to pick up fallen fruit promptly, and pick apples with holes that are still on the tree. This will keep future populations down.

Pheromone traps can be hung in isolated trees. But if you have just one apple tree don't bother. You will just attract codling moths to your tree.

Fruit can be bagged for protection, but this method is very labor-intensive. Heavy infestations may require the use of pesticides, but proper timing of sprays is critical. Read the UC Codling Moth Pest Note for more information.

Spray apple, pear, peach and nectarine, apricot trees with horticultural oil during the dormant period to control scale, and aphid and mite eggs. For more information on using horticultural oil to control pests consult:
- UC Pest Note on Scales
- UC Pest Note on Aphids
- UC Pest Note on Spider Mites

Cottony cushion scale on citrus

More information: Cottony Cushion Scale Pest Note

Although there's no such thing as a deer-proof plant, some can be called deer-resistant. Plants with strong smells, woolly or hairy leaves, or prickly or thorny parts are less appetizing to deer. Other strategies include deer repellents, enclosing plants in wire cages, netting, or tall fences. Read more in the UC Pest Note on Deer and use our search for Deer Resistant Plants.

Earwigs are third only to snails and slugs in causing plant damage. While they are beneficial because they eat insects such as aphids, they also feed on soft plants. Earwigs can do quite a lot of damage if there is a high population. They feed at night and hide in moist, tight-fitting places during the day. Trap them by putting out moistened, tightly rolled newspaper or corrugated cardboard in the evening. In the morning dispose of the paper and the trapped insects. Other control methods are described in the UC Pest Note on Earwigs.

This psyllid has been a real problem in California. New leaves on the infected Eugenia look very much like peach leaf curl. The leaves also may become discolored. Thanks to the diligent work of the entomology researchers in biological insect control at UC Berkeley, a parasitic wasp called Tamarixia was released in Santa Clara County in 1993. The wasp is known to go as far as 45 miles and is found throughout the county. It is essential that no insecticide be used on Eugenia species. The Tamarixia wasp cannot do its job if it's poisoned. For more information see the UC Pest Note on Psyllids.

It is spread by insects, rain, or pruning. The bacteria enter through the blossoms and travel down the tree. If left unchecked, fire blight can enter the trunk and kill the entire tree. Prune infected branches back to healthy wood, at least eight inches below visible damage. If the inside of the branch is discolored, you need to cut back still further. Clean pruning tools between cuts so as not to spread the infection. A less effective way to control fire blight is to spray the open blossoms with a copper spray. Planting varieties that are less prone to fire blight is helpful. Always promptly clean up fallen fruit and leaves.

For more information, see the UC Pest Note on Fire Blight.

If your fruits are ready to pick and you don't get to them first, there are critters that will let you know when they are ripe. For example, for citrus, snails leave little holes in the outside peel, rats hollow out the fruit and squirrels carry them off, often leaving partly eaten fruit on a nearby fence.

To determine what’s eating your fruit - start by narrowing down the list of possible pests in your area. Check the damage to determine whether it is likely caused by insects, birds, or rodents. Determine what time of day the damage happens. Squirrels are active during the daytime, and rats are mainly active at night. Look for other signs, such as placement of the damaged fruits, tracks, and possibly droppings, and compare that to the habits of the pest you suspect. When you are sure what the pest is, you can take appropriate action to protect your fruits.

Resources:

• Identify citrus damage from insects and other pests
• Who is eating my tomatoes

This is the most prevalent and damaging tomato disease. It also starts with the yellowing of lower leaves, but the yellowing may be only on one side (stopping at midrib) of the leaf or just one branch or one side of the plant. The older leaves will droop and curve downward. The yellow leaves wilt and die, gradually killing the whole plant. Sometimes a single shoot is killed before the rest of the plant shows any damage.  More information at UC Pest Note on Fusarium Wilt.

If you see white beardlike filaments handing from the leaves of your hibiscus, bird of paradise, or other plants, you may have giant whitefly. Another sign is the distinctive wax spirals on the undersides of the leaves. They can usually be controlled by washing them off or removing affected leaves. For more information see UC Pest Note on Giant Whitefly.

More information: Gopher Pest Note

Botrytis is gray or brownish fuzzy mold that can attack a wide variety of plants. It likes flower petals, ripening fruits and vegetables, as well as leaves and stems. The spores are spread through the air. It is most severe when there's high humidity and may start forming on decaying matter. According to the UC Pest Note on Botrytis Blight, it is important to remove debris and prunings from the ground. You may even have to pick up flowers daily. Avoid overhead watering.

UC also has information about Gray Mold on Strawberries.

Plant resistant species or varieties. Small seedlings can be protected by protective cloth. On plants such as cole crops, lettuce, and spinach, clip off and remove older infested leaves. Place leaves in plastic bag, and put bag in trash. Leafminers are often kept under good control by natural parasites. Insecticides are not very effective for leafminer control. See UC Pest Note on Leafminers for additional information.

Mosaic virus on squash and cucumber plants is a disease spread by aphids and cucumber beetles. The leaves become rough and mottled, the plant becomes stunted and the fruit can be whitish. Pull the plant and put it in the trash. Do not compost. For more information see UC Pest Note on Squash Mosaic Virus.

More information: More about Neem

Some varieties of marigold can suppress certain damaging nematodes while adding color to the garden. The Tagetes species suppress root knot and lesion nematodes. French marigolds, including Nemagold, Petite Blanc, Queen Sophia, and Tangerine, are most effective. Avoid signet marigolds, T. signata or tenuifolia, because nematodes will feed and reproduce on these. Marigolds don’t work well against the northern root knot nematode, Meloidogyne hapla, a species common in areas with cool winters. The effect of marigolds is greatest when grown as a solid planting for an entire season. When grown along with annual vegetables or beneath trees or vines (intercropping), nematode control usually isn’t very effective. To prevent marigold seed from getting in the soil, cut or mow the plants before the flowers open. As with other cultural control methods, nematode populations rapidly will increase as soon as susceptible crops are grown again. Learn more at the UC Pest Note on Nematodes.

This fungal infection, Armillaria Mellea, also known as mushroom root rot, honey fungus and shoestring fungus can cause foliage to become sparse and eventually kill the tree. The white or cream-colored fungus will grow between bark and wood of roots and up into the lower part of the tree trunk. It smells like your typical grocery store mushroom.

In spring or fall there may be mushrooms growing around the base of the infected tree. If the disease has not progressed too far, you can cut out infected tissue and expose the base of the tree and roots to air. Reduce watering especially close to trunk. No chemical treatment is known to be effective.

Plants Resistant or Susceptible to Armillaria mellea, The Oak Root Fungus by Robert D. Raabe contains more detailed information. The UC Master Gardener video Revealing Root Rot can help with detection of two common root pathogens and provide guidance on how to reduce the spread.

This is a pest of California Live Oaks. The adult oakworm moth is tan-to-gray colored with a wing span of about an inch. It lays white eggs twice a year that turn reddish or brown before hatching. The first hatch is in November and overwinters on the leaves, growing and eating more as the weather warms. At full size, the larva is about an inch long with a yellowish green body, dark stripes down the side and large brown head. Outbreaks occur every eight to ten years in the Bay Area. In late March or April, look for little green pellets (droppings) falling from oak trees. A second generation of eggs hatches in mid to late summer. Trees may suffer no permanent damage beyond being unsightly. If you cannot tolerate the worms and their mess, further treatment methods are outlined in the UC Pest Note on California Oakworm.

After deciduous fruit trees have dropped their leaves, paint tree trunks with white latex paint diluted 1:1 with water to prevent sunburn. Preventing sunburn helps trees resist invasion by shot hole borers, which are tiny beetles that boreholes and lay eggs in the cambium layer of the tree. When the eggs hatch, young larvae feed and excavate tunnel galleries in the wood. For more information about identification and control see UC Pest Note on Shothole Borer.

More information: Pear Leaf Curl

This pest can cause damage to California pepper trees. The damage is done when the insect is in its immature stage. They make doughnut-like pits on leaves that wrinkle or twist the leaves as well. Parasitic wasps can provide satisfactory control. See the UC Pest Note on Psyllids for more information.

Poison oak is a California native plant that provides shelter and food for many native birds and other creatures. The downside is that at least 75% of us develop allergic contact dermatitis to the plant. Unwanted poison oak can be pulled or dug up by allergy-resistant friends, remove plants in early spring or late fall when the soil is moist and it is easier to dislodge rootstocks.

A complete list of management options, including herbicide control, is contained in the UC Pest Note on Poison Oak. Under no circumstances should poison oak be burned.

Powdery mildew on squash leaves

Powdery mildew likes warm days and cool nights. Unlike most other fungi, it does not need moisture to thrive. Early symptoms include yellow chlorotic spots on the leaves. The presence of the fungus becomes obvious as it starts to produce spores that look like white powder on leaves. Eventually, the leaves will turn brown and dry.

The best defense is to plant varieties that are resistant to powdery mildew. Also helpful is planting in full sun with good air circulation through the plants. Washing off the leaves, preferably in the morning, can buy some time. Powdery mildew generally affects the older leaves first, and you can remove these when they are too covered to be able to photosynthesize. Fungicides can help but you want to be careful about using them around something you are going to eat. At some point, the plant may decline so much that it needs to be removed.

This is nesting season for many birds, so be sure to check for active nests before pruning trees. Birds are good for natural pest control, as they eat many insects, and they improve your garden’s biodiversity. You can also encourage birds in your yard by providing food and water for them. Just be sure the water stays fresh and clean.

To protect fruit and nut trees from marauding birds, PVC structures covered with netting can save your crop. If netting is placed directly on the tree, birds will still be able to reach much of the fruit. There's more information in the UC Pest Note on Birds on Tree Fruits and Vines.

What is an invasive pest?

If you don't eat your fruit the minute before it's ripe, birds or squirrels will. Once you see signs of damage, either pick the fruit, or find a way to protect it. For example using netting or paper bags. Pick up any fallen fruit as so not to attract rats or other less visible pathogens.

Rats show up when your citrus, tomato, or fruit first starts to ripen. Rats are agile climbers and usually live and nest in shrubs, trees, and dense ground cover like ivy. One management strategy is to prevent access to the tree by cutting branches away from fences or other trees, leaving a gap of at least 2–3 feet, good sanitation is required. Garbage and garden debris should be eliminated. Use tight-fitting lids on garbage cans. Thin out dense vegetation to make the habitat less desirable. Mow ivy once a year to the ground. Climbing ivies on fences or buildings should be removed.

Per the UC pest note (linked below), trapping is the safest and easiest method for controlling rats. Read the pest note for other management strategies as well.

More information: Rat Management

More information: Nematodes Pest Note

More information: Snails and Slugs

The sooty mold fungi grow on “honeydew”, a sticky substance excreted by plant-sucking insects such as aphids, scale, mealybug, and whitefly. They suck the sap out of plants and excrete excess sugars. It exists purely on the surface of a plant and is not a systemic issue. By itself, the fungi cannot kill the plant but it can coat the leaves to the extent that sunlight is prevented from reaching the leaf surface, thus reduce photosynthesis. 

Ants protect the sucking insects from their predators so they can eat the honeydew. Keep ants out of trees and away from honeydew-producing insects by applying a sticky compound such as Tanglefoot on a tape wrapped around the trunk.

More information is available in the UC Quick Tips on Spider Mites.

Unfortunately, the Spotted Wing Drosophila is infesting local cherry, berry, and some other fruits. Look for holes in the fruit. Once the eggs hatch, maggots develop and feed inside the fruit, causing the flesh of the fruit to turn brown and soft. Dispose of infested fruit. Information on identification and control can be found in the UC Pest Note on Spotted Wing Drosophila.

Squash bugs are about ½ long, brownish yellow and flattened like a stink bug. Zucchini is one of their favorite plants. Leaves will blacken and drop as they dry. They can be difficult to control. Placing row cover over young plants helps prevent infestation. Remove nearby vegetation where the bugs can over winter.  More information can be read at UC Pest Note on Squash Bug.

For more information: Pest Note on Tree Squirrels and Pest Note on Ground Squirrels.

Stink bug eggs on an ollalieberry

Green stink bug instar (nymph) stages by Herb Pilcher, USDA-ARS, adult by James Castner, University of Florida

More information: UC Pest Note on Stink Bugs

Sudden oak death is a disease of oak trees caused by an invasive plant pathogen, Phytophthora ramorum. It occurs in Santa Clara County as shown on this map. If your oak tree went from healthy (green) to dead (brown) with a full complement of dead lives on the tree, it might have died from sudden oak death. More details about what to do are available at UC Pest Note on Sudden Oak Death.

Fruits and vegetables can get sunburned in the summer heat. This is more commonly called sunscald and it frequently affects peppers, tomatoes, and persimmons. The leaves shield the produce from the sun, so it helps to make sure the plants have sufficient fertilizer and water for a healthy plant. You can cut out the damaged parts and eat the rest of the fruit.

Termites are very common in Santa Clara Valley.  The first sign of infestation may be the presence of winged forms or fecal pellets pushed out of the infested wood. 

See the UC Pest Note on Termites to identify which type is present and the methods of control. Learn all you can before calling in the professionals.

Ticks are abundant after mild winters and they can carry dangerous diseases. To learn more about ticks, see the UC Pest Note on Lyme Disease. Be sure to visit the County of Santa Clara website for information about ticks.

A brown depression on the bottom of tomatoes is usually blossom end rot (BER). This disorder is related to a calcium deficiency aggravated by irregular watering. Since most soils have adequate calcium, watering is usually the problem. Without regular watering, the calcium in the soil cannot reach the plant. Mulching can help. Water tomatoes regularly. Avoid flooding them so the roots sit in water. For more detailed information about BER please view Managing Blossom-End Rot in Tomatoes and Peppers.

Sometimes scarring can be seen on the bottom (blossom end) of tomatoes. This can be caused by weather conditions such as cool and cloudy weather at bloom time, making the blossom stick longer to the small fruit. The fruit is perfectly good to eat with the damaged part cut out. Some large heirloom tomatoes are more susceptible to this condition.

Also known as catfacing.

Tomato russet mites deplete juice from the cells of leaves, stems and fruit. They usually start at the base of the plant and move upward. If not controlled, these pests can kill plants. At first sign of damage, treat with sulfur dust or a spray solution of wettable sulfur and spreader-sticker. More information is found in the UC Pest Note on Tomato Russet Mite.

Verticillium wilt is a soil-borne fungal disease that damages plant veins. The damage is characterized by affecting one side of the plant. The leaves may wilt and turn brown, dying upward from the base of the branch to the tip. Dead leaves often fall, but may not. Mildly affected plants may survive if fertilized and encouraged into vigorous growth. The fungus can live for years in the soil.

Planting tomatoes, peppers, eggplants, and potatoes (all members of the Solanaceae or nightshade family) in the same place no more than once every three years helps reduce the fungal population to non-harmful levels. Soil solarization may eliminate Verticillium wilt from infected soils. Crop rotation with cereals or broccolis can reduce the pathogen. Avoid high nitrogen fertilizers. Plant Verticillium wilt-resistant varieties of tomatoes, potatoes or strawberries. Refer to the UC Pest Note information on Tomato, Potato, and Strawberry for species-specific suggestions.

Vinegar can work as a weed killer, but its effectiveness is limited. Household white vinegar (5% acetic acid) is too weak to kill anything other than tiny seedlings. Stronger horticultural vinegars with 20% acetic acid provide better weed control but typically only for annual, not perennial, weeds. And the higher concentration can be dangerous—including skin burns and eye injury—if not handled carefully. For comprehensive guidance on weed management in landscapes, refer to the University of California’s Weed Management in Landscapes recommendations.

White mold is a distinctive disease that most often affects stems and foliage at the base of cole crops^* and lettuce plants. Affected tissue develops a soft, watery rot and white, cottony mycelium forms on the surface. Plants may wilt if stems are girdled by the decay. As affected tissue dries up, it turns yellow to white, and hard black sclerotia form on the surface or inside the dead stems. Get more information in the UC Pest Note on White Mold.

^* Cole crops include broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi

Whiteflies are tiny, sap-sucking insects that may become abundant in vegetable and ornamental plantings, especially during warm weather. They excrete sticky honeydew and cause yellowing or death of leaves. Outbreaks often occur when the natural biological control is disrupted. Management is difficult once populations are high.

Whiteflies use their piercing, needlelike mouthparts to suck sap from phloem, the food-conducting tissues in plant stems and leaves. Large populations can cause leaves to turn yellow, appear dry, or fall off plants. Like aphids, whiteflies excrete a sugary liquid called honeydew, so leaves may be sticky or covered with black sooty mold that grows on honeydew (See UC Pest Note on Sooty Mold). The honeydew attracts ants, which interfere with the activities of natural enemies that may control whiteflies and other pests.

Management of heavy whitefly infestations is difficult. The best strategy is to prevent problems from developing in your garden or landscape. In many situations, natural enemies will provide adequate control of whiteflies; outbreaks often occur when natural enemies are disrupted by insecticide applications, dusty conditions, or interference by ants. Avoid or remove plants that repeatedly host high populations of whiteflies.

In gardens, whitefly populations in the early stages of population development can be held down by a vigilant program of removing infested leaves or hosing down with water sprays. Reflective mulches can repel whiteflies from vegetable gardens, and yellow sticky traps can be used to monitor or, at high levels, reduce whitefly numbers. If you choose to use insecticides, insecticidal soaps or oils such as neem oil may reduce but not eliminate populations. Systemic insecticides may be more effective but can have negative impacts on beneficial insects and pollinators.

For more information see UC Pest Note on Whiteflies.

These insects can be solitary or live in group nests above and below ground. Yellowjackets can be aggressive when defending their nests. Since they nest in ground holes or inside walls and ceilings, you might encounter them on a hike or in your home. Avoid the area when possible. Paper wasps on the other hand, are relatively unaggressive, but they can be a problem when they nest in a trafficked location.

When eating outdoors, keep foods well covered. One strategy is to put out bait such as a piece of meat or an opened soda can some distance from the table before setting out the human food. Or, use a lure trap around the area you want to protect. Trapping the queens in the spring and workers during the summer can reduce local populations. See the UC Pest Note on Yellowjackets and Other Social Wasps for more information.