Over 70 invasive insect species have been detected in Washington since 1990, and 36% were first spotted by everyday residents like you. Your backyard trees, garden vegetables, and neighborhood landscapes face constant threats from non-native insects that can devastate local ecosystems, agricultural industries, and property values.
The Washington State Department of Agriculture monitors more than 130 invasive pests annually, but early detection depends on community awareness and quick reporting.
You can protect Washington’s environment and economy by learning to identify these damaging invasive insects. Each species profiled below threatens specific plants, trees, or crops across the state, and knowing what to look for empowers you to take immediate action when you spot potential invaders.
1. Emerald Ash Borer
The emerald ash borer (Agrilus planipennis) represents one of the most serious threats to Washington’s urban and rural forests. This small Asian beetle attacks and kills ash trees by burrowing larvae under the bark to feed on sapwood, disrupting the tree’s ability to transport water and nutrients.
Key Insight: The emerald ash borer has been detected in both Portland, Oregon to the south and Vancouver, British Columbia to the north, putting Washington directly in its expansion path. Detection here could happen at any time through transported firewood or shipping materials.
You’ll recognize adult emerald ash borers by their metallic green color and elongated body shape, measuring about half an inch long. The most telltale sign of infestation appears as D-shaped exit holes in ash tree bark, with one side rounder and the other flatter. Heavy woodpecker damage on ash trees also signals possible borer presence, since woodpeckers actively feed on the larvae beneath the bark.
The larvae cause the actual tree damage by creating serpentine galleries under the bark as they feed. Infested trees show crown dieback starting at the top, with branches becoming sparse and leafless over one to four years. You might also notice vertical bark splits and increased sprout growth from the trunk base as the tree attempts to survive.
Never transport firewood from area to area, as this represents the primary method of emerald ash borer spread. Buy firewood where you’ll burn it, or gather it on-site when permitted. If you suspect emerald ash borer activity on your property, photograph the evidence and report it immediately to the Washington Invasive Species Council.
2. Japanese Beetle
The Japanese beetle (Popillia japonica), native to northern Japan, attacks more than 300 plant species including roses, grapes, and hops. Both adult beetles and larvae cause significant damage, making this pest particularly destructive to Washington’s ornamental landscapes and agricultural operations.
Adult Japanese beetles measure about half an inch long with distinctive metallic green bodies and copper-colored wing covers. You’ll notice five small patches of white hair along each side of the abdomen and two larger white hair patches on the rear end. These beetles feed on plant leaves during daylight hours, creating a characteristic skeletonized appearance where only leaf veins remain.
Important Note: The Washington State Department of Agriculture found the first two Japanese beetles in Washington in 2020, and detection numbers have increased in recent seasonal pest surveys. The species is actively establishing populations across the state.
The larval stage, appearing as white C-shaped grubs, feeds on grass roots underground. You’ll recognize larval damage as brown, dead patches in lawns that pull up easily like carpet since the roots have been consumed. The grubs measure up to one inch long with three pairs of legs near the head and a V-shaped pattern of spines on their rear end.
Japanese beetles aggregate while feeding, so finding one often means finding many. They release pheromones that attract other beetles, leading to mass feeding events that can defoliate plants rapidly. Check your roses, grape vines, and fruit trees regularly from late June through September when adults are most active.
Report any Japanese beetle sightings through the WSDA online reporting form with clear photographs showing the distinctive markings. Early detection helps officials implement control measures before populations explode.
3. Brown Marmorated Stink Bug
Brown marmorated stink bugs (Halyomorpha halys) invaded Washington from Asia and established populations by 2014. These agricultural and garden pests feed on a wide variety of fruits and vegetables by piercing plant tissue and sucking out juices, leaving behind damaged, unmarketable produce.
You’ll identify adult brown marmorated stink bugs by their shield-shaped bodies measuring about three-quarters of an inch long. They display mottled brown coloring on top with alternating light and dark bands on the antennae and edges of the abdomen. The smooth shoulder edge when viewed from above distinguishes them from native stink bug species.
These insects pierce fruits and vegetables with their needle-like mouthparts, creating corky, dimpled areas on apples, pears, tomatoes, and peppers. The damage extends beyond the visible surface marks, as feeding causes internal breakdown that makes produce inedible. You’ll also find them feeding on tree fruits, berries, legumes, and corn.
Pro Tip: Brown marmorated stink bugs seek shelter inside homes during fall and winter, congregating in attics, wall voids, and protected areas. They don’t reproduce indoors but their presence creates nuisance problems, and crushing them releases a strong, unpleasant odor.
The nymphs progress through five stages before reaching adulthood, appearing rounder and darker than adults. Eggs appear in clusters of 20-30 on leaf undersides, resembling tiny white barrels arranged in neat rows. Check your garden plants carefully from May through September when populations peak.
Vacuum up indoor stink bugs rather than crushing them to avoid the characteristic odor. Seal entry points around windows, doors, and foundation cracks before fall temperatures drop. Outside, monitor your vegetable gardens and fruit trees for feeding damage and report heavy infestations to help track the species’ spread across Washington.
4. European Paper Wasp
European paper wasps (Polistes dominula) arrived in Washington from Europe and now compete with native paper wasp species. While they provide some pest control by feeding caterpillars to their larvae, their aggressive nesting behavior near human structures creates safety concerns.
These wasps measure about three-quarters of an inch long with yellow and black markings that resemble yellowjackets, but their slender waist and dangling legs during flight clearly distinguish them. European paper wasps have bright yellow faces and antennae that are orange on the underside. Their wings fold lengthwise when at rest, creating a narrow profile.
You’ll find their distinctive umbrella-shaped paper nests attached to eaves, porch ceilings, tree branches, or other protected horizontal surfaces. The gray papery nests lack an outer covering, so you can see the hexagonal cells where larvae develop. Colonies range from a few dozen to several hundred individuals by late summer.
European paper wasps defend their nests aggressively when disturbed, and unlike honeybees, each wasp can sting multiple times. They’re most active during warm daylight hours when workers forage for caterpillars, nectar, and other food sources. The wasps become particularly defensive in late summer as colonies reach peak size.
Common Mistake: Attempting to remove active nests yourself without proper protective equipment and knowledge can result in multiple stings. Contact professional pest control services for nest removal, or wait until winter when colonies die off naturally, leaving only mated queens that overwinter elsewhere.
Monitor your property in early spring when queens begin building new nests. A single founding queen is much easier to manage than an established colony. If you discover a small, early-season nest in a problematic location, wearing protective clothing and removing it at night when the queen is less active minimizes risk.
5. Gypsy Moth / Spongy Moth
The spongy moth (Lymantria dispar) ranks among the worst forest pest insects in North America, devouring leaves from more than 500 tree and shrub species. Washington has experienced more Asian gypsy moth introductions than any other U.S. state, with egg masses arriving attached to ships carrying cargo from Asian ports.
European gypsy moths were first detected in Washington in 1974, and rapid response efforts have prevented permanent establishment. However, the Asian variety poses greater threats since females can fly up to 12 miles and feed on a broader range of host trees, including many conifer species that the European variety ignores.
Adult male moths appear brownish-gray with darker wavy bands across their wings, measuring about one and a half inches wide. Females are larger, appearing white to cream-colored, but European gypsy moth females cannot fly. The caterpillars cause the actual damage, featuring distinctive paired blue dots followed by red dots along their backs, with long hairs giving them a fuzzy appearance.
You’ll notice spongy moth damage as extensive defoliation during late spring and early summer. Heavy infestations can strip entire trees bare, weakening them and making them vulnerable to disease and other pests. The caterpillars produce extensive amounts of frass (insect droppings) that accumulates under infested trees and creates nuisance problems.
Key Insight: The Washington State Department of Agriculture sets thousands of traps statewide each year specifically to monitor for spongy moth introductions. Public reports remain crucial since traps cannot cover every location where egg masses might arrive.
Egg masses appear as tan or buff-colored fuzzy patches about one to two inches long on tree trunks, outdoor furniture, vehicles, or any protected surface. Each mass contains 500-1,000 eggs that hatch in spring. Scraping egg masses into soapy water destroys them before caterpillars emerge.
Check outdoor items before moving them between locations, especially when traveling from areas with known spongy moth populations. Report any suspected sightings immediately, as early detection enables eradication before populations establish and spread.
6. Cabbage White Butterfly
Cabbage white butterflies (Pieris rapae) invaded from Europe and now rank among the most common butterflies across Washington. While their delicate appearance seems harmless, the caterpillar stage devastates cole crops including cabbage, broccoli, cauliflower, and kale in home gardens and commercial operations.
You’ll recognize adult cabbage white butterflies by their white wings with black tips on the forewings. Females display two black spots on each forewing, while males have only one. These butterflies measure about two inches across and flutter actively through gardens from early spring through late fall.
The pale green caterpillars cause the actual damage, blending remarkably well with host plant foliage. They feed on leaves, creating large irregular holes and contaminating heads of cabbage and broccoli with frass. You’ll find them tucked between leaves or boring into the tight heads of developing vegetables.
Cabbage white butterflies complete multiple generations per year in Washington’s climate, so populations build throughout the growing season. Early generation adults emerge in March or April, with subsequent generations appearing every four to six weeks. Peak damage typically occurs in mid to late summer when caterpillar populations are highest.
Pro Tip: Inspect the undersides of cole crop leaves regularly for small, bullet-shaped yellow eggs standing upright on tiny stalks. Removing leaves with eggs prevents caterpillars from hatching and causing damage. Row covers installed at planting time exclude butterflies entirely while allowing air, water, and light penetration.
Handpicking caterpillars provides effective control in small gardens. Look carefully since their coloring provides excellent camouflage. Organic options like Bacillus thuringiensis (Bt) target caterpillars specifically without harming beneficial insects when applied as caterpillars actively feed.
7. Onion Maggot
Onion maggots (Delia antiqua) attack plants in the Allium genus, including onions, garlic, leeks, and chives. These fly larvae tunnel into bulbs and stems, causing significant losses in home gardens and commercial onion production across Washington.
Adult onion maggot flies resemble small house flies, measuring about one-quarter inch long with gray bodies. You’ll rarely notice them since they stay close to the ground and don’t draw attention. The flies emerge in spring when soil temperatures warm, with peak activity occurring during cool, wet weather that onions prefer.
The white, legless maggots cause damage by tunneling into onion bulbs and feeding on plant tissue. Infested plants show yellowing, wilting leaves that collapse as if suffering from drought despite adequate moisture. When you pull affected plants, you’ll discover soft, rotting bulbs honeycombed with tunnels and often filled with maggots.
Important Note: Onion maggots complete multiple generations per growing season in Washington, so vigilance throughout spring, summer, and fall protects your crops. Second and third generation flies emerge from pupae in the soil to attack new plantings or developing bulbs.
Young seedlings are particularly vulnerable, with maggots killing plants before they establish. Older plants survive longer but produce unmarketable bulbs riddled with damage and secondary rot infections. The tunneling also creates entry points for bacterial and fungal diseases that compound the damage.
Rotate your onion plantings to different garden areas each year to break the life cycle. Onion maggot pupae overwinter in soil where onions grew the previous season, so moving crops forces emerging flies to search farther for host plants. Apply row covers immediately after planting to exclude adult flies during egg-laying periods.
Proper timing matters significantly for control. Flies lay eggs at the base of plants during active growing periods, so monitoring weather patterns helps predict when adults become active. Cool, overcast days with temperatures in the 50s-60°F create ideal conditions for fly activity and egg-laying.
8. Cabbage Maggot
Cabbage maggots (Delia radicum) attack the same cole crop family as cabbage white butterflies but cause damage in an entirely different way. These root-feeding larvae tunnel into stems and roots, causing plants to wilt and die despite adequate moisture.
The adult flies closely resemble onion maggot flies, appearing as small gray flies about one-quarter inch long. They emerge in early spring, often before gardeners transplant cole crops, and lay white eggs in soil around plant bases. The larvae hatch within a week and immediately burrow into roots and lower stems.
You’ll notice cabbage maggot damage as stunted, off-color plants that wilt during warm days even when soil is moist. Pulling affected seedlings reveals roots riddled with brown tunnels and white maggots measuring up to one-third inch long. The damage allows secondary organisms to enter, causing soft rot that completely destroys roots.
Multiple generations occur each growing season, with spring and fall generations causing the most significant damage during cool weather. Summer heat reduces activity somewhat, but third-generation flies emerge in late summer to attack fall plantings of broccoli, cauliflower, and cabbage.
Common Mistake: Applying fertilizer and water to wilting plants without checking for root maggots wastes resources and doesn’t address the underlying problem. Always examine roots of struggling cole crops to rule out maggot damage before assuming nutritional or water stress.
Cut collars made from cardboard, tar paper, or foam placed around transplant stems at soil level prevent adult flies from laying eggs near plants. The collars should extend at least two inches in diameter around each stem. This simple physical barrier significantly reduces infestation rates.
Floating row covers installed at planting and sealed around edges exclude adult flies completely. Remove covers when temperatures consistently exceed 75°F or when plants need pollination. Strategic timing of plantings to avoid peak fly emergence periods also reduces damage risk.
9. Hessian Fly
Hessian flies (Mayetiola destructor) target wheat and other grain crops, making them a significant concern for Washington’s agricultural industry. These tiny flies cause millions of dollars in damage annually across grain-growing regions.
Adult Hessian flies measure only about one-eighth inch long with long legs and dark, mosquito-like bodies. They’re weak fliers that remain near wheat fields. Females lay reddish eggs in grooves on the upper surface of wheat leaves, with each female depositing 250-300 eggs during her short lifespan.
The larvae cause damage by feeding between leaf sheaths and stems, weakening plants structurally. Heavy infestations cause wheat stems to break at feeding sites, lodging entire fields before harvest. Even moderate damage reduces grain yield and quality significantly. Infested seedlings often die before tillering, creating bare patches in fields.
You’ll recognize Hessian fly damage in wheat fields by stunted plants with dark green leaves that appear thicker and more rigid than normal. Stems break easily at or just above ground level, revealing small brown pupae called “flaxseed” that resemble flax seeds in size and color adhering to stems beneath leaf sheaths.
Key Insight: Hessian fly populations fluctuate dramatically between years depending on weather conditions and cropping practices. Warm, dry springs favor fly development, while wet, cool conditions reduce survival rates.
Planting wheat varieties with genetic resistance provides the most effective control. Resistant varieties contain genes that kill young larvae shortly after feeding begins. The Washington State University Extension publishes annual recommendations for resistant varieties suited to different growing regions.
Delaying fall planting until after fly-free dates reduces egg-laying opportunities. Adult flies emerge for a limited period, so planting after peak emergence means fewer flies available to infest new wheat seedlings. Crop rotation and destruction of volunteer wheat plants also break the life cycle.
10. European Earwig
European earwigs (Forficula auricularia) invaded from Europe and now occur throughout Washington. While they consume some pest insects and decaying plant material, their feeding damage on flowers, vegetables, and fruit makes them problematic garden invaders.
You’ll instantly recognize European earwigs by the prominent forceps-like pincers at their rear end. Adults measure about three-quarters of an inch long with reddish-brown bodies and short wings that rarely enable flight. Males have curved pincers while females have straighter pincers. Despite their intimidating appearance, the pincers cannot harm humans significantly.
Earwigs are nocturnal feeders that hide during daylight hours in mulch, under boards, in crevices, and other dark, moist locations. They emerge at night to feed on soft plant tissue, flower petals, ripening fruit, and seedlings. You’ll notice irregular holes in leaves and flowers, with corn silk and ripening fruit showing characteristic feeding damage.
These insects aggregate in groups, so finding one often means finding dozens nearby. They particularly favor roses, dahlias, zinnias, and other ornamental flowers, eating petals and creating ragged, unsightly blooms. Sweet corn suffers damage when earwigs feed on developing kernels at the ear tip.
Pro Tip: Rolled-up newspapers, bamboo tubes, or corrugated cardboard placed in gardens overnight provide effective trapping sites. Earwigs crawl into these dark, tight spaces to hide during daylight. Shake trapped earwigs into soapy water each morning to kill them, then reset traps for continued monitoring.
European earwigs thrive in moist conditions with abundant organic matter and mulch. Reducing excessive mulch around vulnerable plants and eliminating hiding places near building foundations decreases populations. Remove boards, pots, and debris that create perfect daytime shelter.
Despite being considered invasive, European earwigs do provide some beneficial pest control by feeding on aphids, insect eggs, and small larvae. This makes management decisions context-dependent based on whether benefits outweigh damage in your specific situation.
Conclusion
Early detection through community awareness represents your most powerful tool against invasive insects. The Washington State Department of Agriculture and Washington Invasive Species Council depend on reports from residents like you to catch new invasions before they establish and spread.
Download the WA Invasives mobile app to report sightings directly from your smartphone. Include clear photographs showing distinctive features, note the exact location, and describe any damage you observe. Even if you’re uncertain about an identification, submit the report—experts review all submissions and understand that some invasive species closely resemble native look-alikes.
Remember that “it only takes one” introduction to cause massive damage across Washington’s forests, agricultural lands, and urban landscapes. Your vigilance protects your property value, local ecosystems, and the state’s agricultural economy worth billions annually. Check your trees, gardens, and outdoor areas regularly, especially during late summer when many invasive insects emerge as adults.
Keep firewood local by buying or gathering it where you’ll burn it, clean outdoor gear and vehicles when traveling between regions, and inspect plants carefully before bringing them home from nurseries. These simple prevention practices reduce the likelihood of accidentally transporting invasive insects to new locations.
Your role as a citizen scientist makes you an essential partner in keeping Washington the Evergreen State. Report suspected invasive insects through invasivespecies.wa.gov/report-a-sighting and help protect your community from these damaging invaders.
