Pennsylvania is home to some amazing wildlife and plants, but it’s also got a sneaky bunch of troublemakers who’ve taken over.
These invasive species don’t just show up for a visit. They crash the party, hog all the food, mess with native critters, and sometimes cause big headaches for farmers, gardeners, and city folks alike.
From birds that swoop in like feathery gangsters to bugs that chew everything in sight, here’s a look at 25 invaders shaking things up across Pennsylvania… in ways that’ll surprise you.
Feisty Invaders in the Bird World
European Starling – The Shakespearean Stowaway
- Big flocks = big crop pressure: They can hit orchards, grain, and livestock areas hard.
- Disease concerns: Droppings around buildings and barns can create health and cleanup issues.
- Nest bullies: They compete aggressively with native cavity nesters.
Imagine someone released birds just because Shakespeare mentioned them. That’s how the European starling arrived in the 1890s.
These shiny black birds now form huge flocks called murmurations, thousands swirling together in an incredible sight.
They raid grain, chase off native birds like bluebirds, and their droppings can create messy, expensive problems in towns and farm country across Pennsylvania.
House Sparrow – The Brown Bully
- Nest hoggers: They push out native bluebirds and wrens.
- Fast population growth: Multiple broods can turn “a few birds” into a whole neighborhood crew.
- Mess makers: Nests can clog vents, signs, and building nooks.
This small brown bird arrived in the 1850s and became a hard-nosed city dweller that never really leaves.
House sparrows take over nest boxes and prime spots under eaves. They live off crumbs, birdseed, and anything humans accidentally serve up.
In Pennsylvania, they’re one of those birds you see everywhere… and once they claim a spot, they don’t give it up politely.
Rock Pigeon – The “City Rat” with Wings
- Messy roosts: Droppings stain, smell, and are costly to clean.
- Health concerns: Dried droppings can be a respiratory irritant, especially in enclosed areas.
- Built for buildings: Ledges, bridges, and rooftops are basically pigeon condos.
You know these gray pigeons hanging around city squares, strutting on statues, and gobbling fries. Originally from Europe, many escaped and made cities like Philadelphia and Pittsburgh their home.
Their droppings corrode buildings and monuments, and their nests can clog gutters and create maintenance problems on rooftops.
Pigeons are tough, loyal to their roosts, and nearly impossible to “shoo away” for good without real management.
Mute Swan – The Elegant Bully of the Waters
- Plant demolishers: They consume and uproot lots of aquatic vegetation.
- Territorial brawlers: They chase off native waterfowl.
- Human conflicts: They can get aggressive around nests and chicks.
At first glance, mute swans look like graceful pond ornaments. But don’t be fooled. These huge white birds eat and uproot massive amounts of aquatic plants, wrecking habitats fish and native waterfowl depend on.
They’re territorial to the point of danger, chasing off native swans, geese, and ducks, and sometimes going after people who get too close to nests. In Pennsylvania, that can mean trouble on popular lakes, parks, and quiet backwater coves where folks kayak and fish.
They’re pretty… but ecologically, they can be a wrecking ball in the wrong waters.
Eurasian Collared-Dove – The Suburban Settler
- Non-native newcomer: A dove introduced outside its native range that has expanded across the U.S., including Pennsylvania.
- Human-friendly: Thrives around neighborhoods, farms, and bird feeders.
- Space taker: Can compete with native doves and other birds for food and nesting spots.
If you’ve heard a steady “coo-COO-coo” in a parking lot or subdivision and thought, “That doesn’t sound like a mourning dove,” you might be right.
Eurasian collared-doves do well around people, and that’s exactly why they spread. They’re comfortable in the same places we build (edges, lots, barns, and feeder-heavy backyards).
They’re not loud trouble like geese or starlings, but they’re a classic “quiet invader” that slowly becomes part of the local bird mix at native species’ expense.
Bugs That Bite Back
Emerald Ash Borer – The Tiny Tree Killer
- Tree assassin: Infestations can kill ash trees in just a few years.
- Neighborhood changer: Whole streets can lose shade canopy fast.
- Big bills: Removal and replacement can be expensive for towns and homeowners.
This shimmering green beetle might be small, but it has caused massive destruction by killing ash trees across Pennsylvania.
Its larvae burrow under bark, cutting off the tree’s nutrient flow. By the time you notice thinning leaves and dead branches, the damage is often already advanced.
It’s one of the biggest “before and after” invasions PA neighborhoods have seen in a long time.
Asian Longhorned Beetle – The Hardwood Threat
- Hardwood menace: Attacks maples and other valuable trees.
- Big exit holes: Round holes and sawdust-like frass can be warning signs.
- Hard to control: Eradication efforts can require removing infested trees.
This black-and-white spotted beetle looks exotic but spells disaster for maples, elms, and other hardwoods.
Once it gets into a tree, it tunnels and weakens the structure from the inside out.
That’s why it’s treated like a “drop everything” pest when found, because letting it spread can mean losing a lot of the trees people count on for shade, value, and wildlife habitat.
Japanese Beetle – The Garden Nightmare
- Plant-eating machines: Chew ornamentals and crops.
- Lawn destroyers: Grubs can damage turf roots.
- Hard to beat: Adults can fly in from nearby areas even if you treat your own yard.
With shiny green heads and copper wings, these beetles swarm in summer to munch on roses, grapes, corn, and a long list of landscape plants.
They skeletonize leaves, leaving that “lacy” look that screams trouble. Meanwhile, the larvae (grubs) chew turf roots underground, sometimes leaving brown patches that look like drought, until a skunk digs it up like a buffet.
Spongy Moth – The Forest Caterpillar Army
- Caterpillar rain: Heavy infestations can feel like leaves and droppings falling nonstop.
- Tree buffet: Defoliates many tree species, especially oaks.
- Hitchhiker eggs: Egg masses can spread on outdoor items and firewood.
Formerly known as the “gypsy moth,” these caterpillars can swarm and devour leaves across huge areas, especially where oaks are common.
One bad year is rough. Multiple bad years can weaken trees enough to make them vulnerable to disease and other pests. Add in itchy hairs and the “caterpillar crunch” sound, and it’s a Pennsylvania summer memory nobody asks for.
Brown Marmorated Stink Bug – The Smelly Pest
- Crop damage: Can scar and ruin fruit and vegetables.
- Unwelcome houseguest: Moves into buildings in fall to overwinter.
- Stink defense: That odor is real, and it lingers.
This brown bug doesn’t just stink, it can damage fruit crops and garden plants. When the weather cools, it slips into homes and garages by the dozens, looking for warmth.
They don’t bite, but squish one, and you’ll regret it. The safest move is usually gentle removal (or vacuum methods people use for pests), because the smell is basically their whole personality.
Mammal Mayhem: Non-Bird, Non-Insect Invaders
European Rabbit – The Lawn-and-Garden Grazer
- Non-native browser: Introduced rabbits can thrive around people.
- Landscape damage: Chews shoots, flowers, and young shrubs.
- Fast growth: Populations can build quickly where food and shelter are easy.
Backyard rabbits feel harmless… until you realize your landscaping is getting trimmed every night by something with no appreciation for what those plants cost.
European rabbits are non-native, and in the right suburban conditions, they can do very well; plenty of hiding spots, lots of tender plants, and not enough natural pressure to keep numbers low.
If you’ve got neat, angled “snips” on seedlings and flowers that vanish overnight, this one belongs on your suspect list.
Norway Rat – The Urban Survivor
- Super breeders: Large litters and fast cycles make them hard to suppress.
- Disease concerns: Can carry pathogens that matter to humans and pets.
- Damage dealers: Chew wiring, wood, and infrastructure.
The brown Norway rat is the ultimate city slicker, living anywhere humans provide food and shelter.
They chew through wood, pipes, and wiring, and they don’t need a “dirty” place to move in, just one easy food source and one hidden route in.
In Pennsylvania, big cities and small towns alike deal with them because the rats live where we live.
House Mouse – The Quiet Indoor Invader
- Non-native and widespread: Lives alongside people almost everywhere.
- Contamination risk: Droppings and urine can create health and cleanup issues.
- Hard to spot: Often discovered after damage, noise, or that “mouse smell” shows up.
The house mouse is a small animal with a huge footprint. It’s non-native, it’s adaptable, and it’s one of the most common “I didn’t know they were here” invasives in Pennsylvania homes, barns, and sheds.
They slip through tiny gaps, build nests in insulation and storage, and can turn a pantry into a nightly snack bar.
They’re not flashy like a snakehead or a stink bug invasion, but they’re relentless, and they spread fast when conditions are right.
Jumping Worms – The Soil Shakers
- Leaf litter vacuum cleaners: They can strip the forest floor’s natural mulch layer.
- Soil structure changers: Soil can become dry, loose, and “coffee grounds” textured.
- Hard to reverse: Once established, they’re very difficult to eliminate.
These worms don’t wiggle, they jump! Native to Asia, they’re spreading in Pennsylvania gardens and forests.
Unlike typical earthworms that mix soil, jumping worms chew through leaf litter fast, leaving soil looking like coffee grounds and making it harder for many native plants to get established.
They hitchhike on potted plants, mulch, and garden tools, so prevention is the real “best weapon” here.
Feral Cats – The Controversial Hunters
- High-impact predator: Outdoor cats kill birds, small mammals, and reptiles.
- Disease concerns: Can spread or carry diseases that affect wildlife, pets, and people.
- Management dilemma: Welfare and conservation concerns often collide here.
Feral cats may look like “just cats,” but ecologically they behave like a non-native, free-roaming predator( because that’s exactly what they are).
In Pennsylvania, feral cat colonies can hit songbirds and other small wildlife hard, especially near parks, greenways, and suburban edges where wildlife already has limited safe space.
One of the most practical, Extension-friendly messages here is simple: keeping pet cats indoors protects wildlife and protects the cats.
Trouble in the Trees and Fields: Invasive Plants Taking Over
Amur Honeysuckle – The Understory Thief
- Dense thickets: Crowds out native plants.
- Early leaf-out: Steals sunlight before natives get going.
- Bird-spread seeds: Berries help it spread into woodlands and edges.
This shrub was introduced and widely planted, but now it can choke Pennsylvania woodlands with thick growth that shades out wildflowers and tree seedlings.
Birds spread the berries, which means it doesn’t just stay in yards; it shows up along trails, creek edges, and forest margins where it doesn’t belong.
Common Buckthorn – The Hedgerow Hog
- Wall of shade: Blocks sunlight and reduces plant diversity.
- Long leaf season: Outcompetes natives by leafing out early and holding leaves late.
- Persistent seeds: Birds spread berries widely, making reinfestation common.
Once popular as a tough hedge plant from Europe, buckthorn can take over woodlands with dense, dark thickets.
It gets a big seasonal advantage by leafing out early and staying green late, which means native seedlings get shaded out during the very weeks they need light the most.
Garlic Mustard – The Forest Floor Invader
- Spring carpet: Outcompetes wildflowers and seedlings.
- Soil disruption: Can interfere with soil fungi many natives rely on.
- Seed persistence: Seeds can remain viable for years.
This herb sneaks into forests and shaded edges, often popping up before native plants are fully awake in spring.
It spreads aggressively by seed, and once it gets established, it can push out the wildflowers that make Pennsylvania woods so beautiful in April and May.
Callery (Bradford) Pear – The Pretty Problem
- From yard to wild: Birds spread seeds far beyond landscaping.
- Thorny thickets: Wild offspring can form dense, prickly stands.
- Storm damage: Weak branch structure leads to breakage as trees age.
Famous for spring blooms, these trees were sold as “the perfect ornamental” until they started spreading into fields, roadsides, and open woods.
Wild Callery pear often grows thorns and forms dense patches that crowd out native plants. It’s one of those invasives that looks harmless… right up until it isn’t.
Japanese Knotweed – The Riverbank Wrecker
- Thicket builder: Forms dense stands that crowd out everything else.
- Streambank trouble: Can worsen erosion by replacing deep-rooted natives.
- Hard to kill: Small fragments can regrow and spread the infestation.
If you’ve ever walked a Pennsylvania creek, railroad edge, or roadside and seen tall “bamboo-looking” stalks taking over, you’ve probably met Japanese knotweed.
It forms dense walls that choke out native plants, and it spreads by fragments, meaning a small piece can start a whole new patch.
It’s a classic example of an invasive that thrives on disturbance: floods, mowed edges, construction sites, and anything that gives it an opening.
Invaders of Pennsylvania’s Waters: Fish, Mussels & Plants
Hydrilla – The Underwater Choker
- Mat maker: Forms thick underwater tangles that choke out native plants.
- Boat blocker: Snags propellers, fishing lines, and paddles.
- Spreads easily: Tiny fragments can start new infestations.
Hydrilla is one of those aquatic invasives that doesn’t “move in” quietly. It grows fast, forms thick mats, and can turn good boating water into a green tangled nightmare.
It spreads by fragments, so a little piece stuck to a trailer, prop, or paddle can help it hop from one Pennsylvania waterbody to the next.
That’s why “clean, drain, dry” matters, because hydrilla doesn’t need much help to spread.
Zebra Mussel – The Tiny Water Invader
- Infrastructure problems: Can clog pipes and water intakes.
- Native mussel impacts: Can smother native species.
- Sharp shells: Painful for swimmers and shoreline users.
These fingernail-sized mussels hitch rides on boats and equipment, then multiply fast once they’re established.
They can alter aquatic ecosystems by filtering huge amounts of water and changing food availability for native species, plus they create very real maintenance headaches for people.
Eurasian Watermilfoil – The Weed That Won’t Quit
- Boat trap: Tangles propellers and fishing lines.
- Native plant killer: Chokes out beneficial aquatic plants.
- Fragment spreader: Small pieces can start whole new infestations.
This underwater plant grows thick mats that clog boats and block sunlight from native aquatic plants.
It spreads by breaking into pieces, which can hitch rides on boats, quickly turning a clear Pennsylvania lake into a green mess.
Purple Loosestrife – The Pretty But Deadly Wetland Invader
- Wetland choker: Displaces habitat-forming native plants.
- Seed machine: Can produce huge numbers of seeds.
- Control options exist: Targeted biological control has helped in many areas.
Those tall purple flower spikes may look nice, but loosestrife can crowd out cattails and other native wetland plants, reducing habitat quality for birds, amphibians, and insects.
The good news is that long-term management is possible, and some areas have seen real progress with carefully chosen control methods.
Northern Snakehead – The Headline Fish Invader
- Aggressive predator: Eats fish, frogs, and other aquatic animals.
- Human-assisted spread: Can expand when people move or release live fish.
- Food chain disruption: Can pressure native fish communities where established.
Snakeheads are the kind of invasive that sound like a campfire story… until you realize they’re real. In Pennsylvania waters, the northern snakehead is a concern because it’s a tough, adaptable predator.
They eat other fish and amphibians, and when they get established, they can put new pressure on already-stressed ecosystems.
The Extension-friendly takeaway is simple: never move live fish, never dump bait, and never “release” aquarium or pond animals into the wild.
Pennsylvania’s invasive species story is one of surprises, challenges, and ongoing work to protect native ecosystems.
Whether it’s birds that outcompete natives, insects that damage trees and crops, or plants that take over forests and wetlands, these invaders are a reminder that small introductions can create big ripple effects.
Stay curious, stay aware, and if you spot any of these troublemakers, spread the word so we can keep Pennsylvania healthy and wild for generations to come.