Garden Guy: Looking For Alien Garden Visitors

Sunday, June 16, 2024

Alien garden visitors
By Bruce Bennett

As the days lengthen and the weather warms, newspapers and magazines are filled with articles and pictures of the newest plants coming into the retail marketplace and designs for sun and shade. By contrast, whenever my group of professional gardening colleagues get together for an alfresco cup of morning coffee and we discuss warming weather, we’re usually discussing the latest warm weather waves of invasive insects that may begin chewing on those newest plants in the very near future.

In North America, more than 3,400 non-native insect species are established (including, of course, the beneficial European honeybee). Fortunately, only about 10 percent of these non-natives are considered ‘invasive’ meaning they cause significant damage to native ecosystems and agricultural crops. 

Naturally, the Pacific Northwest is not immune to the winged beasties and we do our best to combat the culprits who already call the Pacific Northwest home as well as prepare for the hitch-hiking interlopers that are on their way here to chow-down on our juicy plants. As many of these insect infestations are initially found by homeowners and gardeners during the summertime, let’s use this month’s column to update you about which terrors to watch for during the early summer weather.

Japanese Beetle. Photo by

One of the newest invaders to Washington State is the Japanese Beetle (Popillia japonica). They were first noticed in New Jersey around 1916. They have since become established in just about every state east of the Mississippi River and have been making their way to some of the western states. 

Their first Washington appearance was in Sunnyside in 2021 and, this year, reached Yakima. The Cascade Mountains will pose a barrier to them, but, as with the other insects mentioned on this list, it’s only a matter of time until they hitchhike a ride on a commercial vehicle or RV and reach the western part of the state. 

They are most noticeable in their adult forms as fingernail-sized beetles with glistening green and copper colors to their wing shields. They look like bronze scarabs which could easily serve as jewelry for an Egyptian costume. But ours will leave skeletonized leaves in their wakes; LOTS of skeletonized leaves. Why is it always the pretty ones that do the most damage?

In winter and spring, the beetle larvae attack the roots of plants, particularly the roots of grasses. Roses are a particular delicacy for them. These insects were, and still are, the scourge of my Connecticut garden. 

Plants already stressed by our hotter summers may not survive with the added pressure of infestations from these guys. They have been a problem on the east coast and in the midwest for decades. We haven’t seen them in Washington before, but, with our new warmer temperatures, they have recently been found in Idaho, at a few sites south of Portland and near Yakima. So, it’s only a matter of time, I fear.

Depending on their life stage, Japanese Beetles can be controlled in a few ways. With small populations of adult insects, hand-picking, pheromone traps, insecticidal soaps and Neem Oil can be effective in June – September. 

The beetles lay their eggs in early summer, so, in July – September, using HB (Heterorhabditis bacteriophora) beneficial nematodes, the root-feeding grubs can be located and killed. Other options for grubs are BT (Bacillus thuringiensis) which is, to a grub, a stomach poison and Milky Spoor (Bacillus papillae), a bacterium that interrupts the insects’ blood system.

Emerald Ash Borer. Photo by

The Emerald Ash Borer
(Agrilus planipennis) is a small Asian, wood-boring beetle that has already killed millions of ash trees in North America. First discovered in 2002, near Detroit, MI, the adults will be noticed as narrow, half-inch long slivers of metallic green on tree bark. The larvae burrow under the tree’s bark and eat the sapwood which transports water and nutrients throughout the tree. 

Once damaged, the layers can’t transport nutrients causing the leaves and the trees to gradually die. While not yet in Washington, the EMBs were recently noticed just west of Portland, OR. There are a few systemic insecticide soil drenches which have been found to be fairly effective against these borers. However, they do need to be applied every other year.

Spotted Lanternfly. Photo by

Probably, the most colorful insect of the invasive hordes is the Spotted Lanternfly (Lycorma delicatula). Its red, white and black colors remind me of a masked kabuki dancer (or a member of the Sith for you Star Wars fans), although the youngsters are black at first with white spots). 

Another of the vampire-like piercing-sucking drinkers, the Lanternfly hails from eastern Asia. It was first noticed in Pennsylvania in 2014 and has now spread through the mid-eastern coast states and is appearing in California and southern Oregon.

Lanternflies feed on a wide variety of plants, with apples, cherries, grapes, plums and hops being among their smorgasbord preferences. Not good news for home or commercial growers. 

Also, like birch borers and aphids, Lanternflies excrete large amounts of honeydew, which can cover lower plants and promote the growth of sooty mold. And, I don’t even want to imagine what they can do to the outside of a car parked under a ravaged tree. 

For smaller infestations, organic solutions such as insecticidal soaps, Neem and botanical oils appear to be effective. For larger problems, think about using contact poisons and systemic drenches. In both instances, contact the appropriate State agency.

Spongy Moth / Gypsy Moth. Photo by

The Spongy Moth, AKA, Gypsy Moth, (Lymantria dispar) have been in-country for many years. Originally native to Eurasia, and unlike most invasive insect pests, this fuzzy beastie did not stow away on some cargo ship and escape to fertile New England forests. It was actually imported to Boston in 1868 by several entrepreneurs who were interested in developing a silk industry in North America. 

Instead of serving as the foundation of a new business venture, the Spongy Moth has become one of the worst American forest pest insects. It devours the leaves of more than 500 different species of trees and shrubs and causes enormous damage to the environment and the economy. 

Following the banning of DDT, the Spongy Moth reached some of its greatest population numbers, culminating in my home state of Connecticut with the extensive outbreaks of 1971 and 1981. Those outbreaks are legendary in both forestry and arboricultural circles. A wide range of tree species were stripped clean and the woods were turned bare due to the extensive feeding of these insects. The caterpillars covered houses, roads, and sidewalks. 

They were even blamed for traffic accidents, as people would skid on their massed numbers in the roadways.

The Spongy Moth appears in Washington every now and then after hitching a ride on some unsuspecting vehicle, RV or ship. The Department of Natural Resources is currently spraying newly found infestations in different parts of the State. 

While municipalities continually check for infestations, gardeners and homeowners can also do their part. Spongy moth caterpillars can be controlled with applications of the biological insecticide, Bacillus thuringiensis kurstaki (Btk). It must be applied to the foliage of the plants the caterpillars are feeding upon and will control them while they are still small and becoming less effective as they grow larger. 

This bacterial insecticide kills caterpillars that eat it within a week of its application by damaging the insect’s digestive tract. Like Btk, certain foliar spray products can pose fewer risks to other insects. This group includes insecticidal soaps, horticultural oils, pyrethrins, and spinosad. The soaps, oils, and pyrethrins only kill when sprayed directly onto spongy moth caterpillars and are very short-lived. 

While they can be a good option for smaller plants, these ingredients are not a good option for larger trees.

Giant Asian Hornet / Murder Hornet. Photo by

While more commonly known as the Giant Asian Hornet (Vespa mandarinia), this publicized ‘Murder Hornet’ (cue the music from ‘Jaws’) ranks as the world’s largest hornet and can grow to two inches long, with a wingspan of some three inches. 

These hornets need meat to feed their young and they are pretty direct about getting it. Among protein sources, they prefer honeybees. A single Murder Hornets can decimate a honeybee hive in a day. They do so by biting the heads off the much smaller honeybees. 

Unlike honeybees, Murder Hornets can sting multiple times and have venom several times more potent than local bees and wasps. Fortunately, they are not terribly aggressive around humans unless their nest seems threatened. 

They arrived in Washington in 2020 and, thus far, they have only been sited and exterminated north of us in Bellingham, Blaine and British Columbia. This insect, with its half-inch stinger, is not something gardeners should fool around with. The best eradication advice is to contact the Washington State Department of Agriculture or the Washington Invasive Species Council and let the pros handle the problem.

Brown Marmorated Stink Bug

A Brown Marmorated Stink Bug (Halyomorpha halys) is another native of East Asia and was first introduced in Allentown, PA in 1998, possibly having arrived in a shipping crate. It arrived in Washington in 2010. These half-inch long suckers will attack a large variety of plants, including many fruits and vegetables. It leaves small necrotic patches on any plant it eats, rendering produce inedible. 

As a homeowner, you will likely notice an invasion before anyone else, because this stink bug initially will attack vegetable gardens and landscape plants. The damage they do to crops and landscapes, as well as the efforts to control them are costly. These beasties will then spend the winter in homes and other structures. 

Look for them on the south side of your homes as the weather cools. Presently, there are no viable strategies for control of the Brown Marmorated Stink Bug, although insecticidal soaps and neem oil seem to be effective against stink bugs, especially early in the season. Remember to read and follow the directions and safety precautions on the labels.

It is homeowners, campers and hikers who are usually the first to notice all of these new invasive insects. Trees, lights and standing water are great places to look for insects. Checking yards regularly plays a critical role in protecting Washington’s unique environment from the pests. Citizens can help spot infestations when they are easy to handle. 

This not only saves time and money, but limits the damage these beasties can cause to our gardens, local and national parks and farms and forests. Residents who find a suspected invasive insect are encouraged to take a picture and report it via the Washington Invasive Species Council or the Washington Department of Agriculture. Be aware of what organisms, beyond the plants, are living in your little pieces of verdant heaven.

Finally, with the coming of May, most of the King County Master Gardener Clinics opened to assist the residents of their surrounding communities. 

For the readers of the Shoreline Area News, the closest the closest clinics may be found at the 
  • Lake Forest Park Town Center Ace Hardware (17171 Bothell Way NE, Sundays, 10:00 AM – 2:00 PM) and the 
  • Shoreline Farmers Market (18821 Aurora Avenue N, some Sundays, 10:00 AM – 2:00 PM). 

Happy gardening all!

Further Reading
  • Bradley, F.M. et. al. Natural Pest and Disease Control. 2009. New York, NY: Rodale, Inc. 
  • Cranshaw, Whitney. Garden Insects of North America. 2018. Princeton, NJ: Princeton University Press. 
  • Entomology Society of America Web Page, 
  • Washington Invasive Species Council Web Page, 
  • Washington State Department of Agriculture Web Page,

Bruce Bennett
Gardening columnist, Bruce Bennett, is a WSU Certified Master Gardener, lecturer and Seattle-area garden designer. 

If you have questions concerning this article, have a gardening question or two to ask concerning your home landscape or want to suggest a topic for a future column, contact Bruce at

Previous Garden Guy articles can be seen here.


North City Reader,  June 17, 2024 at 12:45 PM  

i always enjoy reading Bruce's column. The photo's in this one are amazing! Thanks for the heads up on these wicked invaders!

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