By Andrew Scott, The Gardens on Spring Creek

If your garden has roses, grapes, hibiscus, hollyhocks, or cannas, they could be in for a world of hurt in the coming months. Active from mid-June through August, Japanese beetles (Popillia japonica) are an unfortunately persistent pest in Northern Colorado with a voracious appetite for several ornamental perennials, annuals, and food crops.

These destructive beetles first arrived in the United States in 1916, likely through a shipment of Japanese nursery stock that landed in New Jersey. It was thought they would remain an East Coast pest, unable to contend with the arid conditions of the West. While this was technically true, beetle eggs hitched yet another ride in nursery stock out west to where irrigated gardens and turf gave them the oases they needed. By the early 1990s, they were established in Colorado.

Adults emerge from the soil in mid-June, and populations peak in July. They are thankfully easy to identify; about the size of a fingernail, these scarabs have shiny green heads and midbodies, bronze wing covers, and white spots/tufts around their sides. They also conspicuously skeletonize plants, eating all the tissue between the leaf’s vasculature and leaving lacey ghosts. After they have fed and mated, a female beetle will lay about 50 eggs in moist turfgrass areas throughout her life. After hatching, grubs will feed on turf roots and cause dead patches, hibernate over winter, and pupate the following spring to start the whole cycle over again. 

So what can you do? As with all pest management, there are mechanical, chemical, and biological controls available. One commonly recommended method is to hand-pick beetles off plants in the morning, when they are less active, and knock them into a bucket of soapy water to drown. This is a great option for homeowners with small infestations. 

While traps are commercially available, I wouldn’t recommend them. The sexual pheromone they use works a little too well, attracting beetles from all over the neighborhood to your yard, where they’ll stop at the leafy buffet and mate with others. According to retired CSU entomologist Dr. Whitney Cranshaw, the idea that crushing beetles releases an attractant pheromone is a myth; they’re actually drawn to compounds the plant releases when it’s being munched.

For larger infestations, chemical applications come in liquid or granular formulations and can effectively control grubs in the soil. However, be sure to read the label and know the active ingredient you’re applying to your landscape. Insecticides based on diamides are generally safer to use around beneficial insects like bees visiting flowering turf weeds, but neonicotinoids and pyrethrins are lethal to all insects. While you can spray affected plants with chemicals, this method generally only controls beetles for a few weeks before degrading, allowing another population to move in and resume the damage.

If you don’t wish to use synthetics, Dr. Cranshaw notes that a bacterial strain, Bacillus thuringiensis var. galleriae (Btg), is on the market, but it also has a short effect.  Additionally, there is a bacterium that causes milky spore disease commercially available to control grub populations in lawns, but according to Dr. Cranshaw, its effectiveness is marginal at best. More promising is a fungus called Ovivesicula poppilliae (O.P.) that has around a 30% infection rate and gives beetles and larvae what is essentially a chronic kidney infection —one that increases overwinter grub mortality, shortens adult lifespan, and reduces reproductive capability. While this fungus isn’t yet commercially available, it is being utilized by municipalities up and down the Front Range. Some cities have also worked with the Colorado Department of Agriculture to release two species of wasps and a fly that parasitize Japanese beetles, though the establishment of these predators will take time.

The battle against this beetle has been raging in Colorado for decades, but serious progress has been made thanks to researchers and gardeners like you. Whether treating your lawn for grubs in the spring or starting your summer mornings by drowning beetles, every bit helps to bring this exotic pest under control.