Abundant rainfall and warm temperatures over the last several weeks fostered a profusion of blossoms on my Shasta daisies and coneflowers, two of my favorite plants for observing pollinators. However, over the past week or so, petals of these blossoms disappeared in bits and pieces down the gullets of small scarab beetles known as Oriental beetles. We met other members of this destructive branch of the scarab clan in the previous episodes “Midnight marauders: Asiatic garden beetles, Maladera castanea” and “Misery named the Japanese beetle, Popillia japonica”. Oriental beetles are yet another exotic invasive pest introduced into the US sometime before 1920. Originally detected in Connecticut, Oriental beetle now ranges from Maine to the Carolinas and west to the Heartlands.
Grubs of this little rascal, commonly known as white grubs, have been rooting about in my flower beds and lawn since their mothers deposited eggs in the soil early last summer. Last year conditions for survival of white grubs were spectacular and Bug of the Week warned that the scarab clan would likely be problematic this year. Unfortunately for our plants, the prediction seems to be coming true. By munching roots of grasses, and annual and perennial plants, white grubs can be important pests. Oriental beetle grub counts can approach as many as 60 per square foot of turf in heavy infestations. Here in Maryland by late spring larvae have completed development, pupated, and adults make their grand appearance in the first half of June. They will be present for the majority of the summer punishing blossoms in my landscape beds.
After completing development in spring, Oriental beetles emerge from the turf and ravage flowers and leaves of some of my favorite pollinator plants. How rude!
Managing Oriental beetles usually focuses on destruction of grubs in the turf and several products are available. One group of insecticides widely used for controlling white grubs, including the larvae of Japanese beetles and Oriental beetles, are the neonicotinoids. These compounds have received much recent attention due to their impacts on pollinators, particularly bees. A fascinating study by Dan Potter and his colleagues at the University of Kentucky revealed that turf treated with the neonicotinoid insecticide clothianidin was not the best place for bumble bees to forage. White clover in plots treated with clothianidin expressed the insecticide in their blossoms, which in turn intoxicated the bees. However, once the clover blossoms had been removed by mowing, the exposure to bees was mitigated. A second white grub-killing insecticide evaluated in the study, chlorantraniliprole, presented no detectable adverse effect for the bumble bees.
In addition to synthetic insecticides, many folks recommend the use of nematodes to control white grubs in turf. Nematodes enter the grub and release a lethal bacterium. There are many different species and strains of nematodes. Dave Shetlar of the Ohio State University suggests that products containing strains of Steinernema carpocapsae are a bit less effective against beetle grubs than species in the clan named Heterorhabditis. You must wait until late July or August when grubs are in the soil, if you go the nematode route. Pheromone traps are also available to help detect Oriental beetles and capture the males. For me, plucking the beetles from the plants and either crushing their small bodies or giving them a swim in a cup of soapy water provides some retribution for their miscreant behaviors. Unfortunately, I fear that another soggy spring and summer portend increases in Oriental beetles not only this year but in 2020 as well. Abundant summer rainfall makes our gardens lush but, alas, also benefits many species of scarabs.
Excellent references such as the article “Assessing Insecticide Hazard to Bumble Bees Foraging on Flowering Weeds in Treated Lawns” by Jonathan L. Larson, Carl T. Redmond, and Daniel A. Potter, and the textbook “Destructive Turfgrass Insects: biology, diagnosis, and control ” by Daniel Potter, were used in preparing this episode.