Dear Dana,

Thank you for reaching out to the UVM Extension Master Gardener Program with your question.

It sounds like you are confident that you are dealing with Rose Chafers (Macrodactylus subspinosus). As you know these are tan, slender beetles with a reddish head and spiny reddish legs. They feed on the upper surface of leaves and petals leaving a skeletonized appearance to the leaf. They feed on a wide variety of host plants including flowers (such as roses and peonies), vegetables, fruits (such as apples, raspberries, strawberries, grapes), and trees. The beetles are poisonous to birds and small animals.

The flying adult beetles emerge in late May/ early June and live for approximately one month. The females lay eggs, preferably in sandy soil, that hatch yielding white grubs that feed on the roots of turf and other plants. As the temperatures drop the grubs move deeper into the soil to overwinter. In early spring they migrate upward and pupate yielding the flying adults. There is only one generation per year.

I understand how frustrating it is to deal with an infestation in your garden especially when it effects the quality of your fruits. I would still urge you to try to practice Integrated Pest Management (see IPM overview below). It is usually not practical to eliminate a pest completely but you may be able to decrease the population to a level where there will be minimal damage. Knowing the life cycle of the pest helps in planning the management.

You asked specifically about the use of beneficial nematodes. This intervention is directed at the grub phase of the Rose Chafer. There are a variety of nematodes available so you need to ensure that the nematodes that you use are active against the Rose Chafer grub. As the grubs move toward the soil surface in early spring that would be the time to apply the nematodes. If the temperatures are still cool the grubs may be too deep in the soil to be accessible to the nematodes. You will need to keep the soil moist after the nematode application as nematodes will die if the weather is too dry and warm. Even if you treat the Rose Chafer grubs in your yard, adult Rose Chafer beetles can fly to your landscape from sites outside of your property.

Scouting for the adult beetles on your plants in early May is a first step. You can cover your at-risk plants with row cover to prevent access. As the adult flying Rose Chafer feeds during the day you can hand pick the beetles and drop them into a bucket of soapy water to kill them. These techniques may not be effective for heavy infestations. There are a number of potential chemical controls but chemical control should be reserved as a last resort. The chemical compounds can be toxic to bees and other beneficial insects. Heavy infestations may require repeated chemical applications for control.

Here are some references :

https://www.uvm.edu/~entlab/Landscape%20IPM/WhitegrubsinsoilJune2023.pdf

https://content.ces.ncsu.edu/rose-chafer

https://extension.umn.edu/yard-and-garden-insects/rose-chafers

https://www.uvm.edu/sites/default/files/Extension-Community-Horticulture/Resources/IPMOverview2023.pdf

Unfortunately Japanese beetles appear later in the season so their damage can start after the Rose Chafer damage has ended. Good Luck!