In July, female beetles spend 2–3 weeks laying up to 60 eggs each in the soil. The eggs hatch about 2 weeks later. These first-stage grubs feed on roots for most of August. The grubs are small, feeding close to the surface, and vulnerable to biological insecticides. At this time. spraying beneficial nematodes (Heterorhabditis bacteriophora) into the soil in August helps control populations considerably. In early spring I applied milky spore powder to the soil which helps kill grubs before they pupate and become resistant. Round two with more nematodes begins in two weeks.
The nematodes and milky spore need a few years of consistent application to give maximum beetle control. Meanwhile, I still have a problem with adult beetles eating my prize roses. Hand picking helps but I have too many roses and too little time to hand pick more than once a day. I needed a better solution fast. I use organza bags for potpourri and bath salts. They are tightly woven and have a cinch tie. I tied these 4X5 inch bags around my rose buds and hoped for the best. HA! IT IS WORKING!!! The beetles can't get to the flowers. Their mangy little legs get caught on the bags and I can pluck them off into their soapy water death bucket when I have time.
The bag trick isn't practical for those of you who grow thousands of roses. But if you have fewer than 100 bushes you might want to give it a try. The beneficial nematodes and milky spore will help control beetle populations no matter how many roses you grow.