How to protect plants for beetle season

What’s knocking out Amy’s knock outs?

Amy in Springfield writes: “I planted a row of knock out roses last spring which are doing well except something is eating the leaves. I don’t see any larvae or beetles, could it be aphids? I have some ‘Oil Away’ that I could use — would that be a good option?”

No Amy, horticultural oil (like your “Oil Away”) and insecticidal soap sprays work by smothering the pests involved and are only effective if you spray the soap or oil directly on the pest.

And your problem is not aphids. Although a notorious pest of roses, aphids are very visible, clustering on the plants in large numbers — and generally on the unopened flower buds, not the leaves. And aphids don’t “eat” leaves or flowers; aphids are sap suckers that literally suck the life out of the poor plants under attack.

Could it be the dreaded midnight beetle?

Amy in Springfield writes that something is eating the leaves of her roses, but she doesn’t, “see any larvae or beetles.”

Many things can eat the leaves of roses, Amy. If the culprit is a member of the scarab beetle family, the damage will be very distinctive — generally only the tastiest portions of the leaves are first consumed, leaving behind a kind of lacework-like skeleton.

Although most members of this leaf-chewing family — like the notorious Japanese, June and rose chafer beetles — are daring daylight damage-doers, the Asiatic beetle flies by night, and the only thing you generally see is their damage.

If lacy leaves you have and daylight beetles you don’t see, make the area as dark as possible; these late night rose-ravagers are attracted by light.

A Beetle Trap Will Tell the Tale

We are entering beetle season, Amy — specifically the time of year that scarab beetles begin to love making rose leaves lacy (then yes, they move on to chew the flower buds as well).

Most scarabs, like Japanese and June beetles, are day fliers. You can always see them working your plants in the middle of the day. But the Asiatic beetle does its damage by night and often goes unseen.

One way to identify the intruder is to put a single Japanese beetle trap near your roses and check it every evening and every morning. If it was empty at dinnertime but contains some bad actors by breakfast, you have Asiatics. Take the trap to an area with a bright light that’s far away from the roses and turn off all lights near the roses. The combination of the light and chemical lures attached to the top of the trap should lure the beetles away from your blooms and into the trap.

Never put beetle traps near the affected plants because you’ll just increase the damage.

Bring Microscopic Grub Busters to the Fight

Amy in Springfield might be seeing the first signs of beetle damage on her roses, although she does not yet see any actual beetles. Ah, but those beetles are coming crawling toward the surface of the soil in their grub stage and transforming into their armored rose-ravaging adult shape as we speak.

That makes this the perfect time to attack those big grubs with beneficial nematodes. Available by mail order and at better garden centers, a small package contains many millions of the microscopic benefits. Here’s one species that’s available from Gardens Alive.

You can maximize your chance of success by doing the following things:

  • Do a late afternoon watering of the area in question. Really soak it, even if rain has been recent.
  • Open the packet and drop it as instructed into an ordinary watering can three-quarters full of water, wait a minute or two, stir, and then water the nematodes into the area where you want grub control.
  • Be sure to do this in the early evening, not in the morning or the heat of the day, as sun and heat are the enemies of these beetle busters. One exception is that you can apply them in the morning or afternoon if it’s raining and will continue to rain.
  • The nematodes will enter the soil to seek out and destroy the grubs without harming pets, people, birds, earthworms or anything else. Beneficial nematodes only affect grubs, and are the safe and sane method of grub control.

Slug it Out

We’re telling Amy in Springfield to use a single Japanese beetle trap to see if beetles are eating her rose leaves.

If you don’t bag any beetles, or the damage is not their distinctive laciness, put out saucers of fresh beer at the base of the plants in the early evening, not “stale beer,” and not in the morning or afternoon. You want it to be fresh for slug happy hour, which begins an hour or so after sunset.

If the saucers are filled with dead, drunken slugs in the morning, that’s your foe. You can continue drowning them in beer traps (think cheap beer) or apply one of the new organic slug baits that use iron phosphate as their active ingredient.

Basic Rose Care Counts!

And don’t neglect proper care — pests are attracted to weakened plants, and roses that are fed chemical fertilizers or mulched with any kind of wood or bark are always going to be under increased levels of attack. Chemical plant foods especially cause unnaturally fast, weak growth that pests of all kinds find very attractive.

Stop any artificial feeding, remove any wood mulch near the plants and replace it with two inches of premium compost — not composted manure (if the soil is bare, just spread the compost).

The compost will feed the plants naturally and deter the diseases that roses are prone to.

Mike McGrath was Editor-in-Chief of ORGANIC GARDENING magazine from 1990 through 1997. He has been the host of the nationally syndicated Public Radio show “You Bet Your Garden” since 1998 and Garden Editor for WTOP since 1999. Send him your garden or pest control questions at

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