Have bags, will travel: Leaf-munching bagworms (Thyridopteryx ephemeraeformis) spend their time toting cocoons, or bags, camouflaged with bits of plant material. On roses (Rosa spp.), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10, the leaf-covered, spindle-shaped bags may resemble tiny, green cigars. Each bag has an opening at its top end, where a worm exits to eat and make bag repairs. The worms are larvae that develop into moths. Bagworm-killing methods vary with the severity of an infestation.
Bag Them with Bugs
The easiest way to kill existing bagworms and prevent a future infestation is to enlist the worms' natural predators in the fight. Narrow-waisted chalcid and ichneumonid wasps inject the worms with poison venom and deposit their eggs in the paralyzed victims, which become food for the wasps' larvae.
Entice the adult wasps to your rose garden by planting their favorite nectar- and pollen-producing herbs and flowers. Fennel (Foeniculum vulgare), hardy in USDA zones 4 through 9, is an option if you curb its invasive tendencies by removing its spent flowers before they go to seed. As annuals, colorful marigolds (Tagetes erecta) attract the wasps from spring until the first fall frost.
Employ Hand-to-Worm Combat
If you want to kill only a few bagworms, wait until fall when the roses drop their leaves before they enter dormancy. The bags will be easier to see, and so you can remove them by hand.
The male bagworms have brown pupal cases sticking out of the lower ends of their bags. If you have lots of roses to hand-pick, keep the males' bags in place. When adult male moths emerge from the bags in spring, they'll leave your roses in search of females.
The females' bags -- the ones without pupal cases at their lower ends -- contain up to 1,000 eggs apiece. Seal them in plastic bags for disposal; leaving them on the ground gives the eggs a chance to hatch a new generation of rose-infesting worms in spring.
Beat Them with Bacillus
Treat a serious bagworm problem with organic Bacillus thuringiensis var. kurstaki spray -- more conveniently known as Bt. It contains bacteria lethal to the young bagworms but won't harm predatory wasps, honeybees or other beneficial insects. Bt damages the bagworms' digestive tracts and eventually kills them by starvation.
Wear eye protection, waterproof gloves, a long-sleeved shirt, long pants, socks, closed-toe shoes and a hat when preparing and using Bt to keep it from irritating your skin and eyes.
Use a hand sprayer to apply Bt to rose bushes In the hand sprayer, dissolve 4 teaspoons, or the Bt label's specified amount, of Bt concentrate in 1 gallon of water. Bt microbes degrade quickly in sunlight; early-evening treatment is most effective.
Spray the roses until both sides of their leaves are completely wet, and spray the roses each day with freshly mixed Bt until the bagworm eggs stop hatching.
To treat the Bacillus treatment correctly, harvest some female bags in late winter and keep them in sealed jar out of direct sun. When larvae start emerging from the eggs inside the cocoons, it's time to spray the infested roses.