Q: I had problems with bagworms last year on several plants in my landscape, including arborvitae. When do they hatch out, and how can I control them this year?
A: Just to clarify, true bagworms construct homes around themselves with leaves and needles of the plants they are infesting. These “bags” end up resembling small cones by the end of the growing season. Eastern tent caterpillars and fall webworms are sometimes called bagworms, but they construct nests out of silk, not leaves.
Bagworm hatch coincides with the bloom of Japanese tree lilac. Once young caterpillars have hatched, they will look for new plants to inhabit. Young caterpillars often “balloon” just after hatching; that is, they send out a strand of silk to the wind to reach new plants in the landscape. Once ballooning is over, small bagworm larvae enter the “dunce cap” stage, when their abdomens are covered with a cone-shaped silk bag, covered with their sawdust-like frass (frass is a polite term for excrement). When in this stage, bagworm larvae are easily killed with insecticidal soaps, horticultural oils, or your caterpillar pesticide of choice, including Bt. Once they reach the true “bag stage,” about ½-inch long, you’ll have to rely only on traditional insecticides, especially the pyrethroids.
Q: My very large light green hostas by the end of the summer look like lace. They have holes that look as if one used a punch to make them. The blue hosta and striped ones do not get these. What causes the holes and what can I do now to prevent them?
A: Female leafcutting bees are likely at work in your garden. These docile solitary bees remove round and oval pieces of soft leaves from plants like hosta, rose, maple, redbud, epimedium, itea and others. They also cut discs from petals, all with their mouthparts. Each leaf or petal disc is rolled up between the adult bee’s legs and flown back to a hollow twig, where the discs are used to create a soft cell (imagine a stuffed grapeleaf) inside the twig. The female then visits flowers in your garden to collect pollen and nectar. The leafcutting bee lays an egg on this watery pollen solution that will feed her offspring. The female may provision a dozen or so individual cells inside the hollow twig.
The leaf discs removed from your hosta and other plants only cause cosmetic damage. I enjoy seeing the missing discs as evidence that native bees are busy in my garden, even though I rarely see the adults. These bees are important pollinators of many summer crops, so I don’t recommend using insecticides on plant leaves to kill them.
Q: My hollyhocks are in trouble! I have some kind of caterpillar feeding on the leaves, with the resulting leaves looking like lace. Last year I had a fast-moving gray insect on the flower buds. Many of the buds turned yellow and fell off. What can I do?
A: The leaf feeding pest is a sawfly, not a caterpillar. I once had a gardener tell me that she didn’t mind the “caterpillars” feeding on the leaves because she loves butterflies. She was dismayed to find out that adult sawflies are related to bees and wasps, not butterflies (they don’t sting). The larvae will eat the region in between the leaf veins, causing the lacy look that you describe. They can be controlled with insecticidal soaps or horticultural oils applied directly to the undersides of the leaves. They can also be removed by hand. Other labeled insecticides can also be used.
The insect on the buds is the hollyhock weevil. You see the gray adult weevil (a beetle with a long snout) in and among the flower buds. Adult females use their snouts to chew holes in developing hollyhock flower buds, where they lay eggs. Larvae feed inside the flower bud, which is then destroyed. Adults also riddle hollyhock leaves with holes.
Because weevils have hard exoskeletons, they are not generally susceptible to insecticidal soaps and oils. To control this pest, you will need to use a synthetic insecticide to protect developing buds.
Q: Three years after planting my asparagus patch, I enjoyed a nice harvest this spring. Now I see a small insect crawling on the plants. Should I be worried?
A: Asparagus plants are attacked by two species of asparagus beetle, the common asparagus beetle and the spotted asparagus beetle. The common asparagus beetle causes significantly more damage because its larvae feed heavily on the asparagus fronds, limiting the ability of the plants to store reserves for next year. Spotted asparagus beetles feed primarily on the small round fruit on female plants.
In spring, around the time common lilac first comes into bloom, adult beetles appear to mate and lay eggs on developing spears. Larvae feed on fronds for about two weeks, then drop off to pupate in the soil. A second generation of adults emerges to lay more eggs; those larvae feed for the remainder of the growing season. In small asparagus patches, hand picking of adults, larvae and eggs can reduce feeding damage. In larger asparagus plantings, insecticides may be necessary to manage this pest. Several synthetic pesticides are labeled for this beetle, as is the organic pesticide Spinosad.
Q: I used to have a large planting of bearded iris, but it has diminished over the years. A neighbor thinks borers are attacking my plants. What should I do?
A: The iris borer is an important pest of bearded iris, and can sometimes attack other types of irises. The pest overwinters as eggs laid on and around last year’s leaves. In spring, tiny caterpillars hatch from the eggs and bore into the emerging iris leaves. The caterpillars feed on the iris leaves as they work their way down toward the rhizomes. As if their feeding weren’t enough, the larvae introduce a bacterial rot into the rhizome, which often results in the total destruction of the plant. When the larvae reach a size of about two inches, they emerge from the rhizome to pupate in the soil. Adult moths emerge in late summer to mate and lay eggs, completing the life cycle.
Because eggs are laid in and around old leaves, thorough clean up of iris debris in fall or in early spring before new growth begins is an important key in managing this pest. Insecticides can be applied as new growth begins in spring to protect young leaves from the caterpillars. A careful gardener can find evidence of borer activity in leaves and crush the caterpillar inside.
If borers are suspected in summer, rhizomes can be dug up to expose borers. Dig out borers with a knife, then dip rhizomes in a bleach-water solution before replanting.
Denise Ellsworth directs the honeybee and native pollinator education program for Ohio State University. If you have questions about caring for your garden, contact her at 330-263-3723 or firstname.lastname@example.org. Her blog is at https://u.osu.edu/thebuzz.