Extension News

Ask the ISU Extension Garden Experts: Wasps, Peonies and Herbs

Note to media editors: Got gardening questions? Call the Hortline at (515) 294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or e-mail us at hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, http://www.yardandgarden.extension.iastate.edu


Several large, black and yellow wasps are digging holes in my flower garden. What are they and how do I get rid of them?

The large, black and yellow wasps are probably cicada killers. The cicada killer wasp is a solitary wasp. Each female lives independently rather than in colonies, though many may choose to nest in close proximity to one another.

Cicada killer wasps are active in July and August. The female digs one or more tunnels in soft soil (often flower beds or gardens). Tunnels are about the size of a quarter and may extend 24 inches or more into the ground. The female flies to nearby trees to capture an annual cicada that she stings to paralyze and then carries back to the burrow. One or two paralyzed cicadas are placed in each cell at the end of the tunnel and a single egg is deposited before the female closes the cell and flies away, never to return. The eggs hatch into legless larvae that feed on the cicadas and develop into wasps that emerge the following summer.

Only female cicada killers have the capability to sting. However, they usually won’t unless handled or threatened. Stings inflicted by cicada killers are usually not severe, but reaction varies with each individual.

Wasps are generally beneficial, and a nest in an out of the way location where it is not likely to be disturbed should be left alone. On the other hand, nests in high-traffic areas may warrant treatment. Cicada killers can be destroyed by applying an insecticide dust (e.g., Sevin or permethrin) into the burrow entrance during the night. Cover the nest opening with a shovelful of soil and reapply in two or three days if necessary.

There are large, brown spots on my peony leaves. What should I do?

Peony leaf blotch is probably responsible for the large, brown spots. Peony leaf blotch is caused by the fungus Cladosporium paeoniae. The disease is also known as red spot or measles. Typical symptoms include glossy purple to brown spots or blotches on the upper surfaces of the leaves. The disease may cause slight distortion of the leaves as they continue growth. Leaf symptoms are sometimes most apparent on the edges of older leaves. On stems, symptoms appear as long, reddish-brown streaks.

Peony leaf blotch is best managed through sanitation. The fungus survives the winter in infected plant debris. Diseased plant material should be removed in fall or early spring (before new shoots emerge). Cut off the stems at ground level. Remove the plant debris from the area and destroy it. Proper spacing and watering can help to minimize the severity of the disease. Space peonies three to four feet apart. When watering is necessary, avoid wetting the peony foliage. Fungicides can be used as a supplement to sanitation and good cultural practices.

When should I harvest my herbs?

Most herbs are ready to be harvested when flower buds appear on the plants, but before the buds open. The leaves contain the maximum amount of volatile oils at this stage of growth, giving the greatest flavor and fragrance to the finished product. Harvest herbs in the morning as soon as the dew evaporates from the foliage.

Leafy annual herbs can be cut back severely when harvested. Using a sharp knife or pruning shears, cut just above a leaf or pair of leaves. Leave approximately four to six inches of the stem for later growth. Do not cut back leafy perennial herbs as heavily as annual herbs. On perennials, remove the top one-third of growth.


Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu 

Del Marks, Extension Communications and External Relations, (515) 294-9807, delmarks@iastate.edu