Greenhouse and Nursery - Corn Borer in Mums

This is a continuation of information for mum growers on common pests and pest management for this profitable fall sales crop.

The European corn borer (ECB) tunnels in the stems of garden mums and cut flowers causing considerable damage each year for some growers. The ECB has been found boring into more than 200 different plants, including asters, cosmos, dahlia, daisies, gladioli, hollyhocks, roses, zinnia and vegetable and field crops. The adult is a yellowish-brown moth. Females lay up to 400 eggs in flat masses on the underside of leaves of the host plant. The eggs resemble tiny fish scales in shape and arrangement. The caterpillars hatch and feed on the surface of leaves for a few days. As the borers mature, they bore into the stalk of the host plant to feed. The larvae push frass from the stem which collects at the opening. ECB overwinters in the stalk, so destroying the stalks of corn, dahlia, mums and weeds in the area should help to suppress next season’s population; however, it is not guaranteed since the moths fly readily. In Delaware there are two to three generations per year.

Control strategies can be adopted with help from the UD Vegetable IPM Program, where the ECB is a major pest of Sweet Corn and other vegetable crops. The IPM program monitors traps to detect when moths are present and then timing control actions to coincide with the insect’s activity depending on the number of trapped moths. For ornamental plants this connection hasn’t been as well-established, but could be used as a guideline particularly in areas where ECB is an annual problem. Vegetable growers have found the traps to be a real asset in making control decisions.

The latest trap captures, conducted by the UD Vegetable IPM Program can be found at the UD IPM web site http://ag.udel.edu/extension/IPM/traps/trapindex.html. The first generation moth flights are over. The second moth flight occurs in July and it is recommended to wait a week after flight begins before beginning treatment. It is important to time applications before the caterpillars begin boring into stems.

In sweet corn, treatments every five days are suggested when moths are detected. This is probably excessive for most ornamental growers and the interval can probably be extended farther where damage tends to be limited and trap counts remain very low. Where numbers are moderate to high and temperatures are warm (more than 80F) the tighter application intervals might be needed.

Treatment options include Conserve (spinosad), pyrethroids and Bt. Conserve works well and also provides thrips control. Pyrethroids include Talstar, Scimitar GC, Decathlon and Astro. Note that Marathon does not provide good control of ECB. Bt (Bacillus thuringiensis kurstaki or Bt aizawai) can also be used, but frequent treatments are probably necessary to get the same results. Check product labels for approved crops, since some plants are sensitive especially to repeat sprays.

Information adapted from the New England Greenhouse Update newsletter.