No. 16/July 10, 1998
Corn Rootworm Adults Common in Many Cornfields
We've received many reports during the past week concerning the abundance of corn rootworm adults in many cornfields. As fields enter the pollination period, attention should be focused on the silk-clipping threat that exists. Fortunately, rootworm adults are less likely to cause reduced kernel sets this year because plentiful soil moisture exists in most cornfields. Plants that are not under moisture stress are typically able to produce silk at impressive rates even when rootworm beetles are present and feeding on silk tissue. Bob Nielsen, Purdue University Extension agronomist, recently provided some interesting facts about the pollination process in the Purdue University Pest and Crop Newsletter (no. 16, July 3, 1998). When making decisions regarding whether or not to treat a field to prevent silk clipping by corn rootworm adults, consider the following points provided by Bob Nielsen.
- The yellow "dust-like" pollen that falls from a tassel represents two to five million individual, nearly microscopic, spherical, yellowish-translucent pollen grains.
- Approximately two to five thousand pollen grains are produced for each silk. Therefore, the amount of viable pollen available is almost never a limiting factor during pollination.
- As silks first emerge from husk, they lengthen as much as 1 inch per day for the first day or two, but gradually slow over the next several days.
- Silk elongation stops about 10 days after silk emergence, regardless of whether pollination occurs, due to senescence of the silk tissue.
- Silks remain receptive to pollen grain germination up to 10 days after silk emergence.
- A pollen grain germinates on a receptive silk and develops a pollen tube, containing the male genetic material, that grows inside the length of the silk and fertilizes the ovule within 24 hours.
- Generally, silk length on injured ear shoots must be at least 1/2-inch in order that a sufficient length of viable silk tissue be exposed for pollen germination.
Our thanks to Bob Nielsen for providing this interesting information on the pollination process.
With so many planting and replanting dates this year, corn rootworm beetles will have many opportunities to move from field to field to find fresh silks and pollen, their preferred food. Western corn rootworm males emerge first, followed by western corn rootworm females. After emergence and mating, about 14 days elapse before the females begin laying eggs. Western corn rootworm beetles are about 1/4-inch in length. The background color for both males and females is yellow, but the two sexes differ somewhat in their markings (Figure 1).
Figure 1. Western (top), northern (bottom left), and southern (bottom right) corn rootworm adults (illustration from Seed Corn Pest Management Manual for the Midwest, Purdue University.
On males, nearly the entire front half of each wing cover is black; only the tip of the wing cover is yellow. Females are usually slightly larger and have three distinct black stripes on the wing covers. Gravid (pregnant) females have distended and swollen abdomens. Northern corn rootworm adults emerge after western corn rootworm beetles. Northern corn rootworms also are about 1/4-inch long, but they have no distinct markings (Figure 1). Newly emerged northern corn rootworms are cream or tan in color, but they gradually become green as they age. As with the western species, the females are slightly larger. You may also observe southern corn rootworm beetles, or 12-spotted cucumber beetles. These migratory beetles rarely cause much silk-clipping injury.
Northern corn rootworm adults.
The first objective of scouting for rootworm adults is to determine their potential for interfering with pollination. In commercial field corn, treatment may be justified if you find five or more beetles per plant, pollination is not complete, and silk clipping is observed. In seed-production fields, a treatment may be justified if the silks on 20 percent of the plants have been clipped to a length of 3/4-inch or less, pollination is still taking place, and rootworm beetles are present. Products labeled for use against corn rootworm adults include *Ambush 2E, *Asana XL, Lorsban 4E, *Penncap-M, *Pounce 3.2EC, Sevin XLR Plus, and *Warrior 1EC (* = Use restricted to certified applicators only). In future issues of this Bulletin, we will provide information about treating cornfields to suppress egg laying by corn rootworm adults. Keep in mind, optimal treatment times necessary to prevent silk clipping and suppress egg laying most often do not overlap.
Mike Gray (firstname.lastname@example.org) and Kevin Steffey (email@example.com), Extension Entomology, (217)333-6652