No. 18/July 25, 1997
Corn Rootworm Beetles and Suppression of Egg Laying
The idea of managing adult corn rootworms to prevent egg laying originated with Union Carbide in the 1970s. They promoted their product, Sevin 4-Oil, as an insecticide that would suppress rootworm beetle densities for at least 3 weeks if sprayed in a timely fashion, based on detailed scouting information. Sevin XLR Plus eventually replaced Sevin 4-Oil as the product of choice. The program worked effectively if farmers were committed to an intensive scouting program. Ideally, Sevin XLR Plus is designed to be sprayed once at the appropriate time; if beetle counts never return to critical levels, economic levels of eggs are most likely not being laid. A well-managed adult-control program eliminates the need for applying a soil insecticide the following spring. Other companies such as Atochem have actively marketed Penncap-M as an insecticide that can be effectively used in adult management efforts. In certain areas of the Corn Belt, particularly in Nebraska, the adult-control concept has been the main focus of adult corn rootworm management efforts for many years. As mentioned in previous issues of this Bulletin, resistance to Penncap-M is beginning to surface in several Nebraska counties.
Growers in some areas of northwestern and north-central Illinois continue to express interest in the use of adult-control strategies to prevent economic losses caused by corn rootworms. More recently, producers in east-central Illinois have raised questions concerning the use of broadcast insecticide treatments in soybeans to prevent egg laying and eliminate the need for a soil insecticide in first-year corn. In soybeans, this strategy is unproven, and additional research is required to develop an economic threshold for western corn rootworm adults in soybeans. In corn, we maintain that the program works only if specific guidelines are followed by experienced scouts and consultants.
The following questions are designed to clarify some issues surrounding the use of broadcast insecticide applications for managing corn rootworm beetles.
If I treat a field to prevent excessive silk clipping, why can't I count on this treatment to eliminate the egg-laying threat of corn rootworm beetles? To protect the pollination process, a threshold of five or more beetles per plant has been suggested, as long as pollination is not complete and silk clipping is evident. It is important not to confuse this threshold recommendation with a management strategy designed to prevent significant egg-laying to reduce root damage the following year. A treatment to prevent silk clipping would generally occur too early in the season to substantially reduce egg laying by beetles later in the summer. Research has shown that fewer than 10 percent of rootworm eggs are laid in continuous corn by August 1. Beetles that are found from mid-August through early September are responsible for more than 50 percent of the egg laying in corn. Because egg hatch was very delayed this season, we should expect oviposition to be somewhat delayed this year. This delay will further separate in time the optimum treatment dates for suppression of egg laying and prevention of silk clipping.
When should I start scouting for beetles, and what monitoring techniques should I use in a beetle-suppression program aimed at preventing significant egg laying? By mid-July, and continuing through early September, you should be committed to scouting for rootworm beetles at least once each week. Determine the average number of beetles per plant by counting beetles on two plants selected at random in each of 25 areas of a field. Count all western and northern corn rootworm beetles each time. The counts take about 45 minutes in a 40-acre field. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including the ear tip, tassel, leaf surface, and behind leaf axils. Record the number of beetles you find per plant. If the average is greater than 0.75 beetle per plant in corn after corn or 0.5 beetle per plant in first-year corn for any sampling date, plan to rotate away from corn, apply a rootworm soil insecticide to corn in 1998, or initiate a program for preventing egg laying. If densities do not exceed these thresholds for any sampling date, rootworm larvae should not cause economic root damage to corn the following spring.
Producers involved in a beetle-suppression program may find an insecticide application warranted if beetle thresholds are reached and 10 percent of the females are gravid (with eggs). If more than 10 percent of the female beetles within a field are gravid, significant egg laying already may have occurred. An attempt at this point to reduce beetle densities to decrease root damage in 1998 may prove less than satisfactory.
Why are the thresholds different for first-year corn and continuous corn? More females are typically found in first-year corn than in continuous corn. Therefore, there is a greater threat of more eggs being laid in first-year corn and, consequently, greater root damage occurring the following year.
How hard is it to tell male and female rootworm beetles apart? Separating male and female western corn rootworm beetles is not difficult. Male western corn rootworm beetles have wing covers that are mostly black, with a small tinge of yellow at the tips. Female western corn rootworm beetles have three widely separated black stripes on yellow wing covers (Figure 1). Northern corn rootworms, both male and female, are about 1/4 inch long and are uniformly green, with no distinguishing stripes.
Figure 1. Western corn rootworm male (left) and female (right).
Is a single insecticide application aimed at reducing the number of egg-laying females all I need to invest in? Even if a field is treated earlier in the season, weekly scouting is vital through early September. Fields may become reinfested several weeks after an earlier insecticide application. Unfortunately, some fields require two treatments. Treating a field twice in an adult-suppression program in 1997 to reduce egg laying and also using a soil insecticide at planting in 1998 is not an economically or ecologically sound approach.
Does planting date have any influence on which of my fields are most suited for participating in an adult-suppression program? Late-planted fields, especially replanted fields, are generally not good candidates for an adult-suppression program. Large numbers of beetles are more likely to move into these fields, which are most apt to have fresh silk and pollen (compared to fields that are drying down more rapidly).
I'm not comfortable with the accuracy of my beetle counts because these insects always seem to be moving. Are there any other monitoring techniques besides counting beetles on plants? During the mid-1980s, research efforts at Iowa State University suggested that yellow sticky traps, commonly referred to as unbaited Pherocon AM traps, could be used successfully to predict an economic infestation of rootworms the following season. Trapping protocols suggested placing 12 traps within a field during the egg-laying period of rootworm beetles. If the traps captured six beetles per trap per day, producers were encouraged to rotate to another crop or apply a soil insecticide to corn the following spring. Because the efficiency of these traps declines over time, fresh traps should be deployed each week.
Mike Gray, Extension Entomology, (217)333-6652