CRW Sampling and Economic Thresholds
Scouting a cornfield for corn rootworm can be a valuable tool to estimate (insect damage) and determine when control measures are necessary. It is an involved process but is beneficial in determining whether or not to manage CRW. The economic threshold is the point at which the potential value lost to CRW in estimated yield, justifies a control method (e.g. pesticide application, host-plant resistance) (Witkowski and Wright, 1997). It is at this point where CRW management should be implemented.
Egg sampling requires collection of soil that is then examined for the presence of CRW eggs. Egg sampling takes a great deal of time, is an involved process, and is not a cost effective method.
CRW larval sampling is done to determine the time of first egg hatch. Sampling usually begins in late May to early June and is continued until the first larvae are found. When the larvae are small (first instar) it is harder to find them due to their size and tendency to burrow into the corn roots. There are two main ways to sample for larvae in the soil. The first method involves sampling when the plants and larvae are young. Five to ten plants throughout the field, including roots, are randomly dug and suspended over a pan of water. As the soil dries the larvae will drop into the water and can be counted. The plants can be left suspended for 24 to 48 hours to be sure all the larvae present have been collected. The second method of evaluating the number of CRW larvae is used when the larvae are larger (2nd or 3rd instar). One plant with the roots and surrounding soil is dug from ten different locations throughout the corn field. Then the soil surrounding the roots of each plant (enough soil to include all of the corn roots) is broken apart and slowly sifted to look for CRW larvae. With either method, often only a small percentage of larvae are found. A general rule is that if an average of two CRW larvae on each plant are found during visual evaluations and an average of eight larvae are found when examining surrounding soil, then a rescue insecticide treatment may be warranted.
From the middle of July to the beginning of September scouting for CRW adults should be done. There are two methods used to scout and sample for adult corn rootworm beetles. The first method evaluates whole plants within the field. The plant is examined from the tassel to below the ear. The ear tip is also examined by pulling away the leaves surrounding the new ear. Since the beetles like to consume the fresh silks, the adults are often prevalent within the silks and are not visible until the leaves are pulled back.
The second method of scouting for adult beetles is the ear zone method. The ear zone method focuses only on the area immediately above and below the ear. This includes the underside of the leaf directly above and the top of the leaf directly below the ear.
When using these methods to scout for beetles it is important to cover the entire field. This is accomplished using a U or V shaped pattern (FIGS. 1a and 1b) over the entire field, of any size, and stopping at 27 sites for the whole plant method and 32 sites when using the ear zone. The whole plant method requires that two plants per site be examined while the ear zone method needs to have five plants examined at each site.The plants that are examined at each site should be several feet apart so the beetles are not disturbed and an accurate count can be taken. “Although the ear zone method takes less time per plant, it may take more time overall because more plants need to be sampled to obtain a reliable estimate” (Wright, et al., 1999).
The economic thresholds for adult beetles in a field vary according to the population of plants and the type of scouting performed. Table 1 lists the different thresholds for CRW and NCR beetles that may cause enough damage to be an economic concern the following year.