Managing Corn Rootworm Adults Robert J. Wright, Extension Entomologist, IPM, South Central Research & Extension Center, Clay Center, NE

Lance Meinke, Research Entomologist, Department of Entomology, Lincoln, NE Updated October 19, 1999

For further information on corn rootworm adults, refer to:

• EC 1563, Corn Rootworm Management
• NebGuide G96-1300-A, Insects That Feed on Corn Ears

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Western and Northern CRW Beetles

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, and the northern corn rootworm (NCR), D. barberi Smith and Lawrence, annually cost United States corn producers millions of dollars. Costs include purchase and application of insecticides and yield losses due to root feeding by larvae (i.e. plant removal, negative impact on corn plant physiology, and harvest losses due to lodged corn).

In Nebraska, adult corn rootworm control programs have been used to manage corn rootworm populations in continuous corn since the 1960's. The goal of adult spray programs is to suppress corn rootworm beetle populations and reduce egg-laying so that larval populations the following season will not cause economic loss. Most of the insecticides used for beetle control are aerially applied in Nebraska, with some application through sprinkler irrigation systems (chemigation). Most of the insecticide formulations currently used are "contact kill" products; beetles are killed if directly hit by spray droplets during application or by walking on a sprayed surface.

Adult corn rootworm management is very knowledge and labor intensive. A good understanding of beetle biology and factors that affect beetle population dynamics and movement in agroecosystems is needed to effectively use an adult management strategy.

When using an adult corn rootworm management strategy in continuous corn, it is important to scout corn fields starting in late June to identify when initial beetle emergence occurs, to identify which corn rootworm species are present over time, and subsequently to determine if beetle populations reach the established economic threshold (0.75 beetles per plant, based on a plant population of 24,000 plants per acre; and 10 percent of the females have mature eggs).

Plant Sampling Methods

There are two methods of sampling rootworms on plants; the whole plant and ear zone methods. The whole plant is examined for rootworm beetles in the first method, taking care to examine all locations including the leaf surface, behind the leaf sheaf, and in the ear tip. The ear zone method samples only the middle part of the plant surrounding the ear (the lower surface of the leaf above the ear, the ear and ear leaf, and the upper surface of the leaf below the ear). Research has shown that the most efficient way to scout for corn rootworm beetles using whole plant counts is to examine two plants per site and to sample at least 54 plants per field (27 sites) in a pattern that results in some samples being taken from each quarter of the field. The two plants per site should be several paces apart so that you do not disturb beetles on the second plant in the process of examining the first plant. The most efficient way to scout for corn rootworm beetles using the ear zone method is to examine five ears per site and 32 sites per field (160 plants total), examining plants from all four quarters of the field. Although the ear zone method takes less time per plant, it may be more time consuming to obtain an equally reliable estimate of the number of beetles in the field because of the greater number of plants that must be sampled. Also, the ear zone method samples a variable percentage of the population. A greater percentage of beetles are in the ear zone when the ear is silking, and a lower percentage of beetles are in the ear zone after the silk has turned brown. Over the whole season the ear zone counts approximately 50% of the total beetle population.

Yellow Sticky Trap Method

Yellow Sticky Trap

Another method to scout for rootworm beetles is to use yellow sticky traps. Research in Iowa has shown that using 12 Pherocon AM^R traps spaced out over a field adequately samples rootworm beetle numbers. These traps are placed at ear level on corn plants; the yellow color is attractive to beetles and a sticky adhesive coating on the trap captures them after they land on the trap. The traps are checked weekly and the number of beetles trapped counted. An economic threshold for using these traps would be 6 beetles/trap/day. Some advantages of using sticky traps are that they sample over a several day period and average out effects of time of day or short term weather changes that may influence visual counts. Also, some people question their ability to adequately count beetles visually. Disadvantages include their cost (ca. $1 per trap) and the possibility of getting the sticky adhesive from the trap on your hands and clothes.

Beetle Emergence Patterns

Several factors related to beetle emergence patterns should be considered to target the optimal time for initial insecticide applications. Female beetles on the average need to feed on high quality food (i.e. pollen, reproductive parts of the plant) for at least 10-14 days before they are capable of laying eggs. Also, during the early beetle emergence period,more males emerge than females. Therefore, it is advantageous not to spray during the first 2-3 weeks of the emergence period (little egg laying during this period) even if beetle populations are high. This will allow more females to emerge before application and reduce the pool of emerging beetles that can contribute to population resurgence after the insecticide application.

Factors That Determine Population Levels

There are many factors that can interact to determine beetle population levels in a continuous corn field at different times during the season. The number of beetles emerging from within a field is often a primary contributor to the total population level in a field. Even if an adultmanagement strategy was successfully used the previous year, or a soil insecticide was applied at planting-time or in conjunction with cultivation, some larvae will be present (few to many depending on the situation) which will eventually emerge as adults.

Beetle movement can also contribute to the population level in a field during a specific time period. Both WCR and NCR beetles are very mobile and short range trivial movement within fields or into neighboring fields occurs during the entire time beetles are present. The amount of isolation (or lack of isolation) of a corn field from other continuous corn fields will influence how large the potential pool of beetle immigrants will be. The WCR has also been shown to have a true migratory phase in which a certain proportion of a population will move long distances. Migration usually occurs early in the WCR lifecycle; migrating females are usually mated but have not yet developed mature eggs. The mobility of corn rootworm beetles also enables them to colonize first year corn fields. Because a high percentage of migrant WCR beetles are usually female, more female WCR are often found in first year corn fields than males. Therefore, economic thresholds in first year corn are often lower than in continuous corn (i.e. ca. 0.45-0.5 beetles per plant, based on a plant population of 24,000 plantsper acre if a first year corn field will be planted to corn the following season). Refer to NebGuide G86-774 for more information on adult treatment thresholds.

WCR and NCR prefer pollen (especially corn pollen/silks) as a food source, so pollinating fields are always very attractive to beetles. The corn growth stage in an individual field in relation to the surrounding corn fields can greatly influence whether the field will be a "donor field", a "receiver field", or a "neutral field". If a field pollinates later than the surrounding corn fields, the field will be attractive to beetles and some beetles may move into the field (receiver field) from fields that have finished pollination (donor fields). If all fields in a local area pollinate about the same time, then each field in relation to surrounding fields would be similar in attractiveness to beetles (neutral fields) thereby lessening the chance that mass movement of beetles into one or more fields will occur. Because receiver fields act as trap crops (increase in female beetle population over time results in more eggs being laid in these fields than surrounding fields) it is more difficult to successfully manage corn rootworm populations using an adult-based strategy in receiver fields than in donor or neutral fields (i.e. greater chance for population resurgence in receiver fields after insecticide application).

Weather Factors

Weather patterns can greatly influence immature and adult corn rootworm survival, the beetle emergence pattern, and the length of beetle activity/egg laying periods. Very wet soil conditions (i.e. waterlogged soils like we saw in some eastern NE sites during 1993) during the larval/pupal period can reduce larval establishment on plants, or kill larvae and pupae thus reducing the number of adults emerging in a field. A cool summer can slow down larval and plant developmental times, which will influence initial beetle emergence dates. Cool weather can also lengthen the beetle emergence period and also lengthen beetle activity (increase survival/longevity) and egg-laying periods (seen at some NE locations in 1992, 1993). Conversely, hot weather can speed up immature rootworm development (seen at some NE locations in 1994), greatly compress the emergence period, and reduce beetle activity and egg-laying periods (greater mortality/shorter life span due to heat stress). It is usually easier to use an adult management approach when the beetle emergence and egg-laying periods are shorter in duration because the critical time fields need to be "protected from excess egg-laying" by insecticide applications will be shorter and there is less chance of population resurgence after insecticide application.

Rainfall (and to some extent sprinkler irrigation) can also influence the residual activity of products that are applied for beetle control. In a cool and wet year (such as 1992 in eastern NE), with prolonged beetle emergence and egg-laying periods, short product residual due to washoff can create a situation where overall control is poor and population resurgence may be fairly rapid. This would be especially true if the management location is a receiver field. Under these conditions it is extremely important to pick the optimal time to apply the product (as related to rootworm biology and weather) as it will be more difficult to economically use an adult management strategy. If a field is acting as a trap crop in a cool wet year it may be more economical to scout the field and, if warranted, use a soil insecticide the following year instead of adult control (too many aerial applications may be needed to effectively manage the beetle population in this particular field).

Corn Rootworm Beetles and Silk Clipping

Severe Silk Clipping

Resultant Poor Ear-Fill

Corn rootworm beetles occasionally interfere with pollination if there are sufficient numbers to keep silks severely clipped during the pollen-shedding period. Control is indicated only when severe silk clipping (silks chewed to within ½ inch of husks) is occurring at 25-50% pollen-shed. In most years few fields will need to be sprayed to prevent silk clipping. Beetles are most likely to cause a problem in late-planted or late-silking or where water stress delays or reduces silking. Silk clipping after pollination causes no problems.

Summary

In summary, when making adult management decisions it is important to consider each management situation separately taking into account the interactions of factors that have just been discussed. This is especially important when selecting the specific product that will be used for beetle control, when determining the appropriate time(s) to apply the product, and when evaluating the efficacy of the product.

 

Suggested Treatments for Adult Corn Rootworm Control

 

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