Corn Rootworm IV: Extended Diapause in Northern Corn Rootworm

 by Murt McLeod and Steve Butzen

Summary

 

·        Corn rootworm (CRW) is a major insect pest of corn in many regions of the world. Western and northern CRW are the two most damaging species of this pest.

·         Populations of northern CRW in some areas have adapted to crop rotation, resulting in damage to corn following soybeans, a phenomenon referred to as extended diapause.

·         Extended diapause has been most prevalent in parts of South Dakota, Minnesota and northwest Iowa. Studies and observations indicate that the problem is expanding.

·         Occurrence of extended diapause problems in affected areas can be unpredictable, as hot spots often move from year to year. The reasons for the rapid onset and decline of problem areas are not completely understood.

·         Management options for CRW in extended diapause areas include longer rotations, soil insecticides, seed-applied insecticides and transgenic CRW hybrids.

·         Crop rotation in these areas must now include two years away from corn to effectively control CRW. Most growers are unwilling to limit corn production to one year in three.

·         Many growers are applying soil insecticides to corn in two-year rotations. Pioneer® brand hybrids with Poncho³ 1250 seed-applied insecticide provide another option in low to moderate risk areas.

·         This Crop Insights discusses CRW biology and damage, his-tory of extended diapause, predicting where outbreaks will occur, and management strategies to counter this problem.

 

Corn Rootworm Biology

 

Western and northern CRW are the two main rootworm species causing economic damage to corn. These species have similar life cycles and both complete one generation per year. Western adults are yellow in color with black or black-striped wing covers. Northern adults are a uniform green to tan color without wing markings (Figures1 and 3.)

 

 

Female CRW beetles lay eggs in the top 8 to 12 inches of soil. The eggs then enter a resting or diapause state to overwinter. After eggs have chilled for a required period of time, diapause is broken, eggs resume development and eventually hatch in late spring. Hatch may begin from late-May through mid-June depending on the location in the Corn Belt and the climate that year.

Larvae initially feed on root hairs and smaller portions of roots. As larvae develop through three stages, they feed externally and internally on larger roots. Third instar larvae are approximately 5/8 inch long with a brown head and a brown plate on the top of their last abdominal segment.

Corn rootworm feeding is most extensive in early through mid-July in most regions of the Corn Belt. When fully developed, CRW larvae form earthen cells in which to pupate, transforming from the larval to adult stage in about a week to ten days. Adult CRW emergence begins in late July and may extend over several weeks. However, adult activity in corn fields, including egg laying, typically persists into September.

Rootworm Damage

 

The primary damage caused by CRW is from larval feeding on the roots of corn. This feeding causes a reduction of root mass, resulting in decreased ability by the plant to transport water and nutrients. Physiological yield loss is usually the result. Reduction in root mass from CRW larval feeding may also cause corn plants to lean or lodge with wet soil conditions and high winds. When corn plants try to right themselves, the result is curved corn stalks, commonly referred to as goose-necking (Figure 2).

Yield losses due to root feeding damage and lodging vary widely. Drought stress or low soil fertility can accentuate the yield impacts of a reduced root system. Root lodging reduces light interception, which can lower yields and stalk quality. Harvest delays and losses are also common in root-lodged fields. The extent of these problems depends on the severity of lodging and environmental conditions throughout the remainder of the season.

History of Extended Diapause

 

Failure of crop rotation as a management strategy for corn rootworms was reported as early as the 1930’s. In 1965 extended diapause was first identified as the cause of many of these CRW control failures (Chiang, 1965). Extended diapause is an attribute of some northern CRW eggs that causes them to delay hatching until after two or more winters in the soil. In a cropping sequence in which corn is planted every second year, extended diapause allows for many CRW larvae to hatch in the corn production year, averting starvation and resulting in damage to corn in rotation.

Figure 3. Northern CRW beetles range in color from green to tan (images courtesy of Donald Specker2).

Scientists now know that environmental conditions such as temperature, time and photoperiod determine when the eggs will break diapause and resume development. The amount of stimulus needed is genetically based, and this requirement varies among individuals in the population. Although most eggs are programmed to hatch after the first winter, many eggs in the northern CRW population remain in the soil for two winters prior to hatch, and a few are still viable after four years in the soil.

In a cropping sequence with corn planted every second year, most CRW larvae hatching during the first growing season after a corn crop fail to survive, while larvae hatching the second year have a high chance of survival. Surviving individuals reproduce and pass extended diapause to the next generation, increasing the frequency of the trait in the population. Significant changes in the northern CRW population are well-documented. When extended diapause was first discovered in 1965, less that 0.5% of the eggs had this trait. Twenty years later, when the problem became rampant in parts of IA, MN and SD, scientists reported that the percentage of eggs with extended diapause was approximately 35%. Recent research suggests that the current percentage is even higher (Ostlie and Potter, 2001).

 

Extended diapause is limited almost exclusively to northern corn rootworms, not having been found to any appreciable extent in the western corn rootworm population. The problem areas have been mostly confined to parts of South Dakota, Iowa and Minnesota. In South Dakota, extended diapause is spreading west and northwest from its epicenter in the mid-east and southeast counties.  In Iowa, problems have been most prevalent in the northwest part of the state. Reports indicate that extended diapause is occurring more frequently there and also spreading dramatically in southwest Iowa. In Minnesota, the problem area has expanded from the southwest to the west central part of the state (Ostlie and Potter, 2001). Producers in all affected areas are increasingly observing yield losses and severe root lodging as a result of extended diapause of northern corn rootworms.

Predicting the Occurrence of Extended Diapause Problems

 

In general, extended diapause tends to be more area-specific than field-specific. One of the difficulties of managing northern corn rootworms in extended diapause areas is predicting which fields will have a problem. Growers and researchers have realized that hot spots often move from year to year. The reasons for the sometimes rapid onset and decline of extended diapause problems are not completely understood, but the following factors have been found to favor its occurrence (Ostlie and Potter, 2001):

·         Presence of the extended diapause trait in a significant percentage of the northern CRW population

·         High population of beetles in the area

·         Relative attractiveness of the field for egg laying

·         Environmental conditions that favor hatch in two years

·         Favorable overwintering conditions during each winter

·         Weather conditions favorable for larval survival and significant root damage

·         Conditions conducive to root lodging

Beetle Counts Alone Not Effective

Using beetle counts to predict the probability of rootworm damage in continuous corn fields has been used successfully in some parts of the Corn Belt. However, in extended diapause areas, using northern corn rootworm adult counts to predict damage has not proven effective, and there remains a great deal of uncertainty about this method. In some instances, beetle counts of 3 to 4 adults per plant resulted in rootworm damage in subsequent first-year corn, whereas in other instances beetle counts of from 14 to 17 per plant did not translate into significant root injury in subsequent first-year corn. This disparity may be due to any number of environmental factors, but the end result is that beetle counts are not a reliable method for predicting the occurrence of extended diapause.

Management of Extended Diapause

 

Crop Rotation

Historically, alternate-year rotation to a crop other than corn was a very effective management strategy against CRW. In extended diapause areas, the cropping sequence must now include two years away from corn to be effective. Most growers in the highly productive areas where extended diapause is a problem have been unwilling to limit corn production to only one year in three.

Soil Insecticides

Many producers have responded to extended diapause by applying soil insecticides to corn following soybeans in two-year rotations. This practice is particularly prevalent in areas where growers have repeatedly observed extensive corn root damage and lodging.
 

Past research studies in both Iowa and Minnesota have shown inconsistent economic returns from use of soil insecticides. But increased damage to corn and continued spread of the extended diapause problem have made this strategy more common. In fact, more agronomists and crop consultants are now recommending it in problem areas.

Poncho 1250 Seed-Applied Insecticide

Pioneer is introducing a new management option for CRW control in 2004 - Pioneer® brand hybrids with Poncho³ 1250 seed-applied insecticide. Growers in areas with low to moderate risk of the extended diapause problems should consider the benefits of this new management tool. Poncho 1250 users will find advantages in convenience, handling, safety, grain marketing and hybrid availability over other corn rootworm management options.

Poncho 1250 comes on-the-seed and in-the-bag, eliminating the cost and time required to install soil-applied insecticide equipment, calibrate equipment and load products during planting, and handle and return insecticide containers. Poncho 1250 also allows growers to “stack” corn rootworm protection with the newest hybrid genetics and corn technologies, including the latest Pioneer® brand hybrids with Herculex⁴ I or YieldGard Corn Borer⁵ insect protection, or the Roundup Ready⁵ trait.

Transgenic Corn Rootworm Hybrids

Transgenic corn hybrids that resist CRW larval feeding are now available. This technology has shown effectiveness in reducing CRW damage and maintaining a larger root mass versus no treatment. Hybrid choices and quantities are currently limited with this new technology.

When considering transgenic CRW hybrids versus other management options in extended diapause areas, growers should pay close attention to the value proposition of each system. The newest hybrid genetics with soil or seed-applied insecticide may be much more profitable than some lower-yielding transgenic hybrids now commercially available.

References

 

Chiang, H. C. 1965. Survival of northern corn rootworm eggs through one and two winters. J. Econ. Entomol. 58:470-472.

McLeod, M., S. Butzen and P. Davis. 2003. Poncho 1250
seed-applied insecticide controls CRW and other insects.
Crop Insights Vol. 13 no. 5. Pioneer Hi-Bred, Johnston, IA. www.pioneer.com/growingpoint/agronomy/crop_insight/1305.jsp

 

McLeod, M., and S. Butzen. 2003. Corn Rootworm I: Biology and Management. Crop Insights Vol. 13 no. 6. Pioneer Hi-Bred, Johnston, IA. http://www.pioneer.com/growingpoint/agronomy/crop_insight/corn_rootworm_biology.jsp
 

 

McLeod, M. 2003. Corn Rootworm II: Damage Assessment and Yield Loss Relationships. Crop Insights Vol. 13 no. 8. Pioneer Hi-Bred, Johnston, IA. http://www.pioneer.com/growingpoint/agronomy/crop_insight/corn_rootworm_damage.jsp

 

Ostlie, K. and B. Potter. 2001. Northern corn rootworm extended diapause. In IPM Stuff – SW MN newsletter. Issue #2001-1, April 18, 2001. University of Minnesota Southwest Research and Outreach Center, Lamberton.
http://swroc.coafes.umn.edu/swmnpest/01News/ipmstuff41801.pdf

 

¹Figure 1 image courtesy of Marlin Rice, Iowa State University Department of Entomology. Used with permission.

²Figure 3 images courtesy of Donald Specker, Pioneer Hi-Bred.

³Poncho is a registered trademark of Bayer AG.

⁴Herculex I Insect Protection technology by Dow AgroSciences and Pioneer Hi-Bred. Herculex is a trademark of Dow AgroSciences LLC.

⁵Registered trademark used under license from Monsanto Co.