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2002 Annual Report

Effects of Cropping Rotations on Beneficial and Pest Insects
at the Central Great Plains Research Station; Akron, CO

M.D. Koch

PROBLEM: In 1986 the Russian wheat aphid Diuraphis noxia (Mordvilko) became a major small grains pest in Colorado. Control methods for this pest include cultural, mechanical, chemical, and biological. Russian wheat aphid control has been most effective using chemicals. However, producers may be able to combine cultural and biological techniques to decrease the need for other costly control measures. This would also diversify production on a given farm. Crops grown in close proximity to one another providing a year-long host of green vegetation may allow predators and parasites to survive and control pests. To test this theory, plots were established at three locations in eastern Colorado. Locations are near Briggsdale, Akron and Lamar.

APPROACH: The Akron, Colorado site was established in the spring of 1996. The previous crop was corn with a small area used for sunflowers. Crops being studied were selected by area production practices. The rotations being analyzed are: winter wheat-fallow; winter wheat-corn-fallow; winter wheat-corn-millet; and winter wheat-corn-sunflower-fallow. The individual plots for this location are relatively large, 90 feet wide by 180 feet in long. Experimental layout was in a complete randomized block with four replications. Every phase of the rotations is present each year in all four replicates. The wheat plots are divided in half lengthwise. One half is a susceptible variety and the other resistant to the Russian wheat aphid (RWA). The varieties used for 2002 were TAM 107 as the susceptible and Prairie Red as the resistant.

RESULTS - AGRONOMIC DATA: Precipitation was less than average for all crop seasons this past year. Moisture received for the wheat growing season of September 2001 through June 2002 accumulated just 5.95 inches. The average amount for this period is 11.73 inches. Spring crops also suffered from dry conditions. The 01 March 2002 through 31 October precipitation total was 7.88 inches as compared to an average of 13.97 inches.

Wheat yields ranged from a wheat-corn-sunflower-fallow rotation of TAM 107 at 22.04 bushels per acre to a low of a wheat-corn-millet rotation plot of Prairie Red at 2.85 bushels. Overall average of the plots was 10.38 bushels. TAM 107 averaged 10.28 and Prairie Red 10.49 bu./acre.

Corn yields were well below average for the Akron area this year. Hail stripped the leaves and damaged stalks of the suffering plants on 01 July. The plots were not harvested at this location. No grain was produced and the plants only grew to thirty inches in height.

Millet plots were not harvested this year. If not for the hail mentioned earlier, yields would have approached five to seven bushels per acre. The plants were short at seven inches in height and the heads were small at two or three inches. Grain production averaged only a few seeds per plant.

Sunflower yields were very low. Last year the wheat-corn-sunflower-fallow rotation yielded 1140 pounds. This year the average plot yield was slightly above 20 pounds per acre.

RESULTS - ENTOMOLOGICAL DATA: Since 1996, no insect populations have reached economic levels. Wheat pests were present in low numbers throughout the growing season. Mites and aphids were the prevalent insect pests. Russian wheat aphids Diuraphis noxia Mordvilko per 100 random tillers did not exceed three when put in Berlese funnels for 24 hours. Four aphids were caught through sampling. Of the four, one was found on TAM107. No Bird-cherry oat aphids Rhopalosiphum padi Linn or Greenbugs Schizaphis graminum Rond were found in the Berlese funnels after sampling. Onion thrips Thrips tabaci Linderman were found in low numbers during the growing season. Ladybird beetles and Lacewings were the primary predators in the wheat.

Pale western cutworms Agrotis orthogonia and Army cutworms Euxoa auxiliaris Grote were present in the Akron plots at slightly higher levels than the past few years. Total counts for three sampling dates showed one Pale western and 44 Army cutworms. Five random sites of one square foot were dug up per wheat variety to find the larvae. The highest population recorded was five per square foot. The average was 0.56 larvae per square foot.

Brown wheat mite Petrobia latens (Müller) infestations were very low this year. Sampling was performed with a Vortis insect sampler. A wooden stake was also used to compare populations. Thirty-one mites in a plot of wheat was the most observed throughout the growing season. The stake method seemed to show a higher density of mites when compared to the Vortis. Variability was high with both methods. There were few predatory mites observed this year.

Insect pests in corn were not of economic importance. There were eight cutworms found this year. Five were Pale western cutworms. The others were Army cutworms. Aphid species present were Greenbugs, Bird-cherry oat aphids, and Corn leaf aphids Rhopalosiphum maidis. Onion thrips were also noted but did not pose a threat to plant health. Corn earworm Heliothis zea, Corn rootworm Diabrotica virgifera LeConte and Sap beetles Carpophilus lugubris M. were all present in the plots. Only two Corn rootworms were found during sampling.

Greenbugs and Bird-cherry oat aphids were the only notable pest insects in the millet crop this year. The number of aphids was well below economic thresholds. Spiders were the main predators seen in the plots. Also, Ladybird beetles were present to help control the pests.

Three cutworms were found while sampling the four sunflower plots. No aphid species were present. Spotted sunflower stem weevil Cylindrocopturus adspersus Leconte did not cause any lodging. Sunflower head moth Homoeosoma electellum Hulst, Banded sunflower head moth Cochylis hospes Walsingham, and Red seed weevil Smicronyx fulvus Leconte were all present in low numbers. Seed head diameter was about two to three inches. Thus seed size was very small. Only two Grey seed weevil Smicronyx sordidus Leconte were noticed while sampling. Predators were scarce in this crop. Sampled plants throughout the growing season showed only one spider and no other beneficial insects.

FUTURE PLANS: This study is scheduled to continue several years. No changes to experimental design or layout are foreseen. Varieties and other agronomical and entomological inputs and data collection methods may vary depending upon new technologies or climatic influences.

The author may be reached via e-mail: M.D. Koch
or phone: 970-345-2259.