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Project description

Economic injury levels, within-plant vertical distribution, population dynamics, and sampling technique for tetranychid mites on corn. (00DS030)
Program UC IPM competitive research grants program
Principal
investigators 		L.D. Godfrey, Entomology, UC Davis
J.J. Cisneros, Entomology, UC Davis
Host/habitat Corn
Pest Spider Mite
Discipline Entomology
Review
panel 		Decision Support
 Start year (duration)  2000 (Three Years)
Objectives Develop an economic injury level for tetranychid mites on both silage corn and corn intended for grain harvest.

Determine the population dynamics and within plant distribution of tetranychid mites on corn throughout the growing season.

Develop a presence/absence sampling technique for mites.

Final report The effects of spider mite injury on corn productivity (silage yield and grain yield) were examined at eight locations over this three-year study (2000-2002). Spider mite populations were effectively manipulated with acaricides, various rates, and active ingredients (both registered and experimental). Mite populations were lowest in 2000, primarily due to the use of June-planted fields. Populations were highest and most consistent among the three sites in 2002 as we refined our knowledge of the system and experimental techniques. Mite populations peaked at 813 (untreated plots) and 367 (Orthene treatment + hand-infestation of mites) mites per 7th leaf from the bottom of the plant at field sites in Winters and Davis, respectively, in 2002. Populations on the same sample dates as these peaks averaged 34 and 53 mites per leaf for the most effective acaricide treatment at the Winters and Davis sites, respectively. Mite-days, an expression of seasonal mite pressure, ranged from 4000 to 24000 at the Winters site and from 1400 to 8000 at the Davis site. Silage, earlage, and grain yields responded to mite density at two, four, and one test, respectively, out of the eight total tests. For instance, the silage yields were 39.3 and 40.7 t/A for the two population extremes in the Winters test in 2002. There was no response of silage nutrient values (crude protein, ADF, and NDF percentages) from any of the locations. For instance, silage nutrient values (crude protein and NDF) from the Winters site were 7.7 and 54.5% for the most effective treatment and 7.6 and 57.6% for the untreated. Results of within plant distribution of spider mites were consistent among the sites and years. The highest mite densities were seen on the middle leaves (leaf positions three to seven) and the lowest mite densities on the youngest leaves. Spider mite populations tended to decline noticeably about four weeks before silage harvest. In some cases, predators were involved in this decline, but other (unknown) factors also appear to be important. Overall, spider mite infestations appeared to have low to moderate (at worst) impact on corn production, particularly silage corn yield, and no consistent effects on nutrient quality.

Third-year
progress 		The effects of spider mite injury on corn productivity (silage yield and grain yield) were examined at three locations in 2002. Spider mite populations were the highest experienced during the three years of this study. Levels were manipulated with acaricides, various rates and active ingredients. Populations at the peak ranged from 35 to 800 mites per 7th leaf from the bottom of the plant and from 50 to 375 mites per 7th leaf at a second field site. Mite-days, an expression of seasonal mite pressure, ranged from 4,000 to 24,000 at site 1 and from 1,400 to 8,000 at site 2. Some slight differences in plant height, color, and vigor were noted, but not substantial differences. Silage and grain yields were not significantly different across the treatments at any of the three sites. Silage yields ranged from 33.8 to 43.4 t/A. Silage nutrient values, crude protein, ADF, and NDF percentages, from the samples collected in 2001 also did not change in response to spider mite damage. Results of within plant distribution of spider mites followed similar patterns at two of the three field sites to that seen in 2001 and 2000 studies. The highest mite densities were seen on the middle leaves (leaf positions three to seven) and the lowest mite densities on the newest leaves. The spider mite population tended to decline noticeably about four weeks before silage harvest. In some cases, predators were involved in the decline, but other factors also appear to be important. At the site with the highest mite population in 2002, the within plant distribution pattern was skewed with high mite levels spread over more leaves and for longer in the growing season.
Second-year
progress 		Results during the past two years of the study show that the within plant distribution of spider mites follows a curve shape with the highest mite densities on the middle leaves and the lowest mite densities on the newest leaves. Data from both 2000 and 2001 demonstrated that the highest densities were located between leaf positions three to seven. The 4th leaf in 2000 showed a positive linear relationship between percentage of infested leaves and the cumulative mite-days (1 mite-day=1 mite per leaf feeding for 1 day), indicating that this leaf may be used as a presence/absence sampling method for mite population estimation. Sufficient correlation was not seen in 2001 to support this relationship. Overall, we were successful in creating different mite densities in the plots using miticide treatments. There was a significant trend for lower mite densities in the plots treated with high compared to low miticide concentrations. Hand-infested and all Comite treated plots in 2001 resulted in mite densities greater than the untreated control. Despite the different mite populations achieved among plots, no significant differences were found among yields for silage in Yolo County. A weak linear relationship was found with increased mite populations resulting in decreased grain yields. It is expected that mite populations have to reach higher densities than those observed during 2001 to obtain more significant negative effects on yields. Mite densities from these remaining two locations are still being quantified from stored samples.

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