In this Issue:
SPECIAL FEATURE
Corn Earworm Trap Catch and Sweet Corn Ear Infestations in Minnesota: Research Update
VEGETABLE NEWS
STRAWBERRY NEWS
The 2006 Strawberry Season: Short, Sweet and Buggy
APPLE NEWS
| Vol 3 No. 7 | June 30, 2006 |
Corn Earworm Trap Catch and Sweet Corn Ear Infestations in Minnesota: Research Update
Eric Burkness and Bill Hutchison, Department of Entomology, University of Minnesota, St. Paul
As the silking stage of early-planted sweet corn approaches in Minnesota, decisions will soon be made regarding when and if insecticide applications should be made to specific fields. Although the silk stage is susceptible to European corn borer (ECB), and in recent years, clipping by corn rootworm beetles, the focus of this article is on corn earworm (CEW). Information that is necessary to make good management decisions for CEW includes trap catch data from pheromone and/or black-light traps, and the growth stage of sweet corn; i.e., when does the vulnerable crop growth stage (or “treatment window”) match up with increasing moth or egg-lay activity? Based on trap efficiency and ease of use (identification of insects and portability), pheromone traps are often a much better choice for monitoring CEW populations in Minnesota, particularly early in the season. Blacklight traps can be used for CEW, but their use is limited to late-season monitoring, when CEW populations are much higher. Another key difference is that pheromone traps only catch male moths (attracted to the synthetic version of the female moth's sex pheromone), whereas light traps catch both males and females. Regardless of the trap type, several questions often arise including: “…what do the trap catch numbers mean? Are 5 moths/night a lot, or should I not worry about that level? In addition to trap catch, another question is which trap is better: Scentry or Hartstack?”
Beginning in 2005, we established several trials to provide more answers to these questions. In this article, we summarize the results from our primary study at Rosemount (Dakota Co.), MN. Here, we set up a trial with 5 planting dates of sweet corn (starting approximately on May 1st, then planting a new plot every 2 weeks); we also used 2 Scentry (nylon-mesh) and 2 Hartstack (wire-mesh) pheromone traps to monitor CEW flights, placed on a field edge next to the sweet corn plots. Traps were checked 3-5 times per week and sweet corn plots were harvested based on ear maturity (ca. 70% moisture). In Fig. 1, the solid lines indicate the average trap catch for both Scentry and Hartstack traps; the symbols indicate the mean number of CEW larvae per ear at harvest for each planting date of sweet corn.
For the first two planting dates, we observed virtually no CEW larval infestations or damage. For the 3rd planting date, we observed a low infestation of about 0.03 larvae/ear. Silk initiation for the first 3 planting dates occurred on July 19th, 22nd, and 27th respectively. The CEW trap catch for that time period peaked at about 1 moth/trap/night in the Scentry trap and 5 moths/trap/night in the Hartstack trap. For the 4 th and 5 th planting dates, silking occurred on August 12th and 26th, respectively. Trap catch just prior to silking in the 4 th planting date was about 3 moths in the Scentry trap and 38 in the Hartstack trap. The resulting ear infestation was 0.2 CEW larvae per ear. Trap catch just prior to silking in the 5th planting date was about 7 moths in the Scentry trap and 50 moths in the Hartstack trap. The ear infestation for the 5th planting date was about 0.8 CEW larvae per ear. |
Figure 1 |
| Based on previous research (Fig. 2, Table 1) we have established that there is a good relationship between the percentage of ears infested with at least one CEW larva and the overall mean CEW larvae per ear. In Table 1, where selected means are summarized for a given percentage ear infestation, it is clear that very low levels of CEW larvae per ear can lead to relatively high percentages of ears infested (e.g., 0.15 CEW larvae per ear = 15% ears infested). So, for the 5 th planting date in our study, trap catches of 7 and 50 moths per night in Scentry and Hartstack traps, respectively, resulted in 0.8 CEW larvae/ear which is roughly equivalent to 60% of the ears being infested (with one or more larvae/ear). |
| Table 1. Relationship between percentage of ears infested with at least one CEW larva and mean CEW per ear in Minnesota sweet corn (based on multiple data sets; 1997-2005). | |
Percentage of ears infested |
Mean CEW larvae per ear |
5 |
0.05 |
10 |
0.10 |
15 |
0.15 |
20 |
0.20 |
25 |
0.26 |
40 |
0.45 |
60 |
0.76 |
75 |
1.10 |
100 |
1.65 |
So what does this information tell us? As the trap catch increases in either trap we see an increase in ear infestation. The trap catch that occurs at silking is related to the final ear infestation. Once trap catch increases above 2 moth/night in the Scentry trap and 4 moths/night in the Hartstack trap, we start to see significant increases in CEW larval infestations at harvest. These results also confirm that that we should be able to determine a threshold for moth catch during the silk stage at which time a management program should begin. Also, both Scentry and Hartstack traps should be useful for predicting CEW populations that will cause significant ear infestations. However, there is a roughly 5- to 10-fold difference in the trap catch between the traps on the same day. A key consideration for evaluating trap performance for IPM purposes, is not only how many moths are caught, but how well the trap catch predicts ear infestations. For both traps, it appears that they perform reasonably well. However, these trials will be repeated in 2006, at more locations to further assess the value of each trap in side-by-side comparisons.
For More Information:
Sweet Corn chapter, by B. Flood, B. Hutchison and S. Pataky (2005), In: Flood and Foster (Eds.), Vegetable Insect Management, Willoughby, OH. (http://www.meisterpro.com/vim/ or call: 1-440-942-2000).
ACKNOWLEDGEMENTS
Co-Editors: Bill Hutchison (hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (woldx018@umn.edu), Department of Entomology, University of Minnesota
The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jean Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us, MDA, 625 Robert St. North, St. Paul, MN 55155-2538. You can access the Newsletter at the U of MN web site in htm format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf format at: http://www.mda.state.mn.us/ipm/ipmnews/
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