Manitoba
Printer Friendly

Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

July 28, 2008                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives,
Crops Knowledge Centre, Phone: 204-745-5668; Fax: 204-745-5690

To report observations of insect activity or control that may be of interest or importance to others in Manitoba, please send messages to the above contact addresses.

To be placed on an e-mail list to be notified when new Manitoba Insect Updates are posted, please contact John Gavloski at the address or numbers listed above.


Summary

Soybean aphids are now present in Manitoba, although at well below economic levels. Diamondback moth levels are becoming more noticeable in some fields in southeast Manitoba. Localized hotspots of grasshoppers are still of concern. Alfalfa weevils continue to be a concern on alfalfa.

Recent Insect Concerns and Observations

Soybean Aphids: Soybean aphids are now present in some soybean fields in southern Manitoba, although at levels well below economic threshold. Last week, soybean aphids were found or reported from fields near Altona, Carman, and Portage la Prairie.

The economic threshold is an average of 250 aphids per plant and the population still increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages. None of the fields where soybean aphid was found last week were anywhere close to this.

The economic injury level, the level of aphids capable of doing enough damage to soybeans to equal the cost of the insecticide and application, is actually about 670 aphids per plant (Journal of Economic Entomology. 2007. pg. 1258-1267). This number is based on data from 19 experiments performed in 6 different locations; so while some thresholds are quite nominal (essentially best guesses) this threshold for soybean aphids is backed up by a lot of data. The economic threshold of 250 (which is well below the economic injury level of 670) was set because if the population was not being adequately controlled by natural enemies, it would take about a week before soybean aphid levels got to about 670 per plant. This provided adequate time to confirm the population is still increasing and get controls applied.

Some have wondered if and how this threshold should be adjusted for high commodity prices. It would likely not be a wise or economical move to start using thresholds below 250 per plant and the population increasing. Aside form the fact that this would be getting quite far from the economic injury level of 670, using thresholds lower than 250 would not be providing proper opportunity for natural enemies to stabilize aphid populations below the economic threshold. Numerous studies have shown that in many situations natural enemies will be able to stabilize soybean aphid levels below economic threshold numbers if given proper opportunity to do so (free control for the farmer). When commodity prices are high, it may be advantageous to apply controls within 3 to 4 days of populations reaching 250 and increasing, instead of delaying for closer to 7 days. But applying controls at levels below 250 per plant (and populations increasing towards 670) is not likely to be in a farmers best interest.

Diamondback moth: Populations of diamondback moth have been getting noticeable in southeastern Manitoba. Although levels have not been reported above economic threshold yet, some populations approaching economic threshold have been noted in canola near Emerson and Altona.

The only way to properly determine if diamondback moth is at economic threshold is to tap the caterpillars out of the plants and count those dislodged or dangling on silk threads after the plants are tapped. Although economic thresholds are sometimes given as larvae per m2, for practical purposes it is going to be easiest to do counts in a foot2 area of canola in several areas of the field. Thresholds are 20 to 30 larvae per foot2 of canola on average. Doing threshold counts this way is less likely to result in erroneous decisions. In the past some have attempted to make decisions based on numbers counted on individual plants. The danger in this is that unless this is at very high replication and completely randomly, it is easy to pick plants that have larvae on them, do counts on those plants only, then conclude the field is above economic threshold. The threshold is the average number per plant, including those plants that have none. And doing counts on larvae shaken from groups of plants in several areas of the field ensures proper replication is done.

Surveys and Forecasts

Bertha Armyworm: Highest trap counts continue to be in the western part of Manitoba. Highest trap counts so far are 883 (Virden), 739 (Minitonas), 493 (Brookdale), 433 (Durban), and 422 (Neepawa). Counts in this range put these regions in a risk category labeled “Uncertain”; where infestations may not be widespread, but fields that were particularly attractive to egg-laying females could be infested. Data from the bertha armyworm adult monitoring program are available at:
https://www.gov.mb.ca/agriculture/crops/insects/bertha/index.html

Trap counts have peaked and are now declining, with the exception of counts from traps in the Northern Interlake. The data based on adult counts has now been accumulated to enable us to predict the risk for this year, and the focus should now turn to monitoring for the larvae. So traps can be taken down after the counts are taken this week.

Overall the risk is much lower this year than it has been the past few years. So hopefully this is an indication that the population is on the decline. However, field scouting is still advised, particularly in the areas that rated as “uncertain” risk.

Banded Sunflower Moth: We have had a pheromone-baited trap set up for the past few weeks north of Carman determining when banded sunflower moths have been present and relative numbers. The trap was set up on June 20th, and we have recorded the following data since then:

Date             Trap Count
June 27              0
July 4                 0
July 14               5
July 15              53
July 21             147
July 25             175

So banded sunflower moth started to emerge around mid-July in the Carman region and we may be getting into the period of peak flight; this will be clearer after the trap is checked this week. So for confection sunflower growers, now is the time to look for banded sunflower moth while scouting your sunflower fields. The following picture show some of the adult banded sunflower moths from the trap.

Banded Sunflower Moths

These trap catches can alert us to when banded sunflower moth populations are emerging, and eventually when peak emergence seems to be peaking. But the traps can not be used to determine economic thresholds or make management decisions. Like other pheromone-baited traps we use, the purpose is to assist in determining the timing and necessity of field scouting within a region. The following factsheet on banded sunflower moth from North Dakota State University provides more detailed information on banded sunflower moth and scouting techniques: http://www.ag.ndsu.edu/pubs/plantsci/pests/e823w.htm

Crop Scouting Reminders and Tips

Staging sunflowers: Some have been asking about resources to help stage sunflowers to properly time scouting and potential insecticide applications in sunflowers. A good resource for this is a picture key published produced by North Dakota State University. The photo key is available electronically at: http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a1145.pdf

Note that R-5.1 stage is the stage that would be most ideal for treating the seedhead insects that are most abundant in confection sunflower fields this year. We are seeing banded sunflower moth and lygus bugs in sunflowers, but seed weevils have not been found or reported (they will be present but in numbers that in most fields will be almost non-detectable). So scout the sunflowers to see which insects are detected, and make management decisions based on the levels and types of insects that are present.

Insect Identification Quiz

Question: While scouting canola, you are finding some of these green caterpillars that move backwards rapidly and often drop from silken threads when you disturb them. You are also finding some of these small silky cases on the plants as well. What are these caterpillars and the silky cases?

Answer: These are both diamondback moth. The green caterpillars are larvae of diamondback moth. They get to be about 12 mm when fully grown. The silky case in the photo is the pupa stage of the diamondback moth. They pupate on the plant. At this time of year, if you look hard enough you can probably find all stages of diamondback moth (eggs, larvae, pupae, and adults) in most canola field. They are certainly something to monitor in canola over the summer, but numbers do have to be high before controlling them would be economical.