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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

August 5, 2008  

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives
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 address.

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Summary

Soybean aphid numbers have been climbing in some soybean fields. Lygus bug and banded sunflower moth are a concern in some fields of confection sunflowers, however populations of sunflower seed weevil are very low.

Recent Insect and Disease Concerns and Observations

Soybean Aphids: Soybean aphid populations have increased in some fields. At least 1 soybean field in the Altona area has exceeded the economic threshold and had insecticides applied to control the aphids. Note that heavy rains are a natural aphid control, and natural enemies can stabilize or even start decreasing soybean aphid numbers. So keep monitoring soybean aphid levels, and use the economic threshold of soybean aphids actively increasing above 250 per plant from the R1 to R5 stage of soybeans.

Note in Figure 1 that the greenish-yellow spots on the leaf are all soybean aphids, the white specks on the leaf is their cast skins. When they are abundant the leaves may get sticky as well from the honeydew they excrete.

Figure 1

Diamondback moth: Populations of diamondback moth are noticeable, and in some cases have been reported to be approaching economic levels in the Emerson/Altona and Brandon areas. Note that diamondback moth is another insect that at times can be reduced by heavy rains or natural enemies.

Insects in Confection Sunflowers: Some growers of confection sunflowers are apparently receiving letters advising them that they should be spraying for sunflower seed weevils. We have been looking in sunflower fields trying to find sunflower seed weevils. We can’t find any sunflower seed weevils! If anyone can find a population of seed weevils that comes anywhere close to even 1 per plant, please let me know. The timing for ideal control of the seed weevil is a bit later than for control of lygus bugs and banded sunflower moth, 2 of the insects we are seeing in the sunflower fields. So to time sprays properly for the seedhead insects, you need to know what you are going after. Aside from the fact that seed weevils don’t seem to be present, one of my concerns with processors sending out letters requesting the growers spray for sunflower seed weevils, is that the growers would be waiting until R5.4 to do the application in order to get maximum control of sunflower seed weevil (which would be the best strategy if seed weevils were the issue). Banded sunflower moth, another insect that makes holes in the seeds and eats the seeds, should ideally be treated when plants are in the R5.1 stage. Waiting until R5.4 could result in damage to the seeds occurring before the insecticide is applied. And if lygus bugs are at economic levels in the field (which is the case in some fields this year), spraying earlier (around R5.1) would also be a better strategy. So it is important to know what the biggest threats to confection sunflower crops are, and if control is warranted, to make sure they are timed correctly. Taking the approach of just automatically applying a spray for seed weevils could result in a poorly timed spray, and insect damage still occurring on the seeds.

Surveys and Forecasts

Grasshopper Survey: Each year in August, populations of grasshoppers are counted to predict the risk for the next season. Getting an estimate of the egg-laying population helps us determine potential hot-spots and the overall risk. Staff from MAFRI traditionally do a lot of the counts for this survey, but if anyone is interested in providing estimates of grasshoppers in their area, or the farms or area they scout, this would be welcome. The protocol, data sheets, and explanation of the grasshopper survey can be found at: https://www.gov.mb.ca/agriculture/crops/insects/fad95s00.html

Crop Scouting Reminders and Tips

Soybean Aphid Scouting Cards: A compact soybean aphid scouting card has been produced to help farmers and agronomists recognize soybean aphids, some common natural enemies of soybean aphids, and a reminder of the economic threshold for soybean aphid. Photos are also provided showing the difference between the R5 and R6 stage in soybeans. The card is available through local MAFRI offices, or contact me to have some cards sent to you or your organization. The card is also available electronically at: http://www.soybean.on.ca/aphid_view.php?id=43 .  

Insect Identification Quiz:

Question: These minute insects can often be found on sunflowers, soybeans and many other crops, sometimes in high numbers. Should you be concerned?

Answer: These are the adult (black and white) and nymph (yellowish-orange) of the minute pirate bug. Both stages are predators. In soybeans they can be a very valuable predator of soybean aphids. Be careful not to confuse the nymphs with red sunflower seed weevils when scouting sunflowers.