Wheat

Wheat Midge

Although wheat midge populations have now declined, and most crops are no longer susceptible, there have been several questions over the monitoring and control of wheat midge that are good to review.

When is Wheat Susceptible to Wheat Midge?

Wheat will become susceptible to wheat midge once the head emerges, but kernel damage decreases by 15-fold to 25-fold between the later stages of heading and early flowering or anthesis (when the first yellow anthers appear on the wheat head). Fields should be inspected from the time wheat heads emerge from the boot until anthers are visible on the heads. Once the anthers are visible, it is too late to be concerned over monitoring and control of wheat midge.

Are “Insurance Sprays” for Wheat Midge a Good Idea?

It is tempting to think that when applying fungicides to wheat that is still in the susceptible stages for wheat midge you may as well tank mix in an insecticide to control wheat midge at the same time. This may seem to be logical as an insurance spray to assure a good crop, but if wheat midge levels are not at least at economic threshold levels you do risk not only not recovering the cost of the insecticide in improved yield and quality, but also damaging the populations of predators and parasites that are keeping the wheat midge and other potential pests below their economic thresholds. This could create a situation where pests (aphids, future wheat midge populations, etc.) are without the natural enemies that will moderate the populations to below economic levels. So the best advice is still to only apply insecticides when pests are above an economic threshold. The thinking that tank mixing in insecticides to control potential pests that have not been found to be above threshold can’t hurt is incorrect, and can cause more problems than not recovering the cost of the product in improved yield and quality. We still seem to have strong populations of parasites keeping wheat midge in check in Manitoba, so may as well try to keep it that way.

Soybeans

Soybean Aphids

Soybean aphids are now showing up in a lot of soybean fields, resulting in some questions regarding making control decisions.

Economic threshold

Although in some types of crops the level of aphids required for spraying an insecticide to be economical is quite low, in soybeans it is just the opposite. The economic threshold for aphids in soybeans is 250 aphids per plant and the population increasing. To determine if a population is increasing means aphid numbers in the field have had to have been estimated more than once. 250 aphids per plant is NOT the damaging level but warns when the field is in danger of reaching damaging levels and needs to be monitored closely and frequently. If a soybean field is near the economic threshold for aphids, check the field every two or three days to see if the population is increasing. If the population is increasing, then treatment may be economical. If the population seems to have stabilized or is decreasing, keep monitoring the field but do not apply an insecticide. Although 250 aphids per plant on average seems a high threshold, and it would be tempting to apply an insecticide at lower numbers, not only would treatment not likely be economical at lower levels, it may do harm by removing the natural enemies that are helping to stabilize the population.

Timing of Control for Soybean Aphids

Research has shown that economic yield benefits to spraying occur once fields reach 250 aphids per plant with increasing populations, and when the crop is in the early reproductive stage (when there is one open flower on a plant) to R5 (beginning seed – when you can feel a tiny bump when you run your thumb over the top pods). Once the crop is in the R6 stage (full seed – when the top pods have seeds in them that are swollen and filling the entire pod cavity) aphid numbers need to be much higher to see a yield response.

Canola

Timing of Control for Bertha Armyworm

Bertha armyworm eggs are hatching, and some in the Swan River Valley are reporting that the larvae are becoming quite visible in some fields. The question of when is the optimum time to control bertha armyworm, should populations get above economic threshold, has come up a few times. Initially bertha armyworm will feed on the leaves on the plants, and as long as there is green leaf material on the plant will prefer to restrict their feeding to the leaves. However, as the crop advances and leaves senesce, or are eaten off, the larvae will move onto the pods to feed. Research from Agriculture and Agri-Food Canada in Winnipeg has shown that little measurable economic loss occurs until pods are damaged. So the ideal timing of an application, should it be economical, would be just as the larvae are starting to move onto the pods to feed. Some defoliation of leaf material or senescence may help improve the penetration of the insecticide into the canopy should an application be necessary. But you do not want much direct feeding on pods to occur. Research from Agriculture and Agri-Food Canada indicates “although control measures for bertha armyworm should not be undertaken before pod damage is observed, a control decision should be made before 20% of the pods are damaged” (Canadian Entomologist 1987: 365-369).
 

Insect Monitoring Programs Update

Bertha Armyworm Monitoring

Click here for the latest map of bertha armyworm trap counts.

Insect Identification Quiz

Question

This week the question is a tough one. This caterpillar was found on sunflowers, and I had a report of them on hemp from the Dauphin area, and received a sample of soybeans with these larvae on them.

Answer

This is the larva of the saltmarsh caterpillar. Although we are seeing more of them than in most years, and they will feed on many crops, numbers do not get high enough for them to be a pest in Manitoba. I have never seen a field of any crop with high numbers of these in Manitoba, but because they are highly visible they often get noticed when a crop scout is in the area.

Insects and Diseases in Vegetable Crops

Visit the Manitoba Weekly Vegetable Report for information on insects and diseases in vegetable crops in Manitoba.