Texas Wheat: Making Aphid, Fungicide Decisions In Hill County

Bird cherry oat aphids on wheat leaf. Photo: Matt Montgomery, University of Illinois

Insects in wheat are starting to pick up, with a few fields with heavy bird cherry oat aphids, most of these fields are South and West of Hillsboro. Bird cherry oat aphids are present in 71 percent of the scouting programs fields, with 31 percent of the fields needing to be treated. These populations are starting to have some winged aphids meaning they could be moving into new fields soon by riding wind currents as they are not strong fliers.

There is a chance that a few of these fields not at threshold yet will need to be sprayed next week depending on how well our beneficial insect population is managing the population. I still have not found a field with a greenbug population yet, but fields should still be checked for them as these populations can build quickly with our current weather pattern.

There is not an established economic threshold for the bird cherry oat aphid in Texas wheat, but John Few, IPM Program Specialist in Williamson County found an article from the University of Nebraska, showing their economic threshold based on the growth stage of the field.

I have also started picking up some English grain aphids, these insects have black antennae, cornicles and legs which helps to tell the apart form greenbugs. They seldom become an economic pest, but are vectors of Barley Yellow Dwarf Virus. Texas does not have an established economic threshold for the English grain aphid.

It is suggested to use the Nebraska economic threshold which is, 5 or more aphids per stem during flowering, 10 or more per stem during milk stage, and more than 10 per stem between milk and medium dough stage.

Currently the English grain aphids are being found in only two fields in the scouting program around the Abbott area.

So, should you spray for aphids now or wait until you need a fungicide? That’s a common question.

Some people are up in the air about spraying for bird cherry oat aphids right now wanting to wait to treat until they spray the field with a fungicide to protect the flag leaf. Three concerns I see with waiting are:

Some fields may lose yield potential before the flag leaf emerges because of the population size.

Not every wheat variety is going to need a fungicide thanks to host plant resistance.

We are not sure our weather conditions after flag leaf emergence is going to be conducive for the development of rust diseases (leaf rust or stripe rust).

A publication from Oklahoma State University found that 20 aphids per tiller before boot can lead to a 5 percent yield loss, and 40 aphids per tiller prior to boot can cause a 9 percent yield reduction.

I have seen field population close to these numbers, and very little beneficial activity in these fields. Based on these observations and the data from OSU, depending on the population, there could be a yield loss of up to 9 percent or greater.

If this potential yield is lost, by the time we need to spray a fungicide, there may not be a high enough yield potential to justify the application of a fungicide.

The second concern is because of host plant resistance to our rust pathogens not every field is going to need a fungicide application, but at the same time very few wheat varieties for the state of Texas has resistance to bird cherry oat aphids. In these varieties we may not need a fungicide application and could see extensive yield loss if the bird cherry oat aphid population is not treated.

In rust-susceptible varieties if aphid populations are not treated now, we may not have a high enough yield potential to justify a fungicide application. The third concern is we can not predict what the weather is going to be like in a month or more. Rust pathogens need specific weather conditions to cause disease.

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So, it is hard to know whether we will need a fungicide to protect the upper three leaves from becoming severely diseased which can lead to yield loss. Based on the potential yield loss that can be caused by bird cherry oat aphids it is recommended to treat fields for aphids once the thresh-old in Table 1 is reached regardless of growth stage to prevent yield loss as we do not know if or when we will need to apply a fungicide.

It is understood that some producers and even consultants want to hold off on treating the aphid population until it is time to apply a fungicide to save on the number of applications to a field which can reduce input cost. We also need to look at is if there is a potential for a cost of application for separate applications of an insecticide now and a fungicide later could still be less that the value of the yield that would be preserved by applying an insecticide now.

Click table to enlarge.


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