Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
As you make plans to plant small grains, be sure to consider the following insects:
Hessian Fly – As indicated in last week’s newsletter, you still need to consider this pest as you make plans to plant small grains. The fly can survive as puparia (“flax seeds”) in wheat stubble through the summer. In our area damage still generally results from spring infestations. These spring populations result from a build-up of populations in the fall. Wheat planted too early can serve as a nursery for the fall generation. The subsequent spring generation can seriously damage any nearby late-planted fields. Plants attacked in the spring have shortened and weakened stems that may eventually break just above the first or second node, causing direct loss from plants lodging near harvest. Warm fall weather conditions can extend fly emergence and egg-laying beyond the fly-free dates, but these dates should still be used as a guideline for planting.
Although not common in our area, we do see fall infestations from time to time. Plants attacked in the fall at the one-leaf stage may be killed outright. Wheat attacked later in the fall will be severely stunted, with the first tillers killed and plant growth delayed. Plants infested in the fall can easily be recognized by their darker than normal bluish coloration and leaves with unusually broad blades. The following link to a YouTube video from North Carolina provides more information on Hessian fly infestations in small grains (https://www.youtube.com/watch?feature=player_embedded&v=Z4WnuXs0QGc)
The following combinations of strategies are needed to reduce problems from Hessian fly: (a) be sure to completely plow under infested wheat stubble to prevent flies from emerging, (b) avoid planting wheat into last season’s wheat stubble, especially if it was infested with Hessian fly, (c) avoid planting wheat next to last season’s wheat fields – the most serious infestations can occur when wheat is early planted into wheat stubble or into fields next to wheat stubble, and (d) eliminate volunteer wheat before planting to prevent early egg-laying, (e) do not use wheat as a fall cover crop near fields with infestations and (f) plant after the fly-free dates: Oct 3 – New Castle County; Oct 8 – Kent County; Oct 10 – Sussex County.
Aphids – You should also consider aphid management as you plan to plant small grains. In general, cooler summer temperatures with adequate rainfall followed by a warm, dry fall favors aphid development in small grains, especially in early planted fields. Early fall infestations of the greenbug aphid are favored by cool, late summer conditions. In the fall, the aphid that can cause direct damage to small grains is the greenbug aphid species. It can inject a toxin into the leaves and cause death of plants. In outbreak years, we have seen entire sections of fields killed by this aphid. When it comes to Barley Yellow Dwarf Virus (BYDV) and its spread, all aphid species occurring in Delaware small grain fields (English grain aphid, bird cherry-oat aphid, corn leaf aphid, and the greenbug) are capable of transmitting BYDV from infected grasses into small grain fields. In the fall, the 2 most common species encountered in Delaware small grain fields are the bird cherry-oat aphid and the greenbug aphid.
Aphid population densities in small grains in the fall are also affected by when the first hard frost occurs in relation to wheat seedling emergence. Crops that emerge long before a hard freeze have a greater potential for aphid infestation (and exposure to BYDV). Planting after the fly free date can help to help to manage aphids as long as the freeze occurs when expected. Aphids arriving in the fall will continue to feed and reproduce as long as temperatures remain above 48°F.
In areas where you have seen BYDV in the past, where you are planting early (before the Hessian fly-free date), or you have seen direct damage by greenbug aphids, a commercial applied seed treatment which includes an insecticide would be a good control option for fall infestations. Another option would be to scout fields and apply a foliar insecticide. Information from Kentucky indicates that planting date is the most important factor determining the intensity of an aphid infestation. The most important time for controlling aphids in the fall is the first 30 days following emergence. The second most important time is the second 30 days following emergence. So it will be important to scout wheat starting at plant emergence if you plan to use a foliar insecticide for fall aphid management.
We will once again evaluate southern thresholds for fall aphid management and will then have 3 years of experience under different fall conditions to determine how these thresholds apply to our area. The thresholds we are evaluating that were developed in states in the southern region are (a) the first 30 days after planting treat if you find an average of three or more aphids per row-foot, (b) from 30-60 days after planting treat if you find six or more aphids per row-foot, and (c) more than 60 days after the plants emerge treat if you find ten or more aphids per row-foot. Information from the south also indicates that if weather conditions remain warm through the fall and early winter favoring increases in aphid populations, two treatments may be needed so fields will need to be scouted longer to determine if a spray is needed under these weather conditions.
Other Miscellaneous Insect Problems – In the fall, we can also see problems, especially in no-till fields, from slugs, fall armyworm and true armyworm. In past years, these pests have significantly reduced stands resulting in the need to re-plant. Unfortunately, no thresholds are available. Scouting is important as soon as plants emerge to be able to apply a management option before significant stand loss occurs.