Part Two: Insects to Watch for in a Late Planting Year (Like This One)

Late planting and preventive plant acres will bring new challenges to agriculture in 2019, which is why we need to stay diligent and do our best to plan ahead. Last week we discussed diseases to scout for this growing season, this week we’re continuing our conversation but exploring insect pressure. Here are some tips for scouting symptoms of insect activity in growing crops.

What issues will delayed planting, slow growth and development cause for the 2019 season? 
While we covered this question in more detail last week, it’s important to remember that delayed planting will ultimately lead to delayed harvest with decreased yields. It will also lead to a greater prevalence of insect pressure.

Why more insect pressure?
The issue is not so much that insect pressure is necessarily higher, but there are two critical factors that will exacerbate its effects this growing season:

1. Less margin for error from decreased yields due to the late-planted crops. Knowing the insect species, growth habit and potential damage as well as diseases they may vector to the crop will become more important this year.
2. The second issue is having more green tissue growing throughout the “insect season” due to late planting, which may lead to more potential issues. Plants will mature at a slower pace due to cooler, wetter conditions in their growing environment, so we may have to weigh decisions on treating insects that we normally wouldn’t worry about.

So how do we determine when to spray?  This information from Iowa State University provides guidance:

“The most effective way to make treatments decisions for pests with chewing mouthparts (e.g., Japanese beetle, bean leaf beetle, caterpillars, and grasshoppers) is to estimate defoliation…Replicated data over multiple growing regions consistently shows economic thresholds for soybean are 30% in the vegetative stages and 20% in the reproductive stages. This threshold applies to the entire field. Sometimes these pests cause significant injury along field margins and perimeter treatments may be more cost effective if practical.”

Determining Percentage Defoliation of Soybean Leaves
Gather 10 to 15 trifoliates from random plants, making sure to pick the first from the bottom of the plant, the second from the middle and the third from the top, then repeat that process. Choose the worst defoliated leaf from each trifoliate. Repeat this process in three to four areas of the field and then estimate the percentage of damage for the entire field. For reproductive damage, choose and estimate pods. 

Northern Region Insects (Corn)
Wireworms
White grubs
Black and dingy cutworms
Armyworm
Corn earworm

Southern Region Insects (Corn)
Japanese beetles
Corn earworm
Western bean cutworm
European corn borer (conventional hybrids)

Management Tip
In corn, look out for root and stalk feeding, silk clipping and grain feeding. For growers who have planted conventional hybrids, watch for boring insects and be ready to harvest early if you have damaged fields. Spraying insects can be tricky because they are able to move and find protection.

Northern Region Insects (Soybeans)
Aphids (leaf feeder)
Bean leaf beetle (leaf feeder, also vectors diseases)
Soybean gall midge (root and leaf feeder, may vector diseases)
Thistle caterpillar (leaf feeder)
Soybean podworm (pod feeder)

Southern Regions Insects (Soybeans)
Aphids (leaf feeder)
Grasshoppers (leaf feeder)
Stink bugs (pod feeder)
Soybean podworm (pod feeder)

Management Tip
In soybeans, watch for leaf feeding and pod feeding. Leaf feeding can be deceiving to the eye when it comes to the extent of damage. Pod feeding insects will cause more significant damage and should be given priority. Utilize local extension service traps for moth flights, egg hatches, etc. Know the enemy that threatens your crop. Remember, 30 percent vegetative defoliation is the economic threshold for chewing insects damaging leaves, and 20 percent pod damage is the economic threshold from piercing and sucking insects. Presence of insects does not mean treatment is warranted. Only when the population becomes unmanageable and economically destructive is treatment necessary.

For more information, contact your local Stine regional sales agronomist or university extension office. Stay tuned next week for Part Three of our series as we explore chemicals showing good crop response in corn and soybeans.