By Phyllis Bongard and Jeff Coulter et.al
Dry conditions continue to intensify across the state and impact crop development and insect pests. Drs. Jeff Coulter, Extension corn agronomist, and Bob Koch, Extension soybean entomologist, addressed corn pollination and soybean insect concerns during the July 12 Field Notes session. They were joined by moderator Anthony Hanson, Extension crops educator, for the wide-ranging discussion.
Corn agronomy update
Overall, corn development is about 8 to 10 days ahead of normal, despite delayed planting in many parts of the state. Warm temperatures during the early summer accelerated corn growth, compensating for the late spring. Generally, above normal temperatures during the vegetative period don’t hurt the crop, but if they occur during pollination and grain fill, yield potential can be impacted. Ideally, temperatures that don’t exceed the 80s F are favorable for the crop and help minimize stress.
Coulter expects that the heart of the pollination period will occur next week. While temperatures are expected to be favorable, it has continued to be very dry across much of the state. Drought stress during this critical period from now to early August has the potential to reduce yield.
Moisture stress during pollination and grain fill
Number of kernels
The most critical time affecting the number of kernels per ear is from about 10 days before tassel emergence through the blister stage. There are two different ways drought stress can affect the number of kernels. The least common way for moisture stress to affect kernel numbers in Minnesota is to delay silking. Tassel emergence, however, isn’t delayed, so the late-emerging silks miss being fertilized and kernels never develop.
In Minnesota, it’s more common for drought stress to affect kernel number after successful pollination. Fertilized kernels, especially near the tip of the ear, have a high demand for moisture. When plants are under moisture stress, the end kernels can dry up and be lost. This type of kernel loss can extend through the milk stage.
Kernel size
By mid-August, the crop should be past the critical pollination period and into the grain-filling period. While not as critical as the pollination period, drought stress during grain-filling can significantly reduce kernel size and consequently yield.
Hail
Hail occurs somewhere in Minnesota each year and 2023 is no exception. From a development standpoint, hail just prior to tasseling is a major concern. Since most the crop’s leaves have already emerged by this stage, there aren’t many more to replace the hail-damaged ones.
A hail-damaged crop could lose quite a few tassels and still have sufficient pollen to fertilize the silks. Each tassel can produce between 2 and 20 million pollen grains while each plant has about 800 silks. If remaining tassels are distributed relatively evenly throughout the field, there should be plenty of pollen for the silks. Similarly, if the primary ear is damaged, the ear below will often develop into a full ear.
In hail-damaged fields, look for the degree of stalk damage, which can cause lodging later in the season. If damage is severe, consider harvesting those fields earlier.
For more information, see Wind and hail damage to pollinating corn.
Overall yield outlook
There has been a lot of variability in crop development around the state. Some fields were planted early and look good, while others were planted later and are somewhat behind. Coulter estimates that we’re currently on track for somewhat below to average yields, depending on the moisture and temperature conditions going forward. However, it’s too early to make predictions with any kind of confidence.
Soybean insect pests
From Koch’s perspective, it’s been an interesting year for entomology. It started with armyworm flights into the state and their resulting damage and has shifted toward pests tied to dry weather. Like crop development, moisture and temperature conditions have a significant impact on which soybean pests become issues and how their populations develop.
Pests favored by dry conditions
Dry conditions favor two key insect pests that growers should pay attention to.
Grasshoppers
Reports and concerns about grasshoppers have been building over the past 2 or 3 years as dry conditions have continued. Grasshoppers often move into fields from surrounding habitats and can cause significant defoliation.
For more information, see Grasshoppers on soybean.
Spider mites
Spider mites are also favored by drought conditions. While numbers remained low early this season, some fields in west central and southwestern Minnesota recently started showing injury symptoms from this pest.
For more information, see Two-spotted spider mites on soybean.
Soybean aphids
Unlike grasshoppers and spider mites, soybean aphids do not thrive at high temperatures, since it slows down reproduction and decreases survival. However, now that temperatures are moderating, soybean aphid population growth may be favored.
In recent years, Koch has noted populations that would increase to about 100 aphids per plant and then plateau. Prior to this, populations would frequently skyrocket to very high levels. What’s causing this new trend? He speculates that the new parasitic wasp or other natural enemies might be knocking the populations down, but more information is needed. The bottom line is that fields need to be scouted to determine which fields have populations that need to be treated and which ones don’t.
Scout fields now!
The earliest planted soybeans will be most attractive to aphids, but as we transition into the later season, start scouting later-planted soybeans, as well. Monitor soybean aphid populations every 7 to 10 days to catch any rapid increases. To get a good field average, walk an M-pattern or zigzag through the field and count the number of aphids on multiple plants.
When the average number of aphids reaches 250 per plant, start lining up an insecticide treatment.
Economic threshold still valid
Research shows that 250 aphids per plant is still a conservative threshold for protecting soybean yield. This economic threshold (ET) is the trigger point for scheduling an insecticide application and is significantly lower than the level where economic injury actually occurs. The ET buys time for making the application.
While it may be tempting to treat soybean aphids when applying an herbicide or at numbers below the ET, there are significant downsides to spraying too early. The few aphids present may be controlled, but there’s no guarantee those aphids would have increased to damaging levels. Furthermore, such applications unnecessarily expose the populations of aphids and other pests to insecticides and can increase the potential for further resistance development. It also kills beneficial insects, which provide natural pest control.
Green cloverworm
Green cloverworm has been the main caterpillar in Minnesota soybean in recent years. They tend to be susceptible to diseases that keep populations in check. However, high numbers were reported in northwestern Minnesota this season and treatment decisions are generally based on defoliation.
The ET for vegetative stages is 30% defoliation and 20% during soybean reproductive stages. To get a good representation, sample plants from throughout the field. Then look at leaves in the top, middle and bottom of the plant and estimate percent defoliation (Figure 1). Average percent defoliation from the 3 different heights and then across all of the plants.
Other soybean insect pests
Soybean gall midge
Soybean gall midge has been in the state for several years, though it’s primarily found in southwestern Minnesota. So far, numbers this season appear to be lower than in previous years.
If you find soybean gall midge larvae, please contact Koch at koch0125@umn.edu.
For more information, see Soybean gall midge in Minnesota.
Soybean tentiform leafminer
This leafminer is native to Minnesota, but only recently decided to feed on soybean. The caterpillers live inside the leaves where they feed, leaving a tented appearance on the upper leaf surface. It prefers soybeans on field edges, particularly near trees.
Source : umn.edu