Friday, August 19, 2011

Corn aphids explode

Starting last week, I've been seeing and hearing about an aphid explosion - in corn! For the last 3-4 years, certain areas in Iowa have had serious problems with aphids infesting corn in August. The areas having problems right now include the NE/NW corners and west central Iowa. Some of these heavily-infested fields have already been sprayed with an insecticide earlier this year. From my observations this week, I noticed aggregated colonies at the end rows.

In the past, corn leaf aphid could be a problem during tasseling. This species aggregated around the ear and silks, and sometimes their honeydew production interfered with pollination. But natural enemies and the environment rarely let them build up past July. So economic thresholds have only been determined for aphids around tasseling and mostly targeted to fields during drought-stressed summers. Now, it seems aphids are colonizing corn later in the summer and are building up to striking levels. They can be found at the base of the stalk, around the ear and sometimes building up colonies above the ear leaf. 

Aphids in corn can build up large colonies, sometimes exceeding 2,000 per plant. 
Photo by Erin Hodgson.

One important observation I've noticed is that most fields have two aphids species - corn leaf aphid and bird cherry oat aphid. They are closely related and look very similar in size and color. The bird cherry oat aphid had an orange-red saddle between the cornicles. Other aphid species can also be found, including greenbug and English grain aphid, but are not as common in corn this year. Species identification isn't that critical for management at this point (an aphid is an aphid). You can see more than one species in a field and even on a single plant. 

All aphids have a piercing sucking mouthpart and feed on the sap from the plant phloem. They excrete sugar-rich honeydew that can cover the aboveground portion of plants. The honeydew can promote a sooty mold that can interfere with plant photosynthesis. You've probably remember seeing grey-looking soybean leaves when soybean first hit Iowa about 10 years ago. We know soybean plants covered with mold and aphids can have serious yield loss, but we don't know the extent of yield reduction caused by aphids in corn. 

Corn aphids infesting the ear and above the ear leaf. Photo by Erin Hodgson.

Currently, there are no treatment thresholds for aphids in corn past tasseling. But regular sampling will help you make educated decisions about a foliar application at this time. Here are some considerations to make before applying an insecticide for aphids in corn:

1. Are 80% of the plants infested with aphids?
2. Do most of the ears have aphids? What about the ear leaf and above?
3. How long has the field been infested and is the density increasing? Sample field-wide (30 plants for every 50 acres) to determine the average density. 
4. Do you see honeydew or sooty mold on the stalk, leaves or ear?
5. Are you seeing winged aphids or nymphs with wing pads? This may be a sign of migration out of the field. 
6. Is the field under drought stress? Dry weather will make amplify potential feeding damage to corn. 
7. Do you see any bloated, off-color aphids under humid conditions? Natural fungi can quickly wipe out aphids in field crops. 
8. What is the corn growth stage? Fields reaching hard dent may be past the point of a justified foliar insecticide.
9. Some insecticides have a 60-day preharvest interval. Check the label and calender.
10. Get good coverage of the application - ideally droplets should make contact with the aphids for a quick knockdown. Don't expect residual to protect the corn from fluid feeders. 

I strongly encourage you to leave an untreated check strip or two in fields that you spray. Try to leave a strip that is a fair comparison to the majority of the field - not just the stunted corn the field edge. If you decide to treat for aphids in corn, I would like to hear about the yield comparisons. Your pooled data will help me formulate treatment guidelines for the future.

Monday, August 15, 2011

It's our big soybean aphid spray day

My lab has the nation's largest soybean aphid efficacy trial. It originally started with Matt O'Neal, but he kindly gave me the program when I started at ISU in 2009. The program has typically focused on foliar insecticides, but last year we also incorporated some seed treatments and the Rag1 host plant resistance gene. This year, we even expanded to three locations (Northwest Farm, Northeast Farm, and the Johnson Farm). Last week the Northwest Farm hit threshold and applications were made. Even though aphids haven't exceeded threshold at the other two farms, we are spraying them this week. The Northeast Farm has all our treatments, 35 this year, so spraying all the plots takes some serious planning with a large crew.

The 2009 crew is all suited up in Tyvek, getting ready to start spraying. 
Photo by Erin Hodgson.

At all our locations, we are definitely seeing a visual difference between susceptible and resistant soybean. Aphids are present, but at low levels in Rag1 plots. I will update the blog on yield data once the beans are cut. If you are curious about what products we used in the past, go to our soybean aphid website. We are trying out some new spray equipment this year, so I will also recap how it worked (or didn't!).

Thursday, August 11, 2011

Learn how to use Speed Scouting for Soybean Aphid

Now is critical time to scout and make treatment decisions for soybean aphid in Iowa. But sampling whole plants during pod set and seed fill can be very time consuming! I created a sampling plan in 2005 called Speed Scouting for Soybean Aphid. It's a binomial sequential sampling plan that should save growers and crop consultants a lot of time out in the beans.

The Plant Management Network is offering a FREE webinar about how to use Speed Scouting. Visit the Focus on Soybean section under the Resources tab to view open access webinars, or simply click here. I spend about 15 minutes going over the sampling plan and show a few examples of how it works.

Go to my ISU faculty website to get blank Speed Scouting forms. I am interested in hearing about your experience with Speed Scouting. Tell me if you like it or even if you don't. There will be a tablet/smartphone app with Speed Scouting available for next summer.

Scouting for aphids is a difficult task in August - try Speed Scouting to make your life easier! Photo by Brent Pringnitz.

Friday, August 5, 2011

Corn rootworm damage is noticable

I've been traveling around the state with our corn entomologist, Aaron Gassmann, the last two weeks. We've been collecting corn rootworm adults and assessing root injury in commercial corn fields. We are visiting fields that seem to have extreme rootworm pressure, even though the corn is expressing Bt. 

  Severe corn rootworm feeding can lead to root pruning which makes plant unstable 
and interferes with nutrient uptake. Photo by Erin Hodgson.
Aaron has been looking at problem fields for three years and evaluating the susceptibility of corn rootworm to different Bt proteins. He is finding a few fields with increased beetle survivorship compared to the general population. Does this mean corn rootworm are becoming resistant to Bt? Well, according to his recently published article - yes. He is finding fields that have more larvae surviving, or tolerating, the Bt proteins. Although his research showed incomplete resistance in just a few fields in Iowa. This is not a widespread problem at this time and has not been reported from other states. 

There are a couple of common scenarios to these problem fields. Most are planted with continuous corn and use the same Bt protein for 2-3 years. The vast majority of beetles are Western corn rootworm. Aaron strongly encourages people to use crop rotation with soybean to knock down the beetle population. He also suggests mixing up the Bt proteins used to delay genetic resistance of the beetles. Also consider using an in-furrow insecticide application for those fields with persistent corn rootworm damage. Of course incorporating the recommended refuge corn will help delay resistance as well.

 Western corn rootworm beetle. Photo by Natasha Wright.

During our travels, we saw a few fields with extremely heavy adult emergence and feeding. I would estimate the density of two fields we sampled to be around 10-14 adults per plant. The adults had destroyed the silks and were infesting the ears. Leaf feeding was also very apparent. Some of the fields were sprayed with a foliar insecticide to kill the adults. Corn rootworm had delayed development this year, and adults are emerging over a gradual period. The residual from the insecticide does not appear to kill the adults that emerge after application. The University of Illinois does a statewide sample of corn rootworm adults, but found low numbers this year even though root damage by larval was increased form 2010.

Growers should be evaluating corn roots for damage, even if lodging isn't obvious. You should be able to see adults feeding and mating at this time as well. Knowing more about the current population will help you make seed selection decisions for next year.