Friday, June 14, 2013

Corn rootworm larvae are hungry

Today, I put out an ICM News article on predicted corn rootworm egg hatch in Iowa. It has a map of the soil  degree day accumulation for the year. Entomologists can predict most insect development based on temperature and we estimate about 50% of corn rootworm eggs should hatch between 684-767 soil degree days (base 52F). The Muscatine area is approaching that important benchmark and many other areas in southern Iowa will in the next week. If we continue to have warm days, expect all insect development to speed up quickly.

Soil degree day accumulation as of 14 June 2013. 

This egg hatch prediction is behind the average date of 6 June and way behind numbers for 2012. People that track egg hatch in Indiana and Illinois also reported delayed egg hatch, due in part to the extreme drought in 2012. Mike Gray, Illinois extension entomologist, said sometimes females will lay eggs deeper into the soil profile in drought conditions. The delayed corn planting throughout much of Iowa means larvae will have a smaller root system to feed on and potentially damage. I've also had people ask me about saturated soils killing rootworms this year. It is possible to suffocate the larvae, but the eggs probably survived water-logged soil. 

I encourage all you scouts and farmers to check your corn roots mid July to assess any corn rootworm injury. It will help determine the ongoing strategies for this unruly beast. Unexpected corn rootworm is possible with all Bt traits in continuous corn production. To read more about corn rootworm management, read this short ISU publication Aaron Gassmann and I wrote over the winter. 

Thursday, June 13, 2013

Aphids in corn?!

I saw something new for the first time today - aphids on V4 corn. Actually, my summer crew found them while scouting for early-season pest activity in a small plot trial at the ISU Johnson Farm just south of Ames, Iowa. Nice job scouting today - way to go!

Meet part of the 2013 crew. I hope to post something just about our lab personnel soon. 

This find is exceptional for three reasons. First, Iowa corn doesn't always get infested with grain aphids. The overwintering potential in Iowa isn't fully understood, but most people think they have to migrate here from the southern U.S. every year (kinda like potato leafhopper and many other insects). So it can be hit or miss if they land here and establish at all. But grain aphids would rather live on small grains, like wheat sorghum, barely, oats, rye, etc. However, some of these aphids will feed on corn and have been known to build up to extremely high populations after tasseling. There was an corn aphid outbreak in much of northern Iowa in 2011. But finding them in mid-June is very interesting (to an entomologist!).

The second curious part of finding aphids in corn this early is that all conventional corn contains an insecticidal seed treatment. My general thinking is aphids shouldn't like to feed on that or stick around long enough to produce nymphs. But it wasn't hard to find alates (winged adults) and nymphs in this small plot trial.

While looking for cutworms and stalk borers, we found aphids instead!

Alates and first instars were easy to find on V4 corn today. 

 The alate was gone, but left behind at least four nymphs on this plant. 

The third thing that was surprising is I identified them as English grain aphids. Of the many species we can potentially find in Iowa, the two most common species are corn leaf aphid and bird cherry oat aphid. English grains aphids are a little larger than either of them, but not as big as a pea aphid. Adults have dark cornicles (tailpipes on the sides of the abdomen), "knees," and "feet" with a pale cauda (little appendage at the very tip of the abdomen). This species does not alternate between a primary, woody host and secondary perennial host (like with soybean aphid). It is capable of vectoring barley yellow dwarf virus in small grains. It can overwintering on true grasses within the Poaceae family. 

English grain aphid, note the dark knees and feet. Photo by Rothamsted Insect Survey.

I should note we didn't see large colonies on any plant. The alates are migrating to these plots and dropping off a few nymphs. We don't know if they will survive or succumb to the insecticidal seed treatment. Of course we are monitoring these plots weekly and I will post an update if they survive. But in general it means that aphids are around in central Iowa. If colonies persist and are allowed to develop over the growing season, it could be significant feeding that results in yield loss. 

Monday, June 10, 2013

Seeing maggots in your beans?

Well, no two summers are the same. This wet spring has caused some fields to have slug problems. Then other fields are now experiencing some seedcorn maggot problems. Crop consultant in southeastern Iowa, Tom Hillyer provided some excellent photos of recent soybean seedling damage.


I don't have much experience with seedcorn maggot, so I extracted some biology and management recommendations from our former extension entomologist, Marlin Rice. Female seedcorn maggots (true flies) deposit eggs in the soil and larvae feed on organic residue. Decaying organic matter, like manure, is especially attractive to females. Larvae overwinter and will complete development in the spring. First generation larvae will feed on germinating seeds and seedlings, and can possibly kill the plant. Several generations are possible in Iowa, but generally are not an economic concern after stand establishment.  

If you suspect seedcorn maggot, carefully dig in the seed furrow and look for evidence. You may not find any maggots, but could see a few feeding scars, tunneling in the seed or stem, or the entire seed destroyed. But the same type of damage could be caused by wireworms, grubs or cutworms. 

Rescue treatments for seedcorn maggot do not exist. Areas with persistent populations should consider using an insecticidal seed treatment to protect corn and soybean fields the following year. Seed treatments are encouraged when planting into cool, wet soils with manure applications.