For most North Carolina soybean growers dealing with corn earworms is an annual problem, but typically by the time the dog days of summer arrive, the third and last generation of these pests has come and gone.
And it’s never been much of a problem past late July- or mid-August at the latest.
Long-time North Carolina Crop Consultant Danny Pierce says this scenario hasn’t played so well the past couple of years.
“Corn earworms on soybeans are an annual problem in many soybean fields in east central North Carolina where I work. This is an area west of Goldsboro and east of I-95,” Pierce says.
Historically the third generation of earworms shows up in late July and early August in very high numbers and many fields need to be treated in a short period of time. This is usually a 14-20 day period.
Pierce says growers are accustomed to scouting fields and making insecticide applications when needed up to early August. After this peak flight, or third flight, very few fields need treating, he says.
A fourth generation of corn earworms emerges in late August but these earworms have trouble surviving the typical heat and rarely develop in numbers large enough to treat.
In Perquimans County, in the northeast corner of North Carolina, veteran County Extension Coordinator Lewis Smith says he’s seen the same pattern in corn earworm flights the past couple of years.
“In 2010, we never had any real heavy flights of moths, but they just kept coming. I caught moths in traps as long as I kept my traps in the field. That was significantly different than what I’d seen in the past,” Smith says.
Last year there was a heavy moth flight early and second flight that was nearly as strong. Then, populations seemed to settle down, more like we are used to seeing. Then after Hurricane Irene came through the area we had these later flights.”
Smith says in two very different years weather wise the same trend of late arriving and surviving corn earworms occurred.
“In our part of the state, we were lucky last year because most of our wheat was planted early the previous year, so most of our double-crop soybeans were planted early and were past the growth stage that is typically damaged by corn earworms,” Smith says.
“For as long as I can remember, up to 2010, late arriving moths lay eggs and many hatch, but worms have very poor survivability. Because of this, scouting has always been relatively easy during that time of year. Many fields, depending on when they bloomed, had to only be scouted three times or so, Pierce says.
Last year (2010) the fourth generation larvae didn’t just go away. In fact, they survived and developed all too well. Pierce who has been scouting crops in North Carolina for a number of years, says 2010 was the first time he had seen this fourth generation of corn earworms cause a problem.
Bad worm year
2010 was the worst worm year on soybeans since 1977, Pierce says. Hoping that 2010 was just an isolated year in which all the conditions were right, the veteran North Carolina crop consultant, who was named Cotton Consultant of the Year a few years back, approached the 2011 cropping season with a little more caution than usual, taking a closer look for these later generation earworms.
“The same thing happened in 2011. The third generation peaked with lots of fields needing spraying during this time as is normal, but after this peak, earworms kept coming and lots of fields had to be sprayed twice.
“In 2010 many fields had to be sprayed twice. These were fields that were not blooming the first time they were sprayed. This was third generation moths. Re-invasion occurred a few weeks later during bloom, which required a second insecticide application. This reinvasion was from fourth generation moths.”
For the second consecutive year this fourth generation emergence during the end of August did too well for most growers.
“Corn earworm developed like I have never seen. This development was responsible for a lengthy and costly "worm season," he adds.
Last year (2011), which was the lightest "worm year" I can remember, no fields needed spraying twice, except for fields with high populations of later than normal occurring corn earworm. Again, this end of August, fourth generation, flight developed well, the North Carolina crop consultant says.
“I have never seen fourth generation corn earworms survive like they have the last two years. Hopefully, this insect hasn't changed and become more adapted to our cropping systems. For whatever reasons, it appears we must scout diligently late in the season. I am hoping its just a "two year thing" and earworms in soybeans will return to normal next year,” he adds.
Dominic Reisig, who is a North Carolina State University entomologist and IPM specialist stationed at the Tidewater Research Center in Plymouth, N.C., says corn earworm definitely did not behave as normal last year.
“Corn earworm densities held steady in eastern North Carolina soybeans after Hurricane Irene hit on Aug. 27-28. Prior to Irene, pheromone trap and light trap catches decreased. After the hurricane, I expected that population densities would crash due to wind and rain.
“This did not happen, but I still expected most caterpillars to cycle through soybeans, pupate and fizzle out. This typical scenario also didn’t happen, as I observed that both pheromone and light trap catches held steady in the week following the hurricane,” Reisig says.
If soybeans are planted on time, the crop should mature ahead of these damaging late-season insects. However, if the insects and crop meet at the wrong time, damage can be severe.
Young larvae feed on flowers and tender foliage. This injury normally occurs on late-maturing varieties prior to seed enlargement. Later in the season, corn earworms prefer to feed on soybean pods, often causing a serious yield loss.
During heavy infestations (six or more per foot of row), most pods may be destroyed, forcing the larvae to become foliage feeders. Entire fields in North Carolina have been stripped of pods and foliage during such heavy infestations.
Avoiding this kind of late season damage to highly valuable soybeans is something growers may need to look at a bit closer, if the past two years are indicative of how corn earworms are going to occur in the future, Pierce stresses.