It is that time of year when I hear about maggots and armyworms in field crops. At least that means people are out there looking for insects! I don't always get great photos of the insect activity or injury, but this week is the exception. Keep 'em coming, everyone!
Brian Lang, ISU Field Agronomist in northeast Iowa, has already found a soybean aphid (actually he almost always finds the first ones every year) AND he found a few prevent plant fields with seed corn maggot in soybean. He seemed to notice more plant feeding in fields that were planted to oats last year compared to radish. He also noted it was a field with mostly naked soybean seed (no insecticidal seed treatment).
Seedcorn maggot and pupae at the base of an emerging soybean plant. Photos by Brian Lang, ISU.
Tom Hillyer, a crop consultant in Iowa, always sends me interesting photos. He manages to stump me on a regular basis with insect identification. But his recent photos included some developed armyworms feeding on corn. He also found a few bean leaf beetle on soybean despite my earlier prediction for high mortality this year.
Yellowstriped armyworm. Photo by Tom Hillyer.
Armyworm. Photo by Tom Hillyer.
Bean leaf beetle. Photo by Tom Hillyer.
Insect mortality happens every winter, even under ideal conditions. However, this winter was the 9th coldest in 121 years and I’ve been getting questions about how the harsh conditions may have impacted overwintering corn rootworm eggs. Maybe we don’t need to care rootworms this year if all the eggs froze to death? We’re probably not so lucky. Many factors besides cold air temperatures influence successful overwintering of insects in Iowa, including our most important field crop pest.
Cold temperatures can kill rootworm eggs; temperatures below 18.5°F can be lethal to eggs (Woodson and Gustin 1993). We know the eggs are deposited into soil cracks and crevices and are somewhat protected to air temperatures. Mike Gray (University of Illinois) provided a nice blog summary of research involving soil temperatures, egg depth and survivorship. To see how cold it really was this winter, I was able to extract a graph showing soil temperatures at three depths near Ames (1 November 2013 – 1 May 2014). There were several dates where the temperature was cold enough to kill eggs in the top 12ʺ of the soil in central Iowa and likely other places throughout the state.
Soil temperature data courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
Egg deposition is highly variable within and between species, but in general eggs have a better chance of surviving if they are placed deeper in the soil. Western corn rootworms tend to lay most eggs 4-8ʺ below the soil surface, compared to northern corn rootworms that tend to lay most eggs in the top 4ʺ (Gray and Tollefson 1988). So the odds are in favor of more westerns surviving the winter just because of where females put the eggs.
Crop residue and snow cover can significantly improve egg survivorship (Godfrey et al. 1995). However, just how much residue/snow cover is needed is not fully understood. Tillage and tillage timing does not significantly reduce egg populations (Gray and Tollefson 1988). Soil texture did not appear to influence egg mortality in a Nebraska study (Godfrey et al. 1995).
Saturated soils do not kill corn rootworm eggs, but they can negatively impact larvae. When soil is saturated, oxygen can be limited and cause suffocation. About 50% of third instar western corn rootworm larvae die in saturated soils after 24 hours (77°F); survivorship is increased in saturated soil with decreasing temperatures (Hoback et al. 2002). So later this summer, saturated soils could reduce larval populations but don’t count on it for eggs.
The bottom line is all these factors had some impact on overwintering egg mortality. There was probably more egg death this winter compared to more normal winter temperatures. I do think some corn rootworm eggs survived in Iowa. In a recent ICM News article, I estimated corn rootworm egg hatch is happening now if they survived. This prediction is solely based on growing degree days in the soil. Research has demonstrated about 50% of the eggs hatch when they accumulate 684-767 degrees (base 52°F, soil). It makes sense that egg hatch starts in southern Iowa every year, with the average hatching date for the state around 6 June. Predicted egg hatch is important because larvae will feed on corn roots for about 3 weeks. I encourage everyone to assess corn root injury as larvae finish feeding. Remember, one node of injured roots means a 15% yield loss (Tinsley et al. 2012). It's called the billion dollar pest for a reason!
Map data courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
ReferencesGodfrey, L.D., L.J. Meinke, R.J. Wright, and G.L. Hein. 1995. Environmental and edaphic effects on western corn rootworm overwintering egg survival. Journal of Economic Entomology 88: 1445-1454.
Gray, M.E., and J.J. Tollefson. 1988. Influence of tillage systems on egg populations of western and northern corn rootworms. Journal of the Kansas Entomological Society 61: 186-194.
Hoback, W.W., T.L. Clark, L.J. Meinke, L.G. Higley, and J.M. Scalzitti. 2002. Immersion survival differs among three Diabrotica species. Entomologia Experimentalis et Applicata 105: 29-34.
Tinsley, N.A., R.E. Estes, and M.E. Gray. 2012. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. Journal of Applied Entomology 137: 161-169.
Woodson, W.D., and R.D. Gustin. 1993. Low temperature effects on hatch of western corn rootworm eggs. Journal of the Kansas Entomological Society 66: 104-107.
Today, two special entomologists came to Ames for a short visit: Dr. Dave Voegtlin (Natural History Survey, Champaign, IL) and Dr. Dave Hogg (University of Wisconsin-Madison). Dr. Voegtlin is an aphid taxonomist, aka, he specializes in aphid biology and identification. There are only a few people like him in the world and I really appreciate his expertise and (Canadian) sense of humor. I met with the "Daves," Matt O'Neal (ISU soybean entomologist) and my postdoc, Thelma Heidel-Baker to talk about the state of affairs in the aphid world. It is easy to absorb their energy about aphids and we could have talked for days about emerging research projects.
The Daves are touring the north central region scouting for soybean aphids. They look at buckthorn every spring and fall. You might be (or should be?) wondering why they were looking for soybean aphid when most of the beans in Iowa aren't planted or haven't emerged yet. They are scouting for aphids on buckthorn, the overwintering host of soybean aphid. Yes, their primary host is a woody shrub commonly found in shelter belts around the north central region. They move to their secondary host, soybean, every summer. Their life cycle is complicated and I won't go into details here, but they want to learn more about their movement between the primary and secondary host.
Dave Voegtlin explained how he searches for soybean aphid on buckthorn. Photo by Thelma Heidel-Baker.
Dave Hogg and Matt O'Neal hoping to find aphids. Photo by Thelma Heidel-Baker.
Close-up of buckthorn, note smooth leaves with reduced venation.
Thelma was practicing her photography skills.
Although we spent some of the morning looking around the ISU campus (there is a LOT of buckthorn on campus because there used to be a breeding program!), we did not find any aphids. Sniff sniff. They did find aphids at previous stops before Iowa and I tried using a special camera lens to take close-up photographs.
Dr. Voegtlin giving me a camera tutorial. It is actually difficult to take pictures of very small animals! Photo by Thelma Heidel-Baker.
My best photo shows a few soybean nymphs on a buckthorn plant.
Seeing these aphids on buckthorn is kinda like seeing a unicorn. I've worked with soybean aphid since 2001 and have NEVER seen spring colonies on buckthorn. So this was a special work day for me - thanks to the Daves for stopping by today!
My technician, Greg, has been rearing a few black widow spiders in my lab for a few months. It all started with an egg sac donated from the Insect Zoo (thanks?). He has an interest in spiders and is the "go to" guy if you need a spider ID. Since I am not a huge fan of spiders, I don't take the time to learn how to identify them or get to know much about them. I will go 5 soybean rows out of the way to avoid an orb-weaving spider in August.
Why do we care about black widow spiders? The females are considered highly venomous, but human deaths are rare compared to the number of people envenomated. People bitten by a female black widow may have swelling, redness, muscle pain, nausea, headache, and cramping. The venom contains several toxins and in general sounds like a painful experience. The good news is they are predators, and eat insects and pretty much anything that gets caught in their web. There are a few species of black widow in the U.S. and they have a wide distribution in the southern states. It is possible for black widows to live in Iowa, but finding them here is more likely because they were accidentally introduced instead of established.
Today Greg let me know that a female and male black widow have reached adulthood. It took constant care to feed them (live) flies and clean their cages every week. The spiders are not large, with females about 1.5 inches long and 0.25 inches in diameter. Females are shiny black with a classic, red marking on the belly (ventral) side of the abdomen.
Adult, female black widow, Latrodectus mactans. Photo by wiki.
I have to admit, males are not impressive (insert game show sound here) - smaller, and dull in color. The adult body is about 0.25 inch in diameter.
Adult, male black widow, Latrodectus mactans. Photo by Center for Invasive Species Research, University of California-Riverside.
It was hard to take a good picture of the male and female spiders through the plastic rearing chambers in my lab. I didn't feel brave enough to take the lids off and get a close-up.
Female black widow spider. Note all the dead flies on the bottom of the cage. She is an efficient predator and quickly kills her prey.
Greg is going to put them together in the same cage tomorrow and see if they mate. Can you hear wedding bells? Male spiders are often wary of females because they are potential prey. So they spin a sperm web and put the packet on one of his palps (kinda like short legs used for bringing food to the mouth). The male will attempt to deposit the sperm in the female before becoming dinner!
I wonder what will happen at the spider wedding tomorrow? Male is currently in the left cage.
If successfully mated, a female can produce several egg sacs that contain a few hundred eggs. There is a lot of cannibalism with spiderlings, and so not many actually survive very long. Males don't live very long after becoming an adult, but females could live a few years. Sounds like we have a pet spider in the lab...
I recently posted an ICM News article about how scouting for alfalfa weevils in southern Iowa. It doesn't take too many accumulated degree days for them to start moving around and laying eggs. If you grow or scout alfalfa, you will likely see any adults that survived the winter moving in southern Iowa now and northern Iowa next week. In the article, I show an updated economic threshold table based on work from John Tooker at Penn State. It has a lot of numbers in it, and at first, looks kinda confusing. In order to make cost-effective treatment decisions for alfalfa weevil management, three things must be known:
1. what is the expected market value of the hay ($/ton)?
2. how much are the control costs ($/acre)?
3. how tall are the plants (inches)?
Take that information and plug it into the table below. Well, I guess you have to actually go out to the field and sample larvae before consulting the table! The larvae are often tucked into stems and expanding terminals. Pull at least 30-50 stems from different areas of the field and madly shake them into a 5-gallon bucket to dislodge them.
Alfalfa larva (top) and adult. Photos by Clemson University, www.ipmimages.org.
Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample (Originally published by JohnTooker, Penn State Extension).
In case the table is still confusing, I highlight two examples. Example 1 (look at the orange arrows): if you are expecting $260/ton, control costs of $14/acre, and plants are 20 inches tall...the economic threshold would be 40 larvae per 30-stem sample. In other words, if you sample 30 stems and get at least 40 larvae, consider taking some management action to protect tonnage and hay quality. Example 2 (look at the red arrows): if you are expecting $380/ton, control costs of $12/acre, and plants are 28 inches tall...the economic threshold would be 24 larvae per 30-stem sample. In other words, if you sample 30 stems and get at least 24 larvae, consider taking some management action to protect tonnage and hay quality.