Did this winter kill all the corn rootworm eggs?

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.

References

Godfrey, 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.

How to read the updated economic threshold table for alfalfa weevil

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.

How do insects survive the winter?

We sure have experienced very cold air temperatures and even colder windchills this winter. I’ve been asked several times “how cold does it have to get to kill insects?” Perhaps it is important to understand why cold temperatures kill insects. Insects are unlike mammals and birds because they must generate their own heat (called ectotherms). Insects die with they are exposed to temperatures below the melting point of their body fluids. If they want to survive our cold Iowa winters, they must avoid freezing or tolerate freezing. Over time, insects have developed several strategies to survive cold temperatures and none of them involve wearing fleece.

Some insects just move into human structures in the fall and keep warm until spring. Think about boxelder bugs and multicolored Asian ladybeetles aggregating on houses every year. Even if they are protected inside, they will likely die before spring if they don’t get food and water. Some insects also migrate to warmer climates to avoid freezing. A classic example is monarch butterflies moving from Canada to Mexico every year. Sounds pretty good about now!

Multicolored Asian ladybeetles mass on structures every fall. 

Photos by Robert Koch. 

But most of our persistent insects in Iowa have to overwinter outside, and two strategies have evolved to survive extreme conditions: freeze avoidance and freeze tolerance. Freeze-avoidant insects keep body fluids liquid and freeze-tolerant insects can handle the formation of internal ice. Wait a minute, what? I know…either strategy seems fantastical.

The main strategy for insects living in the northern hemisphere, where we have cold temperature for long period of time, is freeze avoidance. Freeze avoidance can be achieved a few ways. Sometimes insects enter a “dry” hibernation by getting rid of all the food and water in their body. That way, ice can’t form inside the body and kill them. Water needs food or dust particles in order to crystallize; water can cool down to -42C without freezing if particles are absent. Other insects have a super waxy coating on the exoskeleton that protects against ice formation on the body. Amazingly, some freeze-avoidant insects also produce cryoprotectants, such as glycerol and sugar, to reduce the lethal freezing temperature of the body. So yes, cryoprotectants act like the antifreeze in your car. I can’t make this stuff up! 

Most insects living in the southern hemisphere, where the climate is more variable, employ freeze tolerance. These insects can stand ice formation in the body. Some will actually initiate freezing their body at relatively high temperatures in order to prepare for a longer hibernation. An example of a freeze-tolerant insect is the woolly bear. [Sidenote: Several winter festivals celebrate the woolly bear kinda like Groundhog's Day.]

Woolly bears overwinter as cold-hardy caterpillars. 

Photo by IronChris, Wiki. 

No matter the overwintering strategy, all insects will eventually die if it gets cold enough. However, the lower lethal temperature is different for each species. Insects can overwinter in any life stage - some are belowground and some aboveground. It gets complicated quickly, and so I will save that for another time. 

Find out more about how insects survive the winter from this Wiki page.

Live brown marmorated stink bugs in Iowa

The first specimen of brown marmorated stink bug (BMSB)  in Iowa was confirmed last February in Cedar Rapids. Since then, several other dead BMSB have been confirmed by our ISU Insect Diagnostician, Laura Jesse. Recently, there have been some live detections near Davenport and Bettendorf, Iowa.

Brown marmorated stink bugs have a marbled coloration, and white-banded antennae. Photo by Laura Jesse, ISU. 

These stink bugs are established along the east coast for about a decade. They have been migrating westward in the U.S. When they invade a new area, they are typically considered an urban pest. They attempt to overwinter in human structures similar to boxelder bugs and multicolored Asian lady beetle. But they eventually become a pest of fruit and field crops. Adults and nymphs will probe fruit and leaves, causing necrotic spots. BMSB feeding can severely deform soybean, and cause "stay green" along soybean field edges. 

Stink bug feeding during the seed fill stage can cause deformation, shrinkage and discoloration in soybean. Photo by Galen Dively, University of Maryland. 

At this time, we don't know if BMSB is established in Iowa, or if a few individuals have hitchhiked here. If you suspect BMSB around your home or buildings, submit samples to the ISU Plant and Insect Diagnostic Clinic. Instructions for submitting samples can be found here.

Can insects scream?

Someone recently asked me if aphids make sounds when approached by predators. I thought it was a really interesting question, and not just because it was about my favorite insect! I am not aware of aphids making noise when threatened, although they do have other defense mechanisms I may share at a later time. But it did get me thinking about insects that can make noise and how sound is generated a few different ways.

Cicadas probably come to mind as a well-known noise maker. The males can produce sound, sometimes up to 120 decibels - technically loud enough to cause hearing loss in humans! They have a thin membrane on the outer abdominal wall, called a tymbal. As the abdominal muscles contract and relax, it causes the tymbal to click. Cicadas rapidly vibrate the membrane and the sound amplifies in enlarged air chambers in the abdomen. Each species would have it's own "song" to attract females.

Periodical cicada, www.ipmimages.org.

Most insects stridulate, or rub body parts together, to make noise. In most cases, the males stridulate to attract mates. Crickets and katydids have serrated teeth on the edges of their wings that form comb-like structures. They drag the combs against each other to create sound. But they rub the wings so fast that it just sounds like a chirp. Crickets have four types of chirps: a loud calling song to attract females, a quiet courting song when a female is near, an aggressive song to repel other males, and a brief song after mating. Crickets chirp at different rates depending on the species and temperature. The relationship of cricket chirping is also called Dolbear's Law.

Field crickets use their wings to create sound. Photo by David Cappaert, Michigan State University, www.ipmimages.org. 

Other insects, like grasshoppers and beetles, scrape their legs to make sound. Grasshoppers have stiff peg-like spines on the hind legs that they rub against their wings. Some long-horned grasshoppers will rub their front legs together for their species-specific "songs."

Striped slant-face grasshopper with stiff spines on the hind leg used for stridulation. Photo by Gerry Fauske, North Dakota State University. 

Some insects use their heads or even their mouthparts to make clicking noises. But perhaps the weirdest noise-making insect I discovered was the tiny water boatman. This aquatic insect can "sing" at 99 decibels by rubbing its penis against his abdomen. Don't ask me how scientists thought of researching this idea or how they collected the data.

The tiny water boatman can generate a very loud sound, www.bbc.co.uk. 

I found a few nice websites that have audio clips of insects making noise. Enjoy!

Insect Sounds, www.naturesongs.com/insects.html

Bug Bytes, www.ars.usda.gov/sp2userfiles/person/3559/soundlibrary.html

Songs of Insects, www.musicofnature.org/songsofinsects/index.html

Harvest begins!

In 2012, we worked at several ISU Research Farms, including: Northwest, Northeast, Southeast, Southwest, and Johnson Farms. We were working mostly in soybean but also had several corn trials this year. Plots are drying down quickly and we are preparing for the last step in our summer research - harvest! Usually it goes pretty slow for us because we have to stop often to weigh grain in every plot. Some of the farms have automatic scales on the combine - otherwise we have to weigh grain by hand. 

Beetles: here, there and everywhere

Just like the Beatles song said, "Nobody can deny that there's something there." Many of you probably noticed a lot of little shiny black beetles around your home and even out in the field. I've seen a few different beetles actively moving around. The sap beetle, or sometimes called the picnic beetle, can be found in high numbers in urban areas now. As the common name suggests, they are attracted to human food, like fruit and vegetables. But they are normally seen  feeding on overripe fruit and decaying organic matter. In a few weeks you may see them feeding on corn silks. They are also secondary invaders and will feed in corn husks infested by corn earworm. Most literature suggests they aren't an economic pest.

Sap beetles are black and shiny with four irregular spots on the forewings. Note the antennae are clubbed and the wings don't fully cover the abdomen. Photo by Joyce Gross, UC-Berkeley.

In addition, I've got reports of a few different flea beetles in corn. Corn flea beetles are common to see early in the season. Corn is the preferred host plant where the adults scrape along upper and lower corn leaves. They will also feed on grasses. The feeding itself isn't an economic problem, but they can vector a bacteria that causes Stewart's wilt. Foliar insecticides are not economically justified after V5.

Corn flea beetles are small, shiny and black. Note the enlarged hindleg femurs used for jumping. Photo by Mike Quinn, TexasEnto.net.

Another flea beetle out and about lately is the redheaded flea beetle. They are similar in size to the corn flea beetle, but more oval in shape and have a red head (funny how entomologists come up with common names!). They can feed a several weeds, corn and alfalfa.

 

The redheaded flea beetle also has enlarged hindleg femurs for jumping. Photo by Lewis Veith.

Ukrainian farmer visit

Today I gave a short presentation on corn and insect management to a group of Ukrainian farmers.

They are on a tour to learn more about agriculture in the midwest. Mark Licht, ISU Field Agronomist, organized a full day of talks and tours for the group. Roger Elmore, ISU Corn Agronomist, started the day with a review of corn production - a really interesting topic for them! I followed with a brief discussion on how we manage insects in Iowa. They don't have access to GMO's, and they asked lots of questions about our options in corn. European corn borer is the more important pest for them. I think they were surprised by how much a bag of corn could cost! Next Mark was going to review tillage, machinery and general production practices in Iowa and give them a tour of some ISU Research Farms.

It was my third time working with a Ukrainian farmer group. The translators are always great about getting my messages across. They thanked me with a hand-painted plant from the Ukraine!

Bean leaf beetles are hungry

In March, I predicted low overwintering mortality based on our mild winter. You may have noticed adults become active in alfalfa starting in April. Some soybean research plots around southern and central Iowa have decent numbers feeding on unifoliates. Read more about early-season management of bean leaf beetle here.

 Soybean can compensate for early-season defoliation by bean leaf beetle. Photo by Erin Hodgson (17 May 2012 near Ames, IA).

Circular-shaped defoliation on leaves is an indicator of bean leaf beetle feeding, even if you don't see the beetles. Photo by Erin Hodgson (17 May 2012 near Ames, IA).

Stalk borers on the move

Iowa has been steadily accumulating degree days ahead of schedule in 2012. Over the weekend, some parts of southern Iowa hit an important degree day benchmark for common stalk borer. About 10 percent of stalk borer larvae can begin moving to corn after accumulating 1,300 to 1,400 degree days. Part of southwestern and southeastern Iowa have reached that threshold and we recommend starting to scout this week in corn. Central and northern Iowa should start scouting May 12 to May 18, if warm temperatures continue. This is about three weeks earlier than last year. 

 Growing degree days accumulated (base 41°F) for stalk borer larval movement in Iowa for 2012. Begin scouting around 1,300 to 1,400 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Agronomy. 

For the full ICM News article about stalk borer management, click here.

Insecticidal seed treatments can harm honey bees

Recently, Christian Krupke (field crops extension entomologist at Purdue University) published a paper on how honey bees may be getting exposed to a class of insecticides most commonly used in seed treatments. The authors found neonicotinoids in soil, dandelions, and pollen. Dead and dying bees collected from aparies near agricultural fields had traces of neonicotinoids.

He recommends the following to minimize exposure to honey bees during planting:

1. Farmers should be in communication about planting timing with local beekeepers.
2. Beekeepers should move hives during planting if possible.
3. Use the appropriate amount of talc to plant treated seed.
4. Do not clean planter equipment near fields.

 For more information, read this recent ICM News article.

 Honey bees exhibit neurotoxic symptoms when dosed with neonicotinoids. Dying bees have trouble flying, collecting food and getting back into the hive. Photo by John Obermeyer, Purdue Extension Entomology.

Alfalfa weevils are hatching

Iowa has steadily accumulated growing degree days (DD) all through March. One of the first insects to hatch is alfalfa weevil. In general, we can expect egg hatch around 200 DD south of I-80 and around 250 DD north of I-80. Most of the state has already hit those benchmarks.

 

Accumulated growing degree days (base 48F) in Iowa form 1 January - 2 April 2012. This map is updated daily (click here). Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy. 

 Alfalfa weevil is an important defoliator because adult and larval feeding can reduce tonnage and forage quality. If plants are greater than six inches, use a sweep net to initially sample. Insecticidal treatments are based on crop value and plant height. Read this recent ICM News Article for management guidelines.

Alfalfa weevil adults are brown and hairy, have elongated snouts and elbowed antennae. Photo by Joseph Berger, ipmimages.org.

First black cutworm moth trapped in Iowa

About two weeks ago, we sent out a general request looking for cooperators to trap black cutworm moths in Iowa. We use the adult captures to help us predict larval cutting dates in corn. Last night, the first moth was captured in a trap in Muscatine County, Iowa. This is very early to see migratory moths moving up north. I am not sure what this means for spring populations, but last year had higher-than normal trap numbers. As a result, we saw more damaged corn and soybean during establishment. I will keep you updated on trap numbers for 2012 and predicted cutting dates later this spring. 

Black cutworm moth. Photo by Marlin E. Rice.

Check out these three new insect management articles!

Have you heard about a peer-reviewed extension publication called the Journal of Integrated Pest Management? It's an open-access journal (free for anyone to read and save files!). You can now access research-based management recommendations on all pests - not just insects and not just field crops. The Editors-in-Chief of JIPM are Marlin E. Rice of Pioneer Hi-Bred International (formerly with Iowa State University) and Kevin L. Steffey of Dow Agrosciences (formerly with the University of Illinois).

This new journal will offer a wide scope of peer-reviewed articles, including entomology, plant pathology and weeds. The intended readership for JIPM will be professionals who are engaged in any aspect of integrated pest management, especially those individuals working in crop protection, retailers, manufacturers and suppliers of pest management products, educators and pest control operators. The firs issue was released in October 2010.  and included articles about stalk borers and western bean cutworm. 

Three recently posted articles may be of particular interest to you. Click on the links to access pdf files:

1. Biology of the soybean aphid
2. Management of soybean aphid
3. Biology and management of bean leaf beetle and bean pod mottle virus

Alate (winged) soybean aphid and a few nymphs. Photo by Erin Hodgson

Getting the word out about rootworm resistance

I'm at the ISU Extension Ag Chem meeting in Ames today. My presentation will review the status of western corn rootworm resistance to Bt corn. There are several tactics farmers can use to delay resistance, and I will encourage people to implement in 2012.

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Entomologists are funny, just ask us!

Us entomologists don't really have that many jokes to tell. I still think "frass happens" is pretty good stuff. But this year at the ESA meeting in Reno, they had an insect-related limerick contest. There were many humorous and creative submissions - perhaps we are a funny society after all? The overall winner was Martha Lutz, who wrote a witty poem about fireflies:

Au Naturel Selection: Photinus meets Photuris
A firefly who was benighted
saw a light and became so excited–
he rushed to his fate
while selecting a mate:
lost his head, lost his heart, was de-lighted.

It's actually about an interaction between two firefly species. I encourage you to visit Bug Girl's blog that gives a bit more detail about insect mimicry and the deceptive nature of some fireflies.

Some female fireflies lure in males with false mating signals.  

Photo by J. E. Lloyd.