Agronomic Insect Update – June 1, 2018

David Owens, Extension Entomologist, owensd@udel.edu

Small Grains
We have heard of a few reports of armyworm activity in northern Kent County, with worms clipping barley heads. As we start moving closer to harvest, watch fields that have limited green tissue left for head clipping. Only a couple of products have short pre harvest intervals: Mustang (14 day), and Prevathon (1 day). Good spray penetration with as much water as can be applied by air or a good rate by ground is really important for armyworm; they hide low on the plants, under debris, and at the soil surface during the day and feed from the bottom up. Also, please be aware that Lannate (methomyl) is no longer labeled for small grains in the Mid-Atlantic.

Soybean
This week’s cloudy weather and the few days ahead favor slug activity. If you have a no-till soybean field with high residue, watch your field like a hawk. Soybeans can tolerate some stand loss, but we want to avoid letting a field get to the point that it needs to be replanted due to slug-induced stand loss. Recent tests have shown that the slug baits Deadline and Ferroxx should remain active in fields even with a heavy rain. Please see the WCU article by Phillip Sylvester, Bill Cissel and David Owens titled “Public Enemy Number One: Slugs on Soybeans” from two weeks ago for more slug management information.

I have also run into a large number of ticks when looking at soybean fields, no surprise to most of you I’m sure. According to the CDC, ticks need to be attached 36-48 hours before transmitting disease. Check your pant legs after leaving fields. Keeping a bottle of insect repellent and spraying knees-down will also help.

Bean leaf beetles are active in soybean and seem to be pretty noticeable, probably because there are relatively fewer emerged acres than is normal for this time of year. Grasshopper nymphs are also present in some fields. Last week we posted a diagram of defoliation; once defoliation gets above 20-30% and herbivores are active (2 bean leaf beetles and/or grasshopper nymphs per plant), a spray may be warranted. If defoliation is below this level, keep an eye on it. With warm weather, the soybeans should also quickly outgrow light to moderate leaf feeding.

Four slugs feeding on a cotyledon.

Alfalfa
Continue scouting for potato leaf hopper and hopperburn. Yellowing leaves are a sign that yield has been impacted. New York is reporting their first adult leafhoppers of the season. Thresholds are adjusted for plant height, the smaller the plant, the lower the threshold. As a general guide, use a threshold of 1 leafhopper in 5 sweeps for plants less than 3 inches tall and moving up to 1 leafhopper per sweep for plants greater than 7 inches.

Agronomic Insect Update – May 25, 2018

David Owens, Extension Entomologist, owensd@udel.edu

Although worms tend not to like prolonged wet weather, both black cutworm and true armyworm are active in fields. How much they are suppressed is the open ended question that is almost like predicting a 40% chance of anything. If you see worms in the field, they were not affected.

Corn
As we wrote earlier, if you have a late planted field that has had a recent green cover crop (within the past week), or if you have a corn field that is not yet planted, you will be at greater risk for either pest. We usually don’t worry about these critters in fields destined for soybean. We should expect good protection from cutworms with traits that contain Cry1F or Vip3A. Please see The Handy Bt Trait Table for an excellent breakdown of trait packages that contain them: https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf. A small amount of feeding in a field with the above traits is expected, the Bt is a stomach poison that needs to be ingested. But watch the field to ensure that injury doesn’t progress.

True armyworm is a bit different. Only trait packages with the Viptera gene, (Leptra, Viptera, Trecepta and the 5222 Agrisure Duracade – not 5122) are rated for true armyworm protection. It is possible that the other traits may have some efficacy, but scout your fields and look out for worms in other traits. The photo below was taken by Dr. Kelly Hamby, University of Maryland field crops entomologist from organic (no trait package) field corn. Thresholds for armyworm depend on number, defoliation, and size of the worm. Treatment is warranted if 25% of the plants are infested, 50% defoliation has occurred, and worms are less than ¾ inch long.

True armyworm on organic corn

Wheat
For wheat we use a threshold of 1-2 armyworms per foot of inter-row space. Check the base of the plants. Barley thresholds are a little lower at 1 per row-ft. It seems like harvest is a long way off, but if you have a treatable population be careful with observing the preharvest intervals. The only pyrethroid with less than a 30 day PHI is Mustang Maxx (14 days). The diamide insecticide Prevathon is available for Delaware this year and has a 1 day pre harvest interval.

Soybeans
Some beans went into the ground before the rain. The wet weather we have been having favors slug activity. You will want to watch your soybeans to detect any early issues with slug-induced stand loss. Unlike corn, soybeans can compensate for some stand loss. Fields with prior slug history and no-till fields with heavy residue tend to have greater slug populations.

We are also seeing bean leaf beetle working on seedlings. Beetle activity is concentrated a little more than usual for this time of year due to planting delays and the wet weather suppressing beetle activity last week. Beetle movement really picked up with Sunday’s sunny weather. Beetles may move out, when scouting, check for beetle presence. Count the number of beetles per 6 row-ft in 10 spots per field, and estimate defoliation percentage. Seedling thresholds are 2/ft, 25% stand loss. Once the plants have their second trifoliate, we need to see 2-3 per plant and 25% or more defoliation. Defoliation can be hard to visually estimate, use the below image as a general guide.

Graphic representations of various levels of soybean leaf defoliation.

Guess the Pest! Week #6 Answer: True Armyworm

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to John Swaine, III for correctly identifying the moths as true armyworms and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #6 Answers: True Armyworm
By Bill Cissel, Extension Agent, IPM and David Owens, Extension Entomologist

The correct answer to this past week’s Guess the Pest is True Armyworm (TAW). Adult TAW moths do not cause any direct injury to small grains; however, they can be seen in your fields making sure there is another generation. True armyworm (TAW) larvae damage small grains by clipping flag leaves and small grain heads. It is important to be able to accurately identify true armyworms because there is another “worm” that is also considered a pest of small grains, the grass sawfly. The grass sawfly is in the Order Hymenoptera, meaning it is more closely related to bees and wasps than moths, which are in the Order Lepidoptera. Even though grass sawflies cause similar damage to small grains, management differs between these two species of insects.

There are several reasons why it is important to be able to distinguish between grass sawflies and true armyworms:

1) Grass sawflies are more damaging than true armyworms because they prefer to feed on small grain stems as opposed to true armyworms that typically will feed on leaves before clipping heads. Also, grass sawfly damage usually occurs before the peak of armyworm damage.

2) The threshold for grass sawflies (wheat and barley – 0.4 linear ft of row) is lower than the threshold for true armyworms (barley – 1 per linear ft of row/ wheat-1- 2 –per linear ft of row).

3) Not all products that are labeled for true armyworm control will provide control of grass sawflies.

4) Insecticide rates also differ between the two species for some products.

There are several features that can be used to distinguish grass sawflies from true armyworm.

Grass sawflies larvae are active during the day and can often be found on the plants so “shaking” plants to dislodge larvae is necessary when sampling. They can be identified by their green color, large amber head, and 5-7 pairs of fleshy prolegs legs. Counting the number of prolegs is the most reliable way to determine if the “worm” is a grass sawfly or true armyworm.

Grass Sawfly Larva

True armyworms are active at night and can often be found curled around the base of plants or under crop residue during the day. Larvae have four pairs of fleshy abdominal prolegs not including the pair of legs at the very end of the abdomen. There also appears to be a large gap between the 3 pairs of true legs and the start of the fleshy prolegs.

True Armyworm Larva

If your field is at threshold for grass sawflies or armyworms, there are several things to keep in when selecting which product to apply. Is the insecticide labeled for the correct pest, i.e. if you have grass sawflies, make sure you are using a product labeled for grass sawfly control? What is the days to harvest restriction (this varies among products)? Is the insecticide labeled for the crop (not all products are labeled for all small grains)?

Here is a link with sampling guidelines, thresholds, and insecticide recommendations for true armyworm and grass sawfly: http://extension.udel.edu/ag/insect-management/small-grains/

Agronomic Insect Update – May 4, 2018

David Owens, Extension Entomologist, owensd@udel.edu

Alfalfa
Continue scouting alfalfa for alfalfa weevil damage. With the warm weather we have been having, they are active state-wide. When sampling, collect 30 stems (stems should be taken from several locations in the field), and beat bundles of 5-10 stems to dislodge larvae. Thresholds for alfalfa weevil depend on the number of larvae found, the growth stage of the plant, and the size of the plant. Early harvest may be an option. We have a video demonstrating weevil sampling: https://youtu.be/M983UMsGk0Q an alfalfa weevil fact sheet for thresholds and decision making: http://extension.udel.edu/factsheets/alfalfa-weevil-control-in-alfalfa-2/ and finally, insect control recommendations here: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/25073121/Insect-Control-in-Alfalfa-2018.pdf.

Grain Crops
David Owens and Bill Cissel
Virginia has started seeing large numbers of brown stinkbugs in small grain cover crop and production fields. Stink bugs are not a pest of small grain, but will put their first generation into small grain. Where this becomes a potential concern is later in the season when wheat is harvested and the stink bugs need to go elsewhere. Corn is not susceptible to stink bugs until V3-V4, when the growing point moves out of the soil.

Cereal leaf beetle activity has picked up recently, though I have not yet heard of a field at threshold. Continue scouting your fields. Virginia reports that true armyworms have not been very active yet, and we have only picked up a couple of moths in light traps. Sawflies are just starting to be active. Pay attention to the number of legs. Sawflies have legs on every segment, they are active on plants during the day, and the threshold for them is half that of armyworm. More information can be found in our small grain recommendation guide.

Field Crop Insect Update

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Alfalfa
Continue to sample alfalfa for potato leafhoppers. Sample weekly starting seven days after the first cutting until final harvest. Ten sweep net samples should be taken in 10 random locations throughout the field when the alfalfa is dry. The threshold for alfalfa 3” or less is 20 leafhoppers per 100 sweeps, 4-6” tall is 50 per 100 sweeps, 7-10” tall is 100 per 100 sweeps and greater than 11” is 150 per 100 sweeps. If the field is more than 60 percent bud stage or if it has experienced “hopper burn”, the alfalfa should be cut instead of sprayed.

For more information on the identification, biology, and management of potato leafhoppers, please review our fact sheet: http://extension.udel.edu/factsheets/potato-leafhopper-control-in-alfalfa/

Here is a link to our Insect Control in Alfalfa Recommendations (pure stands only): https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063238/Insect-Control-in-Alfalfa-final-for-2017.pdf

Field Corn
As small grains mature, watch for true armyworm movement into neighboring corn fields. Fields planted into a small grain cover crop, pastures, and weedy fields are also at an increased risk for true armyworm infestations. The threshold for true armyworms in corn is 25% infested plants with larvae less than 1”. Once the larvae move into the whorls and if they are larger than 1”, control will be difficult. Worms greater than 1.25” have completed their feeding.

Here is a link to our Field Corn Insect Management Recommendations for Chemical Control Options: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/13055805Insect-Management-In-Field-Corn-final-20171.pdf

Soybeans
Continue to sample fields for slugs and other defoliators. The past several weeks, I have seen several fields with grasshopper and bean leaf beetle feeding injury. Soybeans can typically withstand a decent amount of defoliation before yield losses occur, however, if stands are being reduced, an insecticide application may be warranted.

A treatment may also be needed if you are finding one grasshopper per sweep and greater than 30% defoliation. (Note: once plants reach bloom and pod fill stages, the threshold for defoliation is reduced to 15%).

Grasshopper feeding injury on seedling soybeans. Notice the irregular shaped holes and leaf feeding from the leaf margins, a good indication of grasshopper feeding.

In addition to grasshoppers, also keep an eye out for bean leaf beetle damage (chewing on cotyledons and small round holes in unifoliate and trifoliate leaves). This can often be confused with slug damage so look for slime trails, slugs, and beetles. The threshold for bean leaf beetles is 2 per ft of row and 25% stand reduction from emergence to 2 trifoliate. After 2 trifoliate, the threshold is 2-3 per plant and 30 percent defoliation.


Bean leaf beetle feeding injury. Bean leaf beetles will often drop from the plants when disturbed and are excellent at hiding in crop residue.


Slug injury on soybean unifoliate leaves. In some cases, this damage can look similar to damage from bean leaf beetles so when diagnosing the cause, be sure to look for slugs, slug slime trails, and for beetles.

Here is a link to our Soybean Insect Management Recommendations: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063934/Insect-Control-in-Soybeans-2017-final.pdf

 

Field Crop Insect Update

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Alfalfa
Begin sampling alfalfa for potato leafhoppers weekly until final harvest. Ten sweep net samples should be taken in 10 random locations throughout the field when the alfalfa is dry. The threshold for alfalfa 3” or less is 20 leafhoppers per 100 sweeps, 4-6” tall is 50 per 100 sweeps, 7-10” tall is 100 per 100 sweeps and greater than 11” is 150 per 100 sweeps. If the field is more than 60 percent bud stage or if it has experienced “hopper burn”, the alfalfa should be cut instead of sprayed.

For more information on the identification, biology, and management of potato leafhoppers, please review our fact sheet: http://extension.udel.edu/factsheets/potato-leafhopper-control-in-alfalfa/

Here is a link to our Insect Control in Alfalfa Recommendations (pure stands only): https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063238/Insect-Control-in-Alfalfa-final-for-2017.pdf

Field Corn
Continue to sample fields for slug injury. Here is a link to information from previous WCU articles on slug management in corn: http://extension.udel.edu/weeklycropupdate/?p=10303

As barley matures, watch for true armyworm movement into neighboring corn fields. Fields planted into a small grain cover crop, pastures, and weedy fields are also at an increased risk for true armyworm infestations. The threshold for true armyworms in corn is 25% infested plants with larvae less than 1”. Once the larvae move into the whorls and if they are larger than 1”, control will be difficult. Worms greater than 1.25” have completed their feeding.

Here is a link to our Field Corn Insect Management Recommendations for Chemical Control Options: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/13055805Insect-Management-In-Field-Corn-final-20171.pdf

Soybeans
Continue to sample fields for slugs and other defoliators. This past week, I have seen several fields with grasshopper injury. Soybeans can typically withstand a decent amount of defoliation before yield losses occur, however, if stands are being reduced, an insecticide application may be warranted. A treatment may also be needed if you are finding one grasshopper per sweep and greater than 30% defoliation. (Note: once plants reach bloom and pod fill stages, the threshold for defoliation is reduced to 15%).

Grasshopper feeding injury on seedling soybeans. Notice the irregular shaped holes and leaf feeding from the leaf margins, a good indication of grasshopper feeding.

Another example of grasshopper feeding injury on seedling soybeans. Again, note the irregular shaped holes and leaf feeding from the leaf margins.

In addition to grasshoppers, also keep an eye out for bean leaf beetle damage (chewing on cotyledons and small round holes in unifoliate and trifoliate leaves). This can often be confused with slug damage so look for slime trails, slugs, and beetles. The threshold for bean leaf beetles is 2 per ft of row and 25% stand reduction from emergence to 2 trifoliate. After 2 trifoliate, the threshold is 2-3 per plant and 30 percent defoliation.


Bean leaf beetle feeding injury. Bean leaf beetles will often drop from the plants when disturbed and are excellent at hiding in crop residue.


Slug injury on soybean unifoliate leaves. In some cases, this damage can look similar to damage from bean leaf beetles so when diagnosing the cause, be sure to look for slugs, slug slime trails, and for beetles.

Here is a link to our Soybean Insect Management Recommendations: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063934/Insect-Control-in-Soybeans-2017-final.pdf

Small Grains
I continue to hear reports of wheat and barley fields that have reached threshold for true armyworms. If you haven’t been scouting your fields, be sure to do so. With barley harvest approaching, make sure you review label restrictions for days to harvest before making an application.

Link to last week’s article on true armyworm in small grains: http://extension.udel.edu/weeklycropupdate/?p=10413

Grass sawfly and true armyworm fact sheet: http://extension.udel.edu/factsheets/grass-sawfly-and-true-armyworm-management-in-small-grains/

Small Grain Insecticide Recommendations: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063827/Insect-Control-in-Small-Grains-final-2017.pdf

True Armyworms in Small Grain

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

There have been numerous reports of small grain fields with armyworms this week. The threshold for armyworms is 1 per linear ft of row in barley and 1-2 per linear ft of row in wheat. Some fields may also have infestations of grass sawflies (wheat and barley: threshold 0.4/linear row ft). Keep in mind, not all insecticides labeled for armyworm control will provide grass sawfly control. With barley harvest right around the corner, also make sure you consider the days to harvest restrictions when making an insecticide application.


True Armyworm


Grass Sawfly

Here is a link to our Grass sawfly and True armyworm fact sheet for more info on sampling and decision making:
http://extension.udel.edu/factsheets/grass-sawfly-and-true-armyworm-management-in-small-grains/

Here is a link to our Small Grain Insecticide Recommendations:
https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063827/Insect-Control-in-Small-Grains-final-2017.pdf

Not sure how to identify true armyworms and grass sawflies? Here is a link to Guess the Pest Week #7 with information on identification:
http://extension.udel.edu/weeklycropupdate/?p=10378

Small Grain Insects: Sawflies, and Armyworms, and Aphids, Oh, My….

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sawflies and True Armyworms
Sample small grain fields for grass sawflies and true armyworms. A sweep net can be a quick and effective way to initially detect grass sawflies. Once you start finding sawflies, beat two rows of small grain together to dislodge any larvae and examine the inner space between the rows. Sample 5 linear feet of inner row space in ten random locations throughout the field. The threshold for sawflies in wheat and barley is 0.4 larvae per foot of row. True armyworms are active at night and seek refuge at the base of plants, under crop residue, and under weeds during the day. To sample for armyworms, examine 5 linear ft of row in at least 10 locations throughout the field. Also note any head clipping and leaf defoliation. The threshold for armyworms is 1 per ft/row in barley and 2 per ft/row in wheat. Fields with mixed infestations of armyworms and sawflies may require treatment even if worm counts of each pest do not exceed threshold levels.

For information on identification, biology, and management, view our Grass Sawfly and True Armyworm Fact Sheet:
http://extension.udel.edu/factsheets/grass-sawfly-and-true-armyworm-management-in-small-grains/

For chemical control options, refer to our Small Grain Insecticide Recommendations below.

Aphids
Watch out for aphids moving into small grain heads. The warm winter/early spring favored aphid reproduction. The English grain aphid is the only species that will infest small grain heads. Its feeding injury can result in shriveled kernels and reduced test weight. The threshold is 15-25 aphids per head when beneficial insect activity is low. One beneficial insect (lady beetle adults and larvae, syrphid fly maggots, lacewing larvae, damsel bugs, and parasitic wasps) per 50-100 aphids is often sufficient to keep aphid populations in check.

Aphid control in small grains in the spring Fact Sheet:
http://extension.udel.edu/factsheets/aphid-control-in-small-grains-in-the-spring/

Small Grain Insecticide Recommendations:
https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063827/Insect-Control-in-Small-Grains-final-2017.pdf

Agronomic Crop Insects – April 22, 2011

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Economic levels of both can be found in alfalfa fields at this time. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75- 100 per stem. Although beneficial insects can help to crash aphid populations, cooler temperatures will slow their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. As soon as temperatures increase, we will start to see a significant increase in feeding damage from alfalfa weevil. As alfalfa approaches harvest, the decision to cut instead of treat may be considered. However, this option should only be used if you plan to cut shortly after you find an economic threshold level since damage can occur quickly. Cutting should only be considered as a management option if you can cut within 3- 5 days of finding an economic level. Also, the effectiveness of using cutting as a management strategy is affected by temperatures after cutting. If the temperature remain cool, it has not always been effective. Since you need “stubble heat” to get control. As you get close to harvest, be sure to check labels carefully for time between application and harvest.

Field Corn
As soon plants emerge, be sure to check for cutworm feeding, even if an at-planting insecticide or a Bt corn was used for cutworm control. The wet soil conditions this spring have resulted in a higher level of grey garden slugs being found under residue in no-till fields. Although we see more problems in seedling corn when temperatures remain cooler and soil remains wet, it is generally during the warmer days of April when we start to see egg hatch. You will need to sift through previous crop residue and look at the soil surface for slugs. We are generally finding eggs and adults at this time; however we have started to see the first juveniles as well. The eggs, which are clear and about half the size of a BB, are often found in clusters within crop residue or at the soil surface. Although no thresholds are available, past experience in the Mid-Atlantic has indicated that pre-plant levels of five or more grey garden slugs per square foot can indicate the potential for a problem. In 2010, DuPont issued a 2ee recommendation for Lannate LV for slug management (http://www.cdms.net/LDat/ld183004.pdf); however, we have limited experience with the use of Lannate for slug management. Most of our experience has been with the use of a broadcast application of Deadline M-Ps at the low end of the labeled rate (http://www.cdms.net/LDat/ld7CL003.pdf). For more information on slug biology, sampling and management, please refer to the following fact sheet from Ohio State University: http://ohioline.osu.edu/ent-fact/pdf/0020.pdf.

SmartStax Approved for Refuge-In-Bag
Here is a summary of information on recent federal labeling of refuge-in-the-bag (RIB) from the Ohio State C.O.R.N newsletter (Ron Hammond, Extension Entomologist). Commercialization is pending individual state authorizations and notifications, as required.

“Two SmartStax corn products having the refuge-in-the-bag (RIB) concept have received registration from the U.S. Environmental Protection Agency, Genuity SmartStax RIB Complete by Monsanto Company and REFUGE ADVANCEDpowered by SmartStax by Dow AgroSciences. Both of these products are a blend of 95 percent SmartStax corn seed and 5 percent refuge (non-Bt) seed that farmers can plant across their entire field. This means farmers who plant these products no longer need to plant a separate, structured refuge for above-or below-ground pests in the Corn Belt. These new products are the outcome of collaboration between Monsanto and Dow AgroSciences, with both expecting a full commercial launch with broad lineups of hybrids for sale for 2012 planting.”

Small Grains
Although aphid population remain low, weather conditions favoring quick increases in populations include a combination of cool temperatures followed by a quick increase in temperatures. Although beneficial insects can help to crash aphid populations, cooler temperatures will slow their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. Since barley heads are starting to emerge in some locations, be sure to watch for the movement of aphids into grain heads. In many cases, beneficial activity is still not high enough to take care of populations that can move from the lower canopy of the plants into the grain heads. http://ag.udel.edu/extension/IPM/ExtensionFactSheets/AphidControlinSmallGrainIPM-4.pdf

Cereal leaf populations still remain relatively low but we can now find the first larvae in fields. Refer to our factsheet (http://ag.udel.edu/extension/IPM/ExtensionFactSheets/CerealLeafBeetleFactSheetIPM-5.pdf) as well as the Agronomic Crop Insects article in WCU 19:2 for sampling and treatment guidelines.

Once grain heads have emerged, you should also begin sampling small grains for grass sawfly and armyworm larvae. Although we can see economic damage from local overwintering armyworm populations, we often see significant outbreaks in years when moths coming from the South migrate to our area. Reports from trapping programs in Kentucky are indicating that trap catches for 2011 appear to be following their 2006 & 2008 outbreak levels – so be sure to begin checking for small larvae. http://www.ca.uky.edu/agcollege/plantpathology/extension/KPN%20Site%20Files/pdf/KPN1264.pdf

Remember, armyworm larvae are nocturnal so look for larvae at the base of the plants during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. The first small sawflies have been found by consultants in wheat and barley in Kent and Sussex counties. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half. The higher rates of insecticides are needed for grass sawfly control. http://ag.udel.edu/extension/IPM/ExtensionFactSheets/SawflyandArmywormIPM-6.pdf

 

Agronomic Crop Insects – April 30, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If economic levels of alfalfa weevil were present before harvest and you decide to cut instead of spray, be sure to check fields within one week of cutting for damage to the regrowth. If temperatures remain cool after cutting, there is often not enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.

Small Grains
During the past week, there has been a significant increase in populations of cereal leaf beetle larvae, especially in areas where adult egg laying has been abundant over the last few weeks. In many cases, larvae were very small (about the size of a pin-head) early in the week; however, they will quickly increase in size with the predicted warm temperatures. In addition, they can be found throughout the plant canopy so you need to look at the entire plant when sampling. Damage can occur quickly under these conditions so be sure to scout carefully for cereal leaf beetle larvae. The treatment threshold is 25 eggs and/or small larvae total per 100 tillers.

With the predicted warmer temperatures, we will also see an increase in true armyworm catches. Although true armyworms overwinter in our area, we can also get migrant moths from the South. Therefore, be sure to scout all small grains for armyworms at this time. Remember, on barley, head clipping can occur in a relatively short time. As a general guideline, the threshold for armyworms in barley is one per foot of row and for wheat one-two per foot of row.

In addition to armyworms, do not forget to watch for sawflies since larvae can be found in fields throughout the state. As a review, adult sawflies generally emerge in early April and begin to lay eggs in the leaf margins of small grains. Most egg laying is complete by early May but can be delayed by cooler temperatures. The first small larvae generally feed on the lower leaf blades and larval development takes approximately 21-30 days. Barley and wheat are both damaged by sawflies; however, during years of high population pressure, barley may experience more damage. Sawfly larvae prefer to feed on the stems and can be more damaging than armyworms. Stem clipping often occurs before leaf feeding is complete and/or the grain reaches physiological maturity. Since sawflies can clip heads quickly, be sure to scout carefully for larvae and watch closely for clipped heads. As a guideline, a treatment should be applied for sawflies when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. However, remember if the number of clipped heads is twice the worm count for sawflies then it may be too late to treat for them.

Since aphids feeding in the heads of small grains can result in a loss in test weight, be sure to watch for movement of aphids into the grain heads. As a general guideline, a treatment should be considered if you find 20 aphids per head and beneficial insect activity is low. You need at least one beneficial insect per every 50-100 aphids to help crash populations.

Lastly, before treating for any insect be sure to check the days between last application and harvest when selecting a spray material.