August IPM Insect Scouting Tips

Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu
University of Maryland Extension, Dorchester County

Soybean: Continue to scout for stink bugs, dectes stem borer, and defoliators (bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars). Control may be needed if there is 15% defoliation from bloom through pod fill. Chemical control is not recommended for dectes stem borer since it would require multiple applications to reduce larval infestations, which is not economical. If a high number of adults are found, harvesting that field as soon as it matures will reduce losses associated with lodged plants.

With the upcoming hot weather there is an increased chance of spider mite outbreak in vegetable and soybean fields (Fig 1). Scout by using a hand lens to examine mid to upper canopy leaflets on two plants in twenty locations along the edges of the field. Consider treating if 10 % of plants have heavy stippling feeding damage on ⅓ of their mid and lower leaves.

The next flight of corn earworm (CEW) will be taking place soon. Bean fields with open canopies (wide rows), are drought-stressed, or have recently had an insecticide applied are at higher risk for CEW. CEW larvae can feed on flowers without impacting yields. NC State has a good economic threshold calculator to assist with management decisions: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.

Field Corn: Check for stink bugs around the edges. The threshold is 1 stink bug per 2 plants from pollen shed to blister stage. Treatment is not recommended past the blister stage.

Sorghum: Sugarcane aphids typically show up in fields late July and August. Check underside of leaves for insects. Honeydew will turn leaves shiny and is an easy to see indicator that aphids are present. Sugarcane aphids are light yellow with black cornicles, antennae, and feet. Thresholds depend on plant growth stage; at boot to milk, thresholds are 50 aphids per leaf on 25 – 30% of plants. There is documented resistance to resistance to pyrethroids.

July Insect Scouting Tips 

Emily Zobel, Agriculture Agent Associate
University of Maryland Extension, Dorchester County

Soybean: The usual defoliators are starting to arrive, including bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars. Control may be needed if there is 30% defoliation during the seedling and vegetative stages and 15% defoliation once plants start to bloom through pod fill.

Adult Dectes Stem borer will be emerging over the next several weeks. Chemical control is not recommended since it would require multiple applications to reduce larval infestations, which is not economical. If a high number of adults are found, harvesting that field as soon as it matures will reduce losses associated with lodged plants.

Fields that have an open canopy, drought-stressed, or have recently had an insecticide applied are at higher risk for corn earworm (CEW). CEW larva can feed on flowers without impacting yields because soybeans overproduce flowers. However, feeding during pod development can affect yield. An economic threshold calculator is available to assist with management decisions: https://soybeans.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.

Field Corn:  As corn ears begin to form, check for stink bugs. Stink bugs will gather around the edges of fields, so scouting should be done at least 15 rows in. Thresholds are 1 stink bug per 4 plants when the ear is forming, and 1 stink bug per 2 plants from pollen shed to blister stage. Treatment is not recommended past the blister stage. Japanese beetles are minor defoliators and will clip corn silks, but control is not needed unless silks are cut back to less than ½ inch, and less than half the field has been pollinated.

Alfalfa: Once plants have hopper burn, there is no way to undo it, so continue scouting for leafhopper.  Since infestations are highly variable, individual fields should be scouted. If you are planning on selling your hay for horse feed, check for blister beetle as well since they produce cantharidin, which causes skin blisters on humans and can make horses sick.

Sorghum: Sugarcane aphids were found on the Eastern Shore last year and typically show up in fields late July and August. Check underside of leaves for insects.  Honeydew will turn leaves shiny and is an easy to see indicator that aphids are present.  Sugarcane aphids are light yellow with black cornicles, antennae, and feet. Thresholds depend on plant growth stage; at boot to milk, thresholds are 50 aphids per leaf on 25 – 30% of plants. There is documented resistance to resistance to pyrethroids.

 

Pyrethroid insecticide effects on pests and beneficials in field corn

Maria Cramer, Edwin Afful, Galen Dively, and Kelly Hamby
Department of Entomology, University of Maryland

Overview

Background: Due to their low cost, pyrethroid insecticides are often applied when other chemical applications are made. For example, they may be included in tank mixes with herbicides in early whorl corn and with fungicides during tasseling. These pyrethroid sprays often target stink bugs; however, the timing of these treatments is not ideal for stink bug management. Pyrethoid insecticides may harm beneficial insects that help keep pest populations in check and repeated use of pyrethroids can contribute to insecticide resistance.

Methods: In this study, we examined the effect of Bifenture EC® (pyrethroid active ingredient: bifenthrin) applied with herbicides in V6 corn and with fungicides in tasseling corn. We evaluated impacts on pests and beneficials at both application timings. Yield was measured at harvest.

Preliminary Results:  At both application timings, Bifenture EC® did not improve insect pest management because pests were not present at economic levels. We did not find evidence for flare-ups of aphids or spider mites, but a rainy late summer made it unlikely that we would see many of these pests. There were no yield differences between the treatments.

Background

As a result of the low cost of pyrethroid insecticides, preventative applications are common, especially in tank mixes with other routine chemical inputs, such as herbicides and fungicides. However, lower grain prices and low insect pest pressure make it less likely that pyrethroid applications will provide economic returns. Bt hybrids1 and neonicotinoid seed treatments control many of the pests targeted by pyrethroid insecticides. Because they have broad spectrum activity, pyrethroids can negatively impact natural enemies2 which can result in flare-ups of secondary pests3. Tank mix timings may be less effective than applying when insect populations reach threshold. For example, when pyrethroids are combined with herbicide applications, they are too late to control early-season stink bugs and other seedling pests. When pyrethroids are combined with fungicide sprays at tasseling, few insect pests are present at damaging levels. Stink bugs may feed on the developing ear at this time, causing deformed “cowhorned” ears; however, this is rarely a problem in Maryland and stink bug damage is generally not economic throughout a field because feeding is primarily concentrated at the field edge4. Insecticide applications at tasseling have a high potential to affect beneficial insects, especially pollinators and natural enemies that are attracted to corn pollen.

Objectives: Our objectives were to determine the effect of pyrethroids applied preventatively in tank-mixes on corn pests, beneficials, and yield.

Methods: This study was conducted in 2018 and 2019 at the University of Maryland research farm in Beltsville, MD. For each application timing, we planted four replicate plots of a standard Bt field corn hybrid, DeKalb 55-84 RIB (SmartStax RIB complete Bt insect control in addition to fungicide and insecticide seed treatments) at 29,999 seeds per acre. Standard agronomic practices for the region were used.

The herbicide timing compared two treatments:

  1. Herbicide alone (22 oz/acre Roundup WeatherMAX®, 0.5 oz/acre Cadet®, 3 lb/acre ammonium sulfate
  2. Herbicide (same as above) + Insecticide (Bifenture EC® 6.4 oz/acre)

Treatments were applied at V6/V7. We visually surveyed corn plants for pest and beneficial insects before and after application. We also placed sentinel European corn borer (ECB) egg masses in the field to assess predation rates before and after treatment.

The fungicide timing compared two treatments:

  1. Fungicide alone (Trivapro® 13.7 oz/acre)
  2. Fungicide (same as above) + Insecticide (Bifenture EC® 6.4 oz/acre)

Treatments were applied at green silk. We inspected the ear zone and silks for pests and beneficial insects before application. After application, we recorded the number of ears with pest damage and the kernel area damaged. We also counted stink bug adults and cowhorned ears. Six weeks after application, we visually assessed plants for spider mite and aphid colonies.

Sampling for pests and beneficials (left) and; sentinel European corn borer egg mass (right).
Sampling for pests and beneficials (left) and; sentinel European corn borer egg mass (right).

Results

In the herbicide-timing study in 2019 we observed no effect on beneficial insects from the treatments (Figure 1). The most abundant beneficial species were minute pirate bugs and pink spotted lady beetles, which are very mobile and may have recolonized treated plots after treatment. Similarly, treatments did not affect predation on the sentinel egg masses, suggesting that the pyrethroid application may not have affected predators’ ability to locate and consume eggs. Across the treatments, 30-50% of egg masses were consumed by predators.

Minute pirate bug on European corn borer egg mass.
Minute pirate bug on European corn borer egg mass.

The treatments did not impact the number of beneficials at the herbicide timing (N.S.). The pyrethroid insecticide significantly reduced the number of plant hoppers and plant bugs from less than 4 per plant on average to less than 2 per plant (significantly different p<0.05, *), though these insects are not economic pests at this stage. There were never more than 2 stink bugs per 90 plants, well below the treatment threshold of 13 per 100 plants4.

In the fungicide-timing study in 2019, beneficials, especially minute pirate bugs, were abundant at the time of application (3 in every 10 plants), while stink bugs, the presumed target pest, were very rare (1 stink bug in every 68 plants). In 2018, stink bugs were similarly scarce. Overall pest abundance was low (1 in every 35 plants). After application, there was no difference in the incidence or amount of the corn ear damaged by worms, stink bugs, or sap beetles between treatments. Average stink bug and earworm incidence was roughly 1 in 10 ears, while sap beetle was even less frequent. Cowhorned ears and adult stink bugs were almost non-existent in both treatments.

Six weeks after application we found no differences in aphid or spider mite populations between the treatments, suggesting that pyrethroid applications at tasseling did not cause secondary pest outbreaks. We sampled after a period of dry weather; however, the late summer was rainy at Beltsville, which likely suppressed spider mite and aphid populations. Under drought-stress, reductions in the natural enemy population from pyrethroid use might contribute to flare-ups of aphids and spider mites.

Figure 1. Herbicide timing. July 3, 2019, Beltsville MD. Mean number of insects per 10 plants in V7 corn after treatment. N.S.=not significant. H=herbicide; P=pyrethroid.

Yield

For the herbicide timing and fungicide-timing (Figure 2) studies, treatments did not affect yields in either 2018 or 2019.

Conclusions

Figure 2. Herbicide timing (left) and fungicide timing (right), 2018 and 2019, Beltsville MD. Mean yield per acre under two treatments. Yields were not significantly different by treatment in either study. For the fungicide-timing study, 2019 yields were significantly higher than in 2018. N.S.=Not Significant. H=Herbicide; F=Fungicide; P=Pyrethroid.

Results from the 2018 and 2019 studies suggest that pyrethroid applications do not provide yield benefits in corn when tank-mixed with herbicides or fungicides, likely due to the lack of insect pest pressure at these spray timings. Beneficial insects were abundant in the crop at each of these timings and did not appear to be affected by the pyrethroids in the herbicide plots. Repeated preventative use of pyrethroids in the same field could potentially hinder the natural biocontrol of corn pests.

Lady beetle larva (a predatory insect) in silks.
Lady beetle larva (a predatory insect) in silks.

Sources

1 DiFonzo, C. 2017. Handy Bt Trait Table for U.S. Corn Production, http://msuent.com/assets/pdf/BtTraitTable15March2017.pdf

2Croft, B.A., M.E. Whalon. 1982. Selective toxicity of pyrethroid insecticides to arthropod natural enemies and pests of agricultural crops. Entomophaga. 27(1): 3-21.

3Reisig, D.C., J.S. Bacheler, D.A. Herbert, T. Kuhar, S. Malone, C. Philips, R. Weisz. 2012.Efficacy and value of prophylactic vs. integrated pest management approaches for management of cereal leaf beetle (Coleoptera: Chrysomelidae) in wheat and ramifications for adoption by growers. J. Econ. Entomol. 105(5): 1612-1619

4Reisig, D.C. 2018. New stink bug thresholds in corn, https://entomology.ces.ncsu.edu/2018/04/new-stink-bug-thresholds-in-corn/

 

Guess the Pest! Week #20 & 21 Answer: Trissolcus japonicus

Guess The Pest Logo

 

 

 

 

 

Bill Cissel, Extension Agent – Integrated Pest Management, University of Delawarebcissel@udel.edu

Congratulations to Greg Hawn (week 20 winner) and to Joseph Streett (week 21 winner) for correctly identifying the insect as Trissolcus japonicus 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 #20 – 21 Answer: Trissolcus japonicus
Bill Cissel, Extension Agent – Integrated Pest Management, Joe Kaser, Research Associate, USDA-ARS Beneficial Insects Introduction Research, and David Owens, Extension Entomologist

Trissolcus japonicus, a tiny wasp commonly referred to as the Samurai wasp, is an egg parasitoid of the invasive brown marmorated stink bug (BMSB). This particular species is native to Asia and has been in quarantine since 2007 and under evaluation for potential release as a classical biological control agent. In 2014, wild populations of Trissolcus japonicus, slightly different from the ones that were in quarantine, were detected in Beltsville, MD and since, additional discoveries have been made throughout the region, including Washington, D. C., Virginia, West Virginia, Pennsylvania, New Jersey, Ohio, and Delaware. It is believed that Trissolcus japonicus may have hitchhiked a ride in a BMSB egg mass that was on plant cargo shipped from Asia, but it is difficult to say exactly how it got here. In fact, it appears that the samurai wasp has hitchhiked here more than once!

A single Trissolcus japonicus female is capable of parasitizing an entire BMSB egg mass which typically contains ~28 eggs. When the male parasitoids emerge, they wait on the egg mass for the females to emerge so they can mate. They are capable of having up to ten generations per year.

To help with reducing BMSB populations in Delaware, we partnered with some of the folks at the USDA Beneficial Insects Introduction Research Laboratory in Newark, DE to redistribute Trissolcus japonicas throughout the state. When I share that we are releasing a parasitic wasp to help with BMSB control, the first reaction that I typically get is, “Will it sting me?” If you look at the photo with some wasps on the dime, you will understand why this is not a concern. Hopefully, this tiny wasp will live up to its name as the Samurai wasp and do its part in controlling BMSB.

Fun Entomology Fact: A female Trissolcus japonicus will chemically mark the BMSB eggs that she laid eggs in and defend them against other parasitoids.

Here is a link to a very informative fact sheet from UF on Trissolcus japonicushttp://entnemdept.ufl.edu/creatures/beneficial/wasps/Trissolcus_japonicus.htm

Guess the Pest! Week #15 Answer: Stink Bug

Guess The Pest Logo

 

 

 

 

 

Bill Cissel, Extension Agent – Integrated Pest Management, University of Delawarebcissel@udel.edu

Congratulations Chris Leon for correctly identifying the damage in the photo as stink bug damage 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 #15 Answer: Stink Bug

The damage on the corn stalk is stink bug feeding injury. Stink bugs will use their piercing-sucking mouthparts to probe into the stalk of the plant, removing plant fluids. If the stink bug hits the ear at this stage, the ear will often fail to develop kernels at the feeding site. This causes the ear to develop into the classic “C”-shaped or boomerang-shaped ear. This is why the greatest damage and yield loss potential due to stink bug feeding is prior to pollination. This is also why waiting until after tasseling (pollination), to control a stink bug infestation in field corn is too late. Here is a link to last week’s article discussing stink bug management in field corn: http://extension.udel.edu/weeklycropupdate/?p=12194