Wheat Insects – What to Watch for This Spring

Consider adding some insect scouting to your wheat management routine this spring if you are not doing so already. While damaging insect pest infestations are pretty sporadic in Illinois, missing one can be costly. The first step in managing these infestations is knowing what to look for.

 

Aphids

 

There are several species of aphids that infest wheat in Illinois, and they can be difficult to tell apart without careful examination (the figure captions below provide some tips on how to identify the different species; note that aphids are tiny insects, and you will probably need a hand lens to distinguish them). The primary concern with aphids is their ability to transmit barley yellow dwarf virus (BYDV); however, high populations of aphids feeding on plants can reduce vigor, lead to wilting, and coat the plants in sticky “honeydew” and sooty mold. In addition, greenbug feeding introduces a toxin that can reduce yield through stunting of the plants.

 

Image of two bird cherry-oat aphids

Fig. 1. Bird cherry-oat aphid. Note the brown-ish coloration at the rear of the body and the overall round or “globular” shape compared with the other aphid species. Photo: Frank Peairs, Colorado State University, Bugwood.org

 

image of an English grain aphid

Fig. 2. English grain aphid. This species has darkened cornicles (the “tail pipes” at the rear end of the body) and a narrower overall body shape. Photo: Kansas Department of Agriculture, Bugwood.org

 

image of a greenbug aphid

Fig. 3. Greenbug (or greenbug aphid). Greenbugs can be recognized by the dark green stripe down the back of the body. Greenbugs are less common in Illinois wheat than the other two species, but they are more damaging at lower numbers due to a toxin contained in their saliva. Photo: Frank Peairs, Colorado State University, Bugwood.org.

 

Armyworm

 

Armyworms are a sporadic issue in wheat in Illinois, but under high pressure they can cause substantial damage. The species we see in the spring is often called the “true” armyworm to distinguish it from fall armyworm (which, as the name suggests, arrives later in the season) and several other species. The true armyworm caterpillar has a broad, lighter-colored stripe on either side of the body, a net-like pattern on its head, and dark bands on each proleg. Armyworms feed on leaves, resulting in a raggedy appearance. Leaf feeding itself generally does not have much of an impact unless it is severe. However, occasionally armyworm larvae clip seed heads when leaf material becomes scarce, and this can result in serious yield losses.

 

image of a true armyworm larva

Fig. 4. A true armyworm larvae. Note the light colored bands on the side, the net-like pattern on the head, and the dark bands on the prolegs. Photo: Frank Peairs, Colorado State University, Bugwood.org.

 

(Just a note: the images in this article are used under the terms of a Creative Commons Attribution License, and were obtained through www.ipmimages.org, which is an excellent insect identification resource).

Keep watching the Bulletin for seasonal updates if and when we start to see issues pop up. Until then, happy scouting!

 

Author: Nick Seiter, Research Assistant Professor, Field Crop Entomology

nseiter@illinois.edu | 217.300.7199

 


Cool season viruses in wheat

Currently, most small grains are approaching jointing or just past jointing in many parts of the state.  Now is the time that you most likely will start to see early season viral diseases in some fields, specifically Wheat Soilborne Mosaic Virus and Wheat Spindle Streak Virus. These diseases are transmitted by soil borne microbes that thrive in cool, wet conditions. Infected plants typically are chlorotic and may be stunted. Often affected plants occur in low lying areas of the field or areas suffering from compaction. However, on some occasions entire fields can become symptomatic. Symptoms of spindle streak include necrotic dashes that run along the venation, giving the appearance of a spindle (Figure 1). Additional symptoms of soilborne mosaic virus are less conspicuous, but include mottling of lower foliage (Figure 2). Symptoms, including chlorosis and even stunting can look very similar to nutirent deficiencies.  Symptoms cease once temperatures are above 65°F and may be reduced after fertilization.  Even though symptoms may be reduced, it does not mean the viruses and their effects are removed from the crop.  I consider these diseases hidden yield robbers due to the fact that they often go misdiagnosed as nutrient issues and their effects often go unnoticed.    Confirmation can only be made through specialized testing methods such as ELISA and PCR.  The University of Illinois Clinic and Agdia are two options for having samples tested.

 

How should you send a virus sample?  The key for getting good virus results is to keep the sample cool and avoid excess moisture.  If the samples get warm or are kept too wet microbes can degrade the viruses, resulting in false negative results.    Place 15-20 leaves in a plastic bag with a dry paper towel and place immediately in a cooler on ice.  Make sure to ship samples overnight on ice early in the week so that they are processed immediately and not allowed to set at room temperature.  Our lab is conducting a wheat virus survey in 2019 and we are seeking people to help us sample fields.  If you are interested in helping, send me an email .

Figure 1. Symptoms of wheat spindle streak virus in wheat.

Figure 2. Symptoms of wheat soilborne mosiac virus.

What should you keep in mind? 1) Keep track of fields with these viruses. Once the viruses are established they will be present in those fields from here on out.  2) Try to harvest or work in these fields last to prevent spread to other fields. 3) Avoid compaction. 4) Plant tolerant varieties in fields with a history of these viruses. Unfortunately many varieties are screened for these viruses as a complex, so it may be difficult to determine if you are planting a spindle streak or soilborne mosaic tolerant variety in these cases.  However, the University of Illinois is one of the few places that does conduct screening for these viruses.  More information on wheat varieties for Illinois can be found by clicking here.


Diagnosing disease related issues in the field

Well, it is that time of year where we start to see issues developing in the field.  Questions such as, “What happened?”  and  “Why me?” will become more common.  The key to managing diseases is proper diagnosis, and this starts in the field.  In my recent post on the Field Crop Disease Blog, I provide several tips for diagnosing issues in the field, and distinguishing disease related problems from abiotic issues.  Check out the post, and sign up for updates!


Update on wheat in Illinois

This past week we spent a few days surveying wheat fields throughout the state in order to see how the crop is progressing as well as better understand what disease related issues we may be experiencing.  Most of the crop was near flag leaf emergence (Feekes growth stage 8/9) with a few fields near boot in locations further south.  The good news is that of the 26 fields we looked at, none had any stripe rust, nor have I received any additional reports of this disease in the state.  In general, diseases were minimal.  In southwest portions of the state Septoria leaf blotch (aka speckled leaf blotch) was fairly common.

Septoria (speckled) leaf blotch is often found in the lower canopy when conditions are cool and humid. Photo N Kleczewski

 

This is a residue-borne disease that is favored by cool, wet conditions and can grow and persist on small grain residues.  The disease is often located deep within the lower canopy, and causes irregular brown lesions on the foliage.  At the center of the lesions you will often see black structures that may resemble tiny peppercorns.  These structures are why the disease has the extremely creative common name speckled leaf blotch.  The disease spreads upwards predominantly via rain splash, and seldom causes significant yield impacts.  This typically is due to increased temperatures that do not favor disease development as the crop develops and the flag leaf is produced.  Remember, the flag leaf and green tissues above contribute the majority of carbohydrates for grain fill (over 70% from the flag leaf alone).  Foliar diseases that do not reach these tissues are typically not a major concern.

Similarly, I came across a few fields with light powdery mildew.  Unlike Septoria leaf blotch, powdery mildew is an obligate pathogen and requires a living host to grow and reproduce.  Cool, humid (not wet) conditions favor powdery mildew development.  In general, production practices that favor rapid plant growth and lush, full canopies early in the season favor this disease.  For example, high nitrogen rates or manure use can result in rank growth early in the season.  Powdery mildew can reproduce more quickly than Septoria, and therefore can occasionally impact early season growth or tillering in some instances.  Although I did not see anything that would be of concerns and have not had any reports of severe powdery mildew, management is best achieved through selection of a resistant variety and avoiding excessive nitrogen application.   Early season fungicide applications with nitrogen applications can have some benefit when a field is at high risk for disease (i.e. susceptible variety, heavy N use, disease present early, cool weather forecast for several days/weeks) but are not recommended if disease is low.  Anything in the triazole (FRAC group 3), SDHI (FRAC group 7) or Strobilurin (FRAC group 11) fungicide classes will help control powdery mildew in high risk situations.

As we approach boot and heading you should keep an eye on the Fusarium Head Blight Prediction Center for updates on disease risk.  I will follow up with a post on how to best use this tool on my blog in the next few days. Forecasts are calling for e moderate and potentially rainy conditions over the next 7-10 days  depending on your location.    In the meantime, keep an eye on your fields, and enjoy the weather!

 

Nathan Kleczewski Extension Field Crop Plant Pathologist University of Illinois   email:  nathank@illinois.edu


Stripe Rust in S. Illinois

I received notice of stripe rust in S. Illinois today.  Stripe rust is an important disease affecting wheat.  Please find an article on this disease and management by clicking here.  

If you locate stripe rust in your field please tweet a picture to me (@ILplantdoc) or email (nathank@illinois.edu) with the wheat variety, growth stage, and approximate percent of field infected.  This information will be useful to IL wheat producers this year and in upcoming seasons.

An example of a stripe rust in wheat. Photo N. Kleczewski 2016