Economic Alfalfa Weevil Damage Heading Northward– (John Obermeyer, Rich Edwards, and Larry Bledsoe)

Severe alfalfa weevil damage

Surveys of west central and northwestern Indiana alfalfa fields this past week (see “Alfalfa Weevil Larval Survey”) show that alfalfa weevil feeding continues at high levels. The number of plants with tip feeding has reached as high as 100% (range 40 to 100%) with up to 7.4 weevil larvae per stem (range 1.7 to 7.4).

Producers in all areas of northern Indiana should be evaluating their alfalfa fields, if they have not already done so, as soon as possible. However, controls should not be applied until thresholds are reached. Refer to the following management guidelines for northern Indiana and the heat units (base 48°F) in the “Weather Update.” See Pest&Crop #3 for scouting procedures and issue #5 for recommended control materials.

back to top

back to top

Correction to Insecticide Table for Alfalfa Weevil– (John Obermeyer)

Please omit methyl parathion (Penncap-M) from last week’s (Pest&Crop #5) “Insecticides for Alfalfa Weevil Larval Control” table. Though we always try to provide you accurate information, especially with pesticides, we do make mistakes. Remember, the product label is the law, always check the label before using the product!

back to top

Early Emerging Soybeans Attract Bean Leaf Beetle– (John Obermeyer, Rich Edwards, and Larry Bledsoe)

A small percentage of soybeans have been planted and are beginning to emerge. Overwintering bean leaf beetle become active on warm days in the spiring and seek legumes, e.g., alfalfa and soybean, to feed and propagate. Large numbers of bean leaf beetle are possible this spring because of the last fall’s population and the mild winter.

One of the most critical times for soybean damage is from emergence through the establishment of the first trifoliolate. If cotyledons are being destroyed before the unifoliolate leaves fully emerge or if the growing point is severely damaged, reduced yields are likely.  However, once trifoliolate leaves have unrolled, soybean can tolerate up to about 40% defoliation without yield loss. 

For cotyledon, and unifoliolate stage soybean, refer to the following thresholds*:

back to top

back to top

Wheat Streak Mosaic Epidemic– (Don Hershman and Doug Johnson, University of Kentucky)

The following article is being reprinted with permission from Kentucky Pest News, April 24, 2000.

Last week we started to receive samples and phone calls which suggested that wheat streak mosaic had reared its ugly head again in Kentucky. The last and only recorded epidemic of this virus disease in KY was in 1988. Current information indicates that the greatest incidence of wheat streak mosaic is across the southern portion of the state, extending from east of Bowling Green all the way to Fulton County. However, the full extent of the epidemic has yet to be determined. It is likely that additional counties will report the disease as crop development continues.

Wheat streak mosaic virus is transmitted by the wheat curl mite. This association and why we believe that wheat streak is a problem this spring will be addressed later in this article.

Symptoms: Severely diseased plants are fairly distinctive.  They  will be severely stunted; leaves will be small and will have a “spiked” appearance. Lower leaves and tillers may turn brown and die. Plants may have a flaccid look about them and may look like they are having difficulty standing. Leaves will show extensive yellow streaking, especially from the middle of leaves towards the tips. The typical discontinuous yellow streaks in leaves may be so extensive that leaves will have a yellow, “bleached out”appearance. Some leaves will have a mosaic symptom.

Severe symptoms, such as described above, indicate that infection occurred in the fall or early winter. The prognosis for severely disease plants is not very good and 75% or greater yield loss can be expected. Fields have large percentages of severely diseased plants are candidates for destruction and replanting to alternate crops. In some cases, only portions of fields may severely diseased. In those instances, it may only be necessary to destroy a portion of those fields.

Although, currently, many fields do have a high percentage of severely diseased plants, many other fields are just now beginning to show yellowing and streaking, but are not stunted. If symptom expression occurs early, such as prior to flag leaf emergence, then expect those plants to deteriorate rapidly. However, if plants are in the boot stage or beyond when symptoms begin to be expressed, then expect only moderate to light yield effects. In fact, based on information provided to us by Kansas State University Extension Plant Pathologist, Dr. Bob Bowden, most plants which show late symptoms will have reduced test weight, but that is about it. These fields are certainly NOT candidates for destruction, but they may also not be good candidates for adding additional inputs, such as foliar fungicides or insecticides. Basically, the impact of late-appearing wheat streak mosaic symptoms depends upon the incidence of diseased plants in a field. Obviously, the greater the number of diseased plants, the more impact there will be on crop test weight and, thus, profitability.

So now to the key questions. Why is wheat streak severe this year?  Could it have been prevented? What can be done to reduce future occurrence? To answer these questions we need to look at how the pathogen, wheat streak mosaic virus, gets into wheat.

Certainly we would like to have a clear idea of why we have this problem  After all, it has been 12 years since a wheat streak mosaic epidemic has occurred in Kentucky.  So, lets see if we can construct a usable model to provide an explication for this event.

A complete life history of the mite is given in  Entfact:117 - Wheat Streak Mosaic Virus and the Wheat Curl Mite.  It is available from your county agent or from the Entomology Web site at: <http://www.uky.edu/Agriculture/Entomology/enthp.htm>
When you reach the site select “ENTFACTS’ then “FIELD CROPS” and look for Entfact-117.  Also see Sloderbeck, P.E. 1995. Wheat curl mite. Entomology Wheat Insects (L. D.)  at   <http://www.oznet.ksu.edu/library/entml1/wheatcur.pdf>

• The mite is only about 1/100 of an inch long
  
• Epidemics are always associated with a “Green bridge”.
  
• This mite MUST have green tissue to live on.  It can survive only a few days off the host.
  
• Typically,  the mite moves by being wind blown.
  
• Wheat is by far most important host for the mite but it can survive on other plants that grow in KY
  
• There is no known effective pesticidal control for this mite.

What is different about this growing season?  We probably have the mite, the virus, and alternate hosts here every year but we rarely have an epidemic.

On Tuesday April 18, 2000, Dr. Hershman and I met with several county extension agents, producers, consultants, farm store managers and technical representatives.  These “meetings” took place in  infected fields in and around Logan and Warren Counties.  We were able to view the severity and distribution of affected fields, and perhaps more importantly gather background information from people involved in local wheat production. While we are not able to solve the problem, we do think that between all of us we can now explain what has happened.

Wheat is the ideal host. The mite can live on some other grasses but it does not do well.  Since we do not ordinarily have the problem, our wild hosts are not sufficient to provide an epidemic.  In looking at the literature and talking with people from Kansas, they always see the problem when wheat from a previous cropping season is allowed to live continually through to the next wheat season.  This can happen in several ways.  Wheat from shattered heads at harvest will germinate in the field and then may grow continuously tho the next planting.  Wheat may live continuously in abandoned fields and of course from spillage along roads etc.  The point is whether in or near production fields, in problem areas wheat is growing throughout the time between crop, allowing the mites and virus to survive, move about and increase in number. This is the green bridge.

However, our production system is quite different from those out west. We usually have several months (during the summer) between wheat harvest and planting when wheat is generally not present. So how do we get this Green Bridge?  And what was particular to the1999-2000 season? The answer to this question became very apparent by talking with the local individuals.  Because of the drought stress of the summer of 1999, the soybean crop (especially double crop) did very poorly.  As a result producers were unwilling to provide the additional weed control that normally would have controlled volunteer wheat and other grasses in the soybean crop.  As a result, the area currently suffering from Wheat Streak Mosaic had quite a lot of volunteer wheat.  Hence our green bridge!  This is most likely the core of the problem.  However, the long warm fall and very mild winter certainly allowed the mites to remain active for much of the growing season.

Once everyone was aware of how the mite/virus movement and survival controlled the disease epidemic cycle, it was easy to find examples of where the green bridge had occurred.  This does not explain every individual case, but it certainly does explain the core reasons for why we are seeing this epidemic in this year, in this area.

One of the more difficult questions is why do we find fields that are very evenly infested and do not appear to be near areas of volunteer wheat?  My best explanation for that is to use scattered rain storms as a model.  Once these mites get into the air they may simply “rain” down on an area.  This movement would probably be of longer distance, provide a rather even dispersal on a local level,  and be a matter of chance as to where they might settle out.  You might get rained on and you might not.

So what is to be done?  It appears that this pest is very difficult to detect and it cannot be controlled with insecticides.  The only real remedy is to stop the formation of the green bridge.  If we return to normal production practices this problem may take care of itself.  If however, it appears that fields are going to be abandoned or if other cropping practices evolve that allow volunteer wheat to prosper, we will have to do something to control the volunteer wheat.  This will have to be an area wide effort.  Just doing the right thing on your own farm will not be enough.  Everyone needs to understand that wheat growing during the off season is a threat to commercial production.

Without the cooperation of a large number of individuals working in wheat production, we probably would not have been able to sort out this puzzle. 

back to top

Know Your Friends: Bigeyed Bugs– (Bob Wright, University of Kentucky)

The bigeyed bugs, Geocoris spp., are generalist predators commonly  found in a variety of crops and noncultivated plants nationwide. The common name refers to a characteristic feature of these insects, their rather large, protruding eyes. Bigeyed bugs are members of the family Lygaeidae (seed bugs), but belong to a subfamily containing only  predaceous insects.

There are many different Geocoris species found throughout the U. S. The adults range in color from black and white to tan. The nymphs resemble miniature grayish adults. They are small (3-4 mm long) insects capable of feeding on a variety of types of prey such as insect eggs, aphids, mealybugs, spider mites, leafhoppers, bugs, and small caterpillars or beetle larvae. Each nymph may consume up to 1600 spider mites during its immature stages, and as many as 80 mites a day as an adult. Other laboratory studies have shown that each G. punctipes nymph consumes about 250 soybean looper eggs before it reaches the adult stage. In yet another lab study adult bigeyed bugs consumed up to four Lygus bug eggs per day. In laboratory and field studies they have been observed to feed on dead insects, although when given a choice they prefer live prey.

Geocoris also feed on plants; maximal survival and reproduction in the laboratory occurs when they have a mix of plant and insect food. In the laboratory they can survive for months on sunflower seeds and water. This plant feeding behavior is probably related to their evolution from seed feeding lygaeid bugs, and allows them to survive periods of low insect abundance. Their omnivorous feeding habits allow them to survive in a variety of habitats and contribute to biological control of pest species that become abundant. There is no evidence that their plant feeding causes significant injury to the plant.  However, their plant feeding makes them susceptible to systemic insecticides; studies in cotton have shown reduced Geocoris survival on plants treated with systemic soil insecticides.  

Bigeyed bugs have piercing sucking mouthparts. Field observations indicate that Geocoris usually attack prey simply by walking or running up to a potential prey, extending their beak and quickly inserting it into the prey. Geocoris may lift prey into the air, preventing the prey from attempting escape by running. They may drop to the ground if disturbed.

Like all true bugs, they go through incomplete metamorphosis. The immature nymphal stages resemble the adults, except they lack functional wings. Nymphal stages have similar behavior and feeding habits as adults, but tend to feed on smaller prey. Most bigeyed bugs go through five nymphal stages. Under laboratory conditions nymphal development takes about 30 days at 25xC and 60 days at 20xC. Although not well studied, it is reported that bigeyed bugs overwinter as adults or as eggs depending on the species and location.

Bigeyed bugs are particularly common in cotton, peanut, and soybean fields, and seem to prefer weedy areas in gardens. They are also common in managed turfgrass, such as home lawns.

Bigeyed bugs were effective predators of corn earworm eggs in cotton in experimental mass releases, but are not available for sale commercially. However, recent studies by USDA-ARS scientists have identified new artificial diets for bigeyed bugs based on ground beef and beef liver. This diet costs $2.50 per pound, compared with $300 a pound for rearing with insect eggs. Bigeyed bugs reared on the new diets produced more offspring, often mature faster and are up to 50% larger than field collected insects. Although not commercially available yet, these predators are important in natural control in many agricultural settings.

Midwest Biological Control News   -   Volume IV, Number 12 December 1997

back to top