Watch for Weevil Activity: Alfalfa and Clover leaf weevils

Over the last decade, large-scale alfalfa weevil problems have not been seen in Nebraska.  However, there have been locally damaging infestations in some areas, particularly parts of the Panhandle and Boyd and Holt counties, which were hard hit in 1998 and have had some problems since.  Predators and other natural enemies seem to have stabilized alfalfa weevil populations, but occasionally things can get out of balance (recall the caterpillar explosion of last year).  While we don’t know if alfalfa weevils or clover leaf weevils will be a problem this year, those of you who are growing high quality alfalfa hay should take the time to monitor fields for the next month.

According to growing degree accumulations, alfalfa weevil activity should have begun in most of the state by now.  Weevil larvae usually begin doing noticeable damage at about 350 GDD (48 degree base).

Clover leaf weevils (CLW) are occasionally a problem in dry springs but are very vulnerable to a fungus disease, and so haven’t been pests since the late 80’s early 90’s when spring rains were rare.   Clover leaf weevil larvae will be in the debris around the crowns during day.  Scratching in the soil around the crowns and counting the number of larvae found per crown will help give a better idea of clover leaf weevil infestation.  Their brown heads will help distinguish them from the black headed alfalfa weevil.  Table 1 will compare the alfalfa weevil and the clover leaf weevil.

Both and alfalfa weevil and clover leaf weevils feed on first cutting alfalfa as larvae, and regrowth of the first cutting as adults.  While research conducted in northeast Nebraska has shown that clover leaf weevil larvae feeding does not cause yield reduction to first cutting alfalfa, alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting. (KJ)

Table 1.  Comparison of Alfalfa Weevil to Clover Leaf Weevil.

Alfalfa Weevil	Clover Leaf Weevil
Overwinter primarily as adults	Over winter primarily as larvae
Adults brown with dark brown stripe halfway down back 3/16 inch long	Adults dark brown pitted light brown underneath over 1/4 inch long
Larvae prefer to feed on tips	Larvae feed anywhere on plant
Larvae remain on plant most of the time	Many larvae in soil or debris during daytime hours
Larvae have black heads	Larvae have brown heads
Adults leave fields in June	Adults may remain in fields

Weevil Management

Weevil populations can be reduced by several methods.  At this time of year, economic infestations are usually dealt with by insecticide use or early harvest, depending on the height of the alfalfa.  Other non-chemical methods of control that have been used for alfalfa weevil management include flaming, flooding, crushing of eggs and larvae, disking or harrowing, seeding dates, and the use of resistant varieties.  The easiest way to begin weevil management is to select a resistant or tolerant variety that will yield well in your area.  Grazing of alfalfa in the fall and winter is becoming more popular and will reduce overwintering egg populations, particularly in southern areas where a significant number of eggs may overwinter as well as adults.  In a recent Oklahoma study, cattle reduced the density of weevil eggs by over 60 percent. Grazed alfalfa may not be as attractive to egg laying females in the spring although more research needs to be done to determine the factors involved.

It is essential that fields be monitored for alfalfa weevil feeding now.  Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips.  The larvae are a small (1/16 to 3/8 inch in length), pale yellowish green, becoming a darker green when larger.  These legless worms have black heads and a white stripe the length of the back.  The alfalfa weevil larvae spend nearly all their time on the plant.  They curl into a C-shape when disturbed.

Once the alfalfa is about 4-6 inches or so in height, take a bucket, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of the bucket.  Average the number of weevil larvae per stem.  Use the following charts to aid you in making a decision on whether to control alfalfa weevils.  Each chart has been developed for a different alfalfa value.  To treat or re-sample depends on the average number of weevils per stem, the stem length, and the value of the alfalfa.  When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than to treat.  Insecticides registered to control alfalfa weevil larvae include Ambush, Baythroid, Cythion, Furadan, Guthion, Imidan, Lannate, Lorsban, Mustang, Penncap M, Pounce, Sevin, and Warrior. See the Entomology Website at http://entomology.unl.edu/instabls/instabls.htm to check for use rates. (KJ)

Herbicide tolerant crop: Benefits, Concerns and Risks (part 1)

Herbicide tolerant crops (HTC) represent relatively new weed control technology. Since  introduction,  less than a decade ago, their use has been steadily growing.  Examples of HTC include soybean, corn and canola tolerant to glyphosate and glufosinate.  Growers have readily integrated HTC into their crop production practices.  For example, currently more than 60% of 25 million hectares of soybeans grown in the U.S. annually are glyphosate tolerant cultivars. In some regions as much as 90% of soybeans are glyphosate tolerant varieties. Even though the use of herbicide tolerant crops  may have advantages over regular herbicide programs, there are risks associated with their use. Therefore the objective of this article is to provided a short overview of benefits, risks and concerns with widespread and repeated use of HTC.   

HTC can be produced by either the insertion of a foreign gene or by regenerating herbicide tolerant mutants. The first ones are also commonly known as genetically modified organisms or GMOs, while the second ones are referred to as the non-GMOs.  Examples of GMO crops include canola, soybeans and corn tolerant to glyphosate and glufosinate herbicides.  Examples of non-GMO crops include STS-soybeans, Clearfield corn and Clearfield wheat. HTC is a common weed control tool in the North American cropping systems and their usage is steadily growing, especially in soybean crops.  It is estimated that in the year 2001 more than 80% of soybeans planted in the United States were glyphosate tolerant varieties, compared to 70%, 54%, and 41% in 2000, 1999 and 1998, respectively. Similar increase in use was observed in canola and cotton. About 26% of cotton grown in 1998 was glyphosate tolerant, with an increase to 35%, 46% and 57% for 1999, 2000 and 2001, respectively. However, there is a much slower trend on the use of herbicide tolerant corn (eg. Roundup-Ready, Liberty-Link and Clearfield) than for soybean or cotton.  It is estimated that only about 7% of corn hybrids planted in 1998 were  herbicide tolerant hybrids compared to 8%, 12% and 15% in 1999, 2000 and 2001.  Overall, the most common HTC in the United States is soybean tolerant to glyphosate. Since the HTC are a common part of our cropping system, it brings the question, AWhat is the next HTC?@. 

Development of new HTC is the goal of  industry=s research programs. For example,  speculations are that glyphosate tolerant spring wheat will be available in 2004 and 2005 for Canadian and US market, respectively. The Clearfield winter wheat, which is tolerant primarily to imazamox herbicide, is likely to be released for South Central US in 2002 or 2003.   Glyphosate tolerant alfalfa is currently being evaluated in the field variety testing trials, indicating potential for release within a few years, or sooner.

The trend is also growing towards the use of several genes in a single hybrid or variety, as commonly referred as Astacked genes or stacked traits@. There are also corn and cotton hybrids containing two genes, (eg. Bt/glyphosate, or Bt/Liberty). As well as corn hybrids with three genes (Bt, Liberty and Clearfield).  In contrary, there are also several types of HTC that may likely be withdrawn from the market due to various reasons. Speculations are that these HTC may include: Liberty-Link corn (Star-Link), STS-soybean, Liberty-Link soybean, SR-Corn, as well as a High-Oil corn.  

Benefits associated with the use of HTC. Considering the fact that some US states have as much as 90% of soybean fields planted to glyphosate tolerant varieties, there must be benefits that the producers see from this technology. The list of most common benefits to the producers may include: (1) broadening the spectrum of weeds controlled, (2) increased margin of crop safety, (3) less herbicide carry over, (4) price reduction for "conventional herbicides",  (5) use of herbicides that are more environmentally friendly, (6) new mode of action for resistance management and (7) crop management flexibility and simplicity. 

Non-selective herbicides such as glyphosate and glufosinate aid in broadening the spectrum of weeds controlled. The systemic activity of glyphosate also helps with the control of perennial weeds and their perennial vegetative structures such as stolons and rhizomes.

Crop safety in general is also improved with the use of HTC. Both glyphosate and glufosinate provide almost no crop injury, compared to some of traditional herbicides (eg. lactofen, clorimuron), especially in soybean crop.

Both compounds also have almost no soil residual activity, because they are tightly bound to the organic particles in the soil. This provides no restrictions for planting or replanting intervals nor injuries to the subsequent crops.

Introduction of HTC also resulted in a price reduction for conventional herbicides.  For example, just few years ago the cost of weed control in soybeans ranged from $40-60 per acre compared to the current $20-30. The price reduction is the result of the market adjustment and an attempt for companies to remain competitive with their herbicides.

Glyphosate and glufosinate also provide a new mode of action that can aid in resistance management. A single or multiple weed resistance is a serious problem in certain parts of the US and Canada, thus the use of HTC can help with this issue.  

The technology associated with HTC is simple to use. It does not require special skills  nor training.  The technology does not have major restrictions and it is flexible, which is probably one of the reasons for such wide adoption by producers.

Finally the companies that own this technology have benefited financially though the sales of their herbicides and seeds.  Also, the companies save funds by breeding HTC, which is much cheaper than developing new herbicides.                                 

Concerns about the widespread use of HTC:  Major concerns with widespread use of HTC includes: (1) A shift in the philosophy of breeding programs (eg. breeding for herbicide resistance versus yields), (2) performance and quality of yields, (3) cost of planting HTC seeds  (4) farming contracts, and (5) privacy of farmland.

With the introduction of HTC it seems that there is a shift in basic philosophy of many breeding programs. It looks like that the Adriving force@ is not the increased crop yield but the addition of specialty traits.  In general, the traditional objective of a breeding program was to breed for higher yielding hybrids or varieties. However, most of the current breeding programs are actually Abio-tech driven@ programs that produce new HTC, both the GMO and non-GMO types.  The point is that the addition of these genes does not enhance yields. This raises simple questions:  AWho breeds for higher yielding varieties?@ and AAre the current corn/soybean yields at its maximum?@  There was an overall yield increase of 1-3% per year from 1960-1990 in dryland corn and soybean.  However, there was almost no yield increase in the past decade.  It is likely the result of a shift in breeding philosophy, not breeding for higher yields but for specialty traits.

Performance and quality of crop yield is also of concern. In fact this resulted in new terms such as Ayield drag@ and Ayield lag@. Yield drag is a yield reduction due to addition of foreign genes. Yield lag is the potential yield depression due to the age of the variety in which the gene is inserted.  Recent University of Nebraska study concluded that soybean varieties with glyphosate-tolerant gene yielded 5 % less that the sister lines without the foreign gene indicating the yield drag. In the same study the glyphosate tolerant varieties yielded 10% less than the high yielding non-HTC indicating the yield lag.  Public scientists, in most cases, do not have access to the private breeding programs, which is needed in order to conduct independent studies to determine if there is a yield lag or yield drag. This also raises a major public concern and the issue of trust (eg. can we trust large corporations).

There is also a higher cost for seeds of HTC compared to conventional hybrids, which raises the cost of crop production. Also, in order to plant HTC in US and Canada producers must sign contracts, which interferes with the privacy of the farm and it is against the principles of freedom to farm. 

The 2^nd part of this article that talks about potential risks (ecological and economic) associated with the widespread use of HTC will be printed in one of the following issues. (SK)