ndsucpr_L_sm.jpg (11690 bytes)

ent_Logo_Lg.jpg (12173 bytes)


ISSUE 16  August 27, 1998

 

MONITOR GRASSHOPPERS FOR CLUES ABOUT NEXT YEAR

    Many areas of the state continue to deal with grasshopper problems. In some cases field crops are now being monitored as migrating grasshoppers move into these sites. Some farmers are beginning to think about next year and the future outlook for grasshopper problems. The following is a review of management information addressing these concerns.

Grasshopper Egg Laying

    The adult grasshoppers are now moving around, seeking out green vegetation for feeding. The females will also be laying their eggs in the soil. The areas where they feed, are also the sites where they will deposit eggs. The grasshoppers frequently insert the egg pods at the base of plants, among the roots. Egg pods are only one to two inches deep in the soil. To begin planning for effective grasshopper management programs for next year, note those areas where grasshoppers are congregating and laying their eggs. These sites should be monitored early next season to detect when emergence begins and the size of the population. Controlling young grasshoppers in these emergence sites is the most economical way to deal with outbreaks.

Cultural Control Methods

    Cultural control is a preventative approach to insect management. The principal cultural methods for grasshoppers include tillage, crop rotation, early seeding, and trap strips.

   Tillage - Tillage controls grasshoppers by eliminating the green plants on which grasshoppers feed. It is of little value to use tillage with the objective of destroying egg masses in the soil, and is not recommended. Excessive tillage may even be harmful due to increased risk of erosion and moisture loss. Fall tillage to rid weeds from summer fallow during late summer will discourage grasshoppers from depositing eggs in these sites. Preventing weeds and volunteers from growing in harvested small grain fields helps discourage egg deposition in these sites. Spring tillage should be completed early before the grasshoppers hatch to eliminate all green growth on stubble fields to be summer fallowed. If no food is available when they hatch, the young hoppers will starve.

   Crop Rotation - When possible, avoid seeding cereals on stubble fields where infestations are large. Plant them in these sites only when soil moisture is adequate, and one or more applications of an insecticide over the entire field is economical.

    Early Seeding - Early seeded crops that are growing vigorously can withstand more grasshopper feeding than younger plants. It may not prevent damage but allows some additional time for making management decisions.

    Trap Strips - If grasshoppers have already begun emerging next spring when tillage operations begin, eliminate all vegetation except for strips used to concentrate the hoppers into a small area for control. Beginning on the outer edge, create a series of strips alternating bare ground with vegetation. The areas with plants will attract and concentrate the grasshoppers into these sites for control after several days.

    Finally, next year it will be necessary to monitor the early hatching of grasshoppers and determine if a threatening population level is present. Controlling small grasshoppers in their hatching sites is more economical than attempting to control larger grasshoppers that have dispersed throughout the field.

Phillip Glogoza
Extension Entomologist

 

LYGUS BUG FEEDS ON SUGAR BEETS

    The lygus bug, Lygus lineolaris, is also known as the "tarnished plant bug" and has decided to start feeding on beets on the Minnesota side of the valley from north of Ada to south of Moorhead. As of today (8/19/98) it has been reported near Casselton, North Dakota in beets as well. Feeding injury is occurring mostly on the new leaves coming out of the crown where numerous nymphs can be found, along with some adults. The concerns are 1) How much will the feeding damage effect yield and 2) Is the disease that is showing up from the wounding going to cause any problems? The following are some facts and figures that may help in the management of this insect.

    Description: Adults are about 1/4 inch in length and half as wide. They turn from green to yellow to brown as they mature. Older adults are brown with a distinctive mottled coloration of lighter colors. First stage nymphs are only about 1/25 of an inch and green in color. There are five nymphal stages (all green in color) to adulthood, with each getting larger in size up to mature adults. These insects are from the Hemipteran family and have piercing-sucking mouth parts. Adults and nymphs are very active and will hide as soon as you look under the beet canopy. Nymphs stay around the crown, while adults will venture above crown

wpe18.jpg (7568 bytes)wpe19.jpg (9750 bytes)

Tarnished Plant Bug Nymph    Tarnished Plant Bug Adult

    The photo in the "Pests Diseases and Disorders of Sugar Beets" is close but not the same species that we are currently having problems with. The photo in the "Compendium of Beet Diseases and Insects" is a lygus adult, and probably the same species we have but it is immature and still green in color.

   Life History: There are three to five generations possible in one year. Adults overwinter in plant residue, under tree bark, essentially any protected place. Overwintered adults mate in early spring to start the first generation. Females will deposit their eggs in plant tissue. Those eggs hatch and the entire process for the next generation begins.

    Host Plants: There are over 50 cultivated plants listed as hosts. If weed species were included, the host list would probably exceed 400 species. Sugar beets are listed as a host.

   Why The Migration In To Beets? : Lygus have not been considered an economic problem on production beets processed for sugar in North Dakota or Minnesota. They do have a history of migrating from one crop to another, especially when it is dry. The most noted migration occurs when alfalfa hay is cut. Alfalfa hay has taken much of the blame but I believe that since the lygus has such an extended host range, alfalfa is getting too much of the blame. In many instances, roadside ditches with grass, weeds and clovers are probably just as guilty. Bean fields are drying up, and I have been in fields where densities of lygus are heavier the closer you get to the bean field.

    Feeding Injury: Lygus are most noted for being a pest of beets grown for seed production. They are actually a problem on seed production of many crops. Their preference for food is meristematic tissue, embryonic tissue or new growth of any kind. New leaves coming out of the beet crown is the most tender feeding site a lygus could find at this point. The hot dry weather has been pretty hard on a lot of different crops. Lygus stick their mouthparts into the host and start a "pre-digestion pump" where they inject with saliva to start digestion, and then suck the sap into the stomach. This is where the injury comes in. The saliva is toxic to plant tissue and helps break the plant sap into a digestible source. The only other unanswered question would be "why so many"? Obviously they overwintered and got a good start with the early, warm spring. Our best hope is that we are at the tail end of the 5th generation. With the recent rain in some areas, some "other plants" might green up which would help relieve some of the pressure. The lygus have already gone threw two maybe three generations, the question is, how many more will they complete in beets.

    Disease Association: There are two concerns with disease. A dark brown fungus is showing up and is directly associated with the feeding wounds and ooze that is coming from the feeding sites. It is thought at this time that the fungus is saprophytic, only growing as a result of the oozing and not a serious pathogen. Samples have been sent in to see if the fungus can be isolated. There is also some concern that lygus maybe transmitting beet western yellows, a viral disease. Lygus should not be transmitting this disease, however older leaves that were pierced earlier by lygus show similar symptoms of western beet yellows.

    Control: Lygus are not hard to kill according to efficacy data from other crops. The important thing to remember is that good coverage is probably as important as rate. The higher the gallonage the better considering the canopy development. Pay attention to mixing fungicides and insecticides, if in doubt call your field-person. Scout fields for infested plants by randomly selecting a plant, quickly open the canopy with your hands and see if you find one or more lygus adult or nymph. If you see one or more, count it as infested. Counting actual numbers of lygus per plant is impossible. If on average one third of the crowns (new leaves) of 30 to 50 plants are infested with one or more lygus, treatments maybe considered justifiable. This is the most current recommendation we have at this time. One half pint of Lorsban per acre is currently recommended if treatments are warranted. A low rate of Asana (0.03 lb/ai acre) should be sufficient. Carbaryl (Sevin) may also be used at a 1 lb/ai/acre rate. Honey bees are very susceptible to carbaryl. Don’t treat for past injury, treat for current insect infestations. This means walking across a field to check plants and not just the edges.

The other very important thing to remember is the pre-harvest interval for insecticides. Lorsban is 30 days, Asana is 21 days, Carbaryl 28 days.

Scott Armstrong, Entomologist
Department of Entomology

 

RESULTS OF LARVAL WHEAT MIDGE SURVEY 1998

    Wheat fields were surveyed for larvae of the wheat midge during the first week of August 1998. Ten wheat heads, including primary and secondary tillers, were randomly collected from each field. A total of 38 wheat fields were sampled in Bottineau, Renville, McHenry and Ward Counties. Wheat heads were then dissected for wheat midge larvae and counted. The total number of kernels per head was also counted to calculate the percent of kernels infested. This survey was conducted to validate the degree day (DD) guide that is used for planting dates and risk assessment of wheat midge infestation. The guideline is as follows:

* HRSW planted prior to accumulating 200 DD will head before wheat midge emerges.

* HRSW planted from 200 to 600 DD will heading at the time wheat midge are emerging.

* HRSW planted AFTER 600 DD will head after peak emergence and should be at low risk to midge infestation. However, there’s a higher risk of frost damage.

    Results of larval wheat midge counts indicate that the planting dates were more important than the previous crop grown. Overall, wheat planted late - late May to early June, after the 600 DD, had the lowest midge counts; followed by the fields planted before 200 DD - late April, and then by the fields planted between 200-600 DD - early to mid-May. The wide range of counts in the early planting group indicates that wheat midge can infest early planted wheat under the right conditions. However, the majority of the fields planted in the 200-600 DD period had the highest risk of midge infestation. Fields planted late had consistently lower levels of midge infestation. Both the early and late plantings had no fields above the larval economic threshold of an average of 13 larvae per head. However, three out of the 27 fields sampled (11%) in the high risk period (200-600 DD) were above threshold.

Summary of Midge Damage by Planting Date and DD Units

Planting Dates

Degree Day Sum

Fields Sampled

Fields Above Economic Threshold

Avg # Midge per Head
(Range)

Avg
% Infested Kernels/ Head
(Range)

April 24

<200

2

0

5.05
(1.3-8.8)

15%
(4%-25%)

May 3 to May 23 200 to 600

27

3

7.78
(7.2-16.4)

23%
(5%-47%)

May 25 to June 2

>600

9

0

2.02
(0.3-4.3)

5.84%
(1%-12%)

No major differences were observed between fields with different previous crops. The previous crops were barley, canola, crambe, mustard, durum, HRSW, fallow, oats, pea, and sunflower. Fields that were previously planted to wheat had higher midge counts in general. Continuous wheat cropping is known to favor buildup of midge populations. Although pea had the highest midge counts, only one field was sampled making it difficult to compare with the other crops. The following list ranks the crops (except pea) from the highest to lowest midge counts: durum or HRSW; fallow; sunflower; canola, crambe or mustard; oats; and barley.

Summary of Midge Damage by Previous Crop Planted

Previous Crop

# of Fields Sampled

Avg # of Midge
per Head
(Range)

Avg % Kernels Infested
per Head
(Range)

Barley

6

3.4
(1.3-5.6)

10%
(4%-16%)

Canola, crambe, or mustard

10

5.38
(1.1-11.1)

16%
(3%-32%)

Durum or HRSW

7

9.47
(3.6-18.3)

27%
(10%-53%)

Fallow

3

7.57
(4.9-11.9)

22%
(14%-34%)

Oats

5

4.5
(0.3-9.4)

13%
(0.87%-27%)

Sunflower

6

6.98
(0.3-16.4)

20%
(1%-47%)

Pea

1

10.7

31%

    In summary, the timing of the crop development to midge emergence is important in predicting the level of infestation. Crops planted between the 200-600 DD period were at greater risk to wheat midge infestation than crops planted early (before 200 DD) or late (after 600 DD). The previous crop grown did not have as great an affect on the level of infestation, as might be expected, due to the flight movements of adult midge. Additional surveys will help us understand how to better manage the wheat midge, and what other factors may affect local wheat midge populations.


cprhome.jpg (3929 bytes)topofpage.jpg (3455 bytes)tableofcontents.jpg (4563 bytes)previous.jpg (2814 bytes)next.jpg (1962 bytes)