Insect Update for Northeast Nebraska

Grasshoppers: Hatching is underway with a lot of mortality having occurred due to the strong rains of the last couple of weeks. However, hatch is still taking place and there is still some potential for grasshopper problems, mostly west of Highway 81. Start scouting pastures and field margins and prepare to treat in the next couple of weeks if necessary.

Alfalfa weevil: Rain is a wonderful thing. Not only does it benefit crop growth, but also the higher humidity promotes disease, and the physical pounding it gives small insects helps in controlling them. After visiting several alfalfa fields in Holt and Knox counties that had been visibly fed on by weevils, it was thought that some would survive to hold back the regrowth of the second cutting. It still may happen in some fields, but it looks as if many weevils were also killed due to effects caused by the rainfall and/or harvesting. Still, if alfalfa fails to green up rapidly after the hay is removed, scout around the crowns of the plants to look for larvae and adults of the weevils.

Purple Loosestrife - Biology And Control (PART I)

Purple loosestrife ( Lythrum salicoria ), was introduced to North America from Europe in the 1800s. Since then, it has made a slow invasion of wetlands and waterways across the Mid-West including Nebraska. It is estimated that about 12,000 acres of Nebraska's wetlands are infested with this plant, mostly along the main rivers. Therefore, there is a need to find a way to stop the spread of this highly competitive weed.

Lost Wildlife: When wetlands are taken over by loosestrife, the natural habitat is lost and the productivity of native plant and animal communities is severely reduced. Song birds will not feed on the loosestrife seeds. Muskrats cannot use roots for food or shelter. Waterfowl are affected when dense impenetrable stands of loosestrife eliminates nesting sites and open water. Nearby water bodies are also likely to be affected by this or other types of habitat destruction. There is no room for displaced wildlife. The local fish and wildlife population that cannot move is lost forever.

Lost Water Ways : By growing vigorously in irrigation canals, ditches, stream banks and reservoirs, purple loosestrife will clog them. The result will be less water carried to the cropland and further negative effects on agricultural production.

Loss Of Recreational Land: The loss of wetlands and wildlife directly influences other activities of everyday life, especially summer recreation. The funds spent on controlling this weed could be more effectively spent on improving wildlife habitats, boat ramps, camping grounds, etc. With the loss of recreational land, local communities will also lose revenues from tourism.

A Perfect Plant: Purple loosestrife can colonize and thrive easily because it is a prolific seed producer and has a strong perennial root system (rhizome). Each plant can produce up to 2 million seeds in one season. Seeds can be carried far away by water, wind, some birds, and can remain viable for many years. The rhizome grows well in marshy soils, and can help the spread of the species if washed away by the river water.

There are not many birds, fish or animals that like to feed on purple loosestrife. They feed, however, on other plant species that grow around purple loosestrife. By doing this, indirectly, the wildlife population “eats themselves out of house and home”. As native vegetation gets consumed, more space is created for purple loosestrife to spread and produce new plants.

In general, purple loosestrife can grow 3-9 feet tall with several square stalks per plant. Leaves are on opposite sides of stalk, thin and sharply pointed with the base rounded or heart shaped. In Nebraska, it will flower from July to September. Flowers can range from red to rose-purple in color, an easy identifiable feature. The flowers are arranged on 1 to 3 feet long spikes. The fruit is a small oblong capsule with two valves containing many small seeds. There are many capsules within a spike. Also there are at least several spikes per plant and each spike can produce up to one hundred thousand small seeds. The tiny, light seeds are readily moved by wind. Seeds are also extremely viable and will easily germinate when exposed on bare soil. The root system is very strong and when mature, the root branches become thick and woody.

Can We Control It? Purple loosestrife has no natural enemies nor other plant competitors here in North America, therefore its spreading is hard to stop. A single control measure can't provide long term, sustainable, management of this weed. However, if the control practices are integrated in a systematic manner, significant advances can be achieved. The biggest challenge is how to stop the spread of the current 12,000 acres of infested wetland across Nebraska. Methods of control must be based on an integrated management approach, which includes:

1. Prevention and Education: Educating the public should be the major part in preventing the spread of loosestrife. There are many people who are not even aware of this weed. For example, several duck-hunters reported recently that last Fall they used “some plants with purple flowers” to build their duck-blinds and to camouflage their boat. Of course, the boat was used to travel up and down the river. Obviously these duck hunters did not know that the plant was purple loosestrife and that with such an action they actually helped spread the weed. Just to remind the reader, one plant can produce 1 - 2 million seeds. The above was just one example of the need for public education.

2. Manual control : Pulling and digging plants can be very effective for small areas. Pulling is most effective on plants that are 1-2 years old. Loosestrife spreads vegetatively from stems, therefore, regeneration from discarded plants is possible. They should be dried and burned.

If plant pulling is not feasible, flower head removal helps reduce the spread of the seeds. Simply cut the heads in July and August (before the flower sets seeds). Seed formation starts at the bottom of the flower and progresses to the tip. Before cutting the seed head off, check to see that no ripe seeds are present.

3. Cutting: Cutting can actually spread loosestrife if the cuts are not removed because the cut stalk portions can sprout. Therefore all cuts must be removed and burned. Make sure that all plant parts are in a carton or protected site so that they can dry completely without danger of being spread by wind, water, human or animal activity.

Timing of Post-Emergent Weed Control in Soybean

With the advances of herbicide tolerant soybean (eg. Roundup-Ready) there is still a constant dilemma on how to “ time ” post-emergence weed control. To decide whether or not weed control is economically worthwhile, there is a need for understanding if a given weed infestation is likely to reduce yield if left uncontrolled. This establishes the rationale for introduction of the concept of critical period of weed control (CPWC) . The CPWC is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields . This information is essential in making decisions on the need for and timing of weed control and in achieving an efficient use of herbicides.

Research at the University of Nebraska has shown that each crop has a CPWC during which weeds must be controlled to maintain maximum yields. However, we also concluded that the length of such critical period is influenced by the cropping practices, for example row spacing in soybean.

Time of weed removal as affected by soybean row spacing: Studies were conducted in 1999 at Mead, 2000 and 2001 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 70-100 plants per square yard.

The critical time of weed removal was significantly influenced by row spacing. Generally, an increase in row spacing resulted in a need for earlier weed removal, thus a less competitive crop. For example, beginning of the CPWC in the wide-row soybean (30") was approximately at the 1 st trifoliate stage (V1 – Fully developed leaves at unifoliate nodes. A node with a fully developed leaf is one that has a leaflet above it whose edges are not touching) based on a 5% acceptable yield loss level (Table 1). This suggests that in the wide-row soybeans the control measure should start early in the season (at the 1 st trifoliate stage). Beginning of the CPWC in the 15" rows was delayed and corresponded approximately to the 2 nd trifoliate stage, compared to the 3 rd trifoliate stage in soybean grown in the 7.5" rows (Table 1).

Table 1: The beginning of CPWC in soybean based on 5% yield loss expressed as crop leaf stage (eg.V1) and days after crop emergence (DAE) as affected by the row spacing, at two locations in 1999, 2000 and 2001.

Row spacing	Time to control weeds	Time to control weeds
inches	Soybean leaf stage	Days after crop emergence
7.5	V3	19
15	V2	15
30	V1	9

This data implies that reducing row spacing delayed the timing of weed control and increased the tolerance of soybean to weed presence. The mechanism of soybean tolerance needs to be determined yet, although we believe it is related to crop shading effects. Furthermore, from a practical standpoint, these results indicate that a reduction in soybean row spacing increases soybean tolerance to weeds and it may require less intensive weed management programs (eg. weed control measure applied perhaps once or twice) .

Cost of delaying weed control : The commonly asked question among producers is “how much is it going to cost me if I delay weed control”. Possible reasons for delayed weed control may include weather constraints (rainfall, wind), and time constraints due to large acreage to spray. In order to answer the above question the yield loss data from the above studies were pooled among years-locations and graphed against the crop growth stage at the time of weed removal in corn and soybean (Figure 1).

The 2 percent yield loss per every leaf stage of delay past the critical stage of weed control was determined as the cost of delaying weed control in soybean. For example, the time to control weeds in 7.5 inch rows soybean is the V3 stage (third trifoliate-Table 1), if weed control is delayed to the V4 (fourth trifoliate), it will cost a producer about 2 percent in yield losses due to prolonged competition from weeds. The same is true if weed control is delayed past the recommended critical time in other soybean row spacings (Table 1). This recommendation is applicable up to the R3 stage in soybean (beginning pod). If the weed control is delayed further than these indicated stages the yield losses will be much higher than suggested.

In terms of actual economic losses in soybean, it will be about $5 per acre for every soybean leaf stage of delay, assuming a price of $5 bushel and a yield goal of 40 bushels.

Weed Size: Weed size at the time of weed control measure is another concern. If the weeds emerge 4-5 days before the crop or they are taller than crop they will shade the crop so the control should be initiated 4-5 days (1-2 leaves) prior to the beginning of CPWC. If the weeds emerge 5-10 days after the crop they will not shade the crop that early in the season so the control can be initiated 5-10 days (2-3 leaves) after the beginning of the critical period.

The size of weed species will effect the herbicide use rates too, especially the rates of Roundup or any generic glyphosate in Roundup-Ready soybeans. It is well known that Roundup has much better activity on grassy than broad leaf species. Therefore the rates of 16 to 24 oz should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, panicums) that are 3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambsquarters, pigweeds, mustards) that are less than 6 inches tall. For taller grasses and broadleaf species a full rate (32 oz) will be required. Higher rates of Roundup (40 oz - 60 oz) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweed species (lady's thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical use of the CPWC and timing of weed control in herbicide tolerant crops : Roundup-Ready soybeans have received high levels of acceptance in our state. The concept of critical period of weed control is an important part of integrated weed management in answering the fundamental questions IF and WHEN to apply post-emergence herbicide.

A generally sound strategy in Roundup-Ready soybeans will be to apply Roundup tank-mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control for the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum at your farm, we suggest to consult the herbicide efficacy tables from the “Guide for Weed Management in Nebraska (Extension Publication, EC-130).”

Figure 1: Soybean yield loss and beginning of CPWC as influenced by the timing of weed removal and row spacing.