Spring Bean Leaf Beetles
Bean leaf beetles have not been a big problem the last couple of years, but we usually have at least some areas each year that have problems with the bean leaf beetle, and early planted soybean fields always attract some beetles. Because the bean leaf beetle can be a pest of seedling soybean, we'll discuss it in some detail.
Bean leaf beetles have two generations a year in Nebraska . However, since they overwinter as adults, three periods of beetle activity are seen in the growing season: Overwintering colonizers, F1 generation (offspring of the colonizers, the true first generation) and the F2 generation.
Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May) and often can be found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to the seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about forty days and lay from 125 to 250 eggs. After egg laying is complete the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid August and feed on leaf and pod tissues. The pod-feeding F2 beetles are most likely to cause economic damage.
Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about ¼ inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax.
Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury. This has become more of a problem in recent years because planting dates seem to be getting earlier each year. Although the defoliation can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, unless insect populations are large enough to cause more than 50% to 60% defoliation of seedling soybeans, it is unlikely that treatment would be economically
justified. Tables 1 and 2 show economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC - V1. If beetles enter the field right at or during seedling emergence, the thresholds will likely be lower because the beetles do not have leaf tissue to eat and will feed on the growing point, stem, and cotyledons. We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but the thresholds are probably about 1.5 beetles lower than the VC thresholds.
Remember that early-planted soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control, including Asana, Baythroid, Dimethoate, Lorsban, Lannate, Mustang Max, Penncap-M, Pounce, Sevin and Warrior. All will do an adequate job if applied according to label directions.
Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod-damaging F2 generation that emerges in August; however, UNL Extension does not recommend this practice. There are many environmental factors that can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August. Regular scouting and the use of the appropriate economic thresholds are the best way to manage late season bean leaf beetle in soybean. Late-season economic thresholds will be included later this summer.
For similar reasons, we do not at this time recommend treating soybean seed with Gaucho or Cruiser to control bean leaf beetles to prevent defoliation or the spread of bean pod mottle virus unless damage from the virus has been confirmed. Although the beetle is a known vector of the virus, and yield declines have been documented in other states, the relationship in Nebraska is not as well known. Yield trial results from the use of Gaucho and Cruiser in Nebraska have been inconsistent and do not justify the added expense in most fields at this time. (KJ)
Table 1. Early Season Bean Leaf Beetle Economic Thresholds (Beetles per plant VC).
|
VC Soybeans
Management costs |
Crop Value |
$6 |
$8 |
$10 |
$12 |
$5 |
3 |
4 |
4 |
6 |
$6 |
2 |
3 |
4 |
5 |
$7 |
2 |
3 |
3 |
5 |
$8 |
2 |
2 |
3 |
4 |
$9 |
2 |
2 |
3 |
3 |
$10 |
1 |
2 |
2 |
3 |
Table 2. Early Season Bean Leaf Beetle Economic Thresholds (Beetles per plant V1).
|
V1 Soybeans
Management costs |
Crop Value |
$6 |
$8 |
$10 |
$12 |
$5 |
4 |
5 |
7 |
8 |
$6 |
3 |
4 |
6 |
7 |
$7 |
3 |
4 |
5 |
6 |
$8 |
3 |
3 |
4 |
5 |
$9 |
2 |
3 |
4 |
4 |
$10 |
2 |
3 |
3 |
4 |
Spring System Checkup
Performing regular maintenance of your irrigation equipment includes a pre-season checkup. Component wear results in less uniform water application, and increased energy use. In order to reduce the risk that wear and tear will result in untimely breakdowns, worn components should be identified and replaced now.
One of the best ways to identify worn components such as sprinklers, pumps or irrigation systems is to keep good records. Recording the outlet pressure, flow rate and energy use on a regular basis (at least once per month) provide an excellent means of evaluating pump and motor performance. This information should be recorded when the pivot is in the same position each time since the pump produces different outlet pressure when the system is facing uphill than when the system is facing downhill.
Each irrigation system will have a number of areas to lubricate, or parts that may need to be replaced prior to the first irrigation. It is impossible to list them all, but the list should include:
1) Change the engine oil and filter,
2) Replace the air and fuel filters,
3) Grease drive shafts on pump, and motor,
4) Check spark plugs on gas, propane or natural gas motors,
5) Drain, flush and refill the cooling system,
6) Refill the drip oil reservoir and allow about a gallon of oil to drain into the drip line,
7) Insure that the gear drive is free moving and clean and lubricate non-reverse pins,
8) Run the motor at 1000 rpm for 45 minutes,
9) Walk along system looking for malfunctioning sprinklers,
10) Check chemigation pump and safety equipment operation. Check hoses and replace if cracking is noticeable.
Each system is equipped with a number of safety switches to shut the system down in the case of failure. Now is the best time to insure that all these controls function properly. Run the system through a set of conditions that would cause each of the system safety controls to function.
A walk by inspection of the system can identify sprinklers/nozzles that are not operating properly. Be sure to replace malfunctioning nozzles with ones listed on the computer printout you received when the system was installed. Nozzle wear depends on the quality of the water and the system operating pressure. As a rule of thumb, sprinkler replacement should be considered after approximately 10-12,000 hours of operation. (BK)
Western Ragweed Control in Pasture
Western ragweed (Ambrosia psilostachya) is a commonly found native weed in northeastern Nebraska 's rangeland, prairies and disturbed sites in all soil types. It is a perennial forb from the sunflower family (Asteraceae) that reproduces both by seeds and rhizome. The rhizome is a horizontal creeping root system growing within the top 5-10 inches of the soil surface. The plants usually grow in sparse groups (patches or clusters). The stem is very erect, up to 3 ft tall, with many branches and long hairs giving the stem a coarse feeling. The leaves are alternate on the upper part of the stem, opposite on the bottom, with many divisions and teeth. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. Western ragweed, in Nebraska , can flower from July to October, with greenish-yellow flowers positioned on the top of the main stem and branches, and produces an inch long bur-like fruits with a single seed within each bur.
Western ragweed provides forage for deer and the fruits are an important food source for upland game-birds, wild turkeys and songbirds. Native Americans also made a tea from the whole plants to treat colds and cramps. Western ragweed has almost no value to livestock because of its low palatability. With other forage limited, it may be eaten. Pollen produced in late summer causes late summer hay fever in many people, due to presence of volatile oils, which can also cause skin irritation.
Due to its low value for livestock forage, it is a concern to livestock producers and ranchers. This weed can be controlled by various means. Mowing the plants when they are 4-6 inch tall can reduce the ragweed population considerably for the season. Mowing can be done one or two times per season depending on the amount of rainfall during the season. One mowing done in mid June is effective if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August. Herbicides can be also very effective in providing season long control. Herbicide application should be conducted when ragweed plants are 3-5 inches tall. The list of effective herbicides and their rates per acre includes: Salvo (12 oz/acre), 2,4-D-Ester (1 qrt/acre), Grazon P+D (32 oz/acre), Weedmaster (32 oz/acre), Ally (0.25 oz/acre), and Vista (22 oz/acre). (SK)
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