To Scout For Alfalfa Weevil

On April 22, 2019

To Scout For Alfalfa Weevil

Alfalfa weevil will be active across Pennsylvania in the coming weeks, particularly on warmer, south-facing slopes, and the recent bouts of warmer weather undoubtedly moved them along. Extension Entomologist John Tooker tells us young alfalfa weevil larvae cause pin hole-sized damage to leaves near the tips of plants often on unfolded leaves. Much of this early season feeding does not result in economic loss, and indeed the great majority of alfalfa fields in Pennsylvania do not require insecticides to control alfalfa weevil. In many fields, introduced parasitoid wasps control populations of alfalfa weevil pretty effectively.

Nevertheless, scouting fields will help you to determine if weevils have colonized fields and if damage is occurring. Economic thresholds for alfalfa weevil are determined from the size of plants, the value of the hay, the cost of insecticidal treatment, and the number of larvae per 30 stems of alfalfa. To sample weevil larvae, systematically select 30 stems from across a field and shake them into a bucket. If the number of larvae exceeds the threshold, a treatment is likely warranted.  See our fact sheet for thresholds and more details on alfalfa weevil biology ( Many insecticides are available for controlling alfalfa weevil, but remember that an economic return on insecticide use is unlikely unless populations exceed economic thresholds, so do not spray without scouting!

To Manage Soil Fertility In Grass Forages

Soil fertility management is an ongoing process through the life of the forage stand. Extension Agronomist Casey Guindon explains prior to, as well as during the establishment period, there is the most flexibility in the addition of fertility inputs for a healthy crop. It is during this period that it is critical to draw soil pH, phosphorus, and potassium levels into ideal ranges. In subsequent years, soil testing and fertility maintenance can help to maintain optimum yields.

The soil pH can have an enormous impact on potential crop yield. Low pH soils can bind nutrients and make it more difficult for plants to take them up. At extremely low levels, plant root growth can be significantly inhibited. Soil testing is the best method to use to determine the need for amendments to improve soil pH. If it has been more than 3 years since you pulled a soil test for the field, it is in your best interest to take another this spring. In Pennsylvania, many climatic factors lead to the natural acidification of our soils. The addition of some nitrogen fertilizers, acid rain, and even crop nutrient uptake can all contribute to soil acidity. It is imperative to act as soon as possible to remediate soil pH problems in the field. If your soil pH is below 6, you will reap the benefits of applying limestone to improve your soil pH.

Phosphorus recommendations are also based on a soil test. Again, if it has been longer than 3 years since your last soil test, it is to your benefit to pull another test to make sure you have an adequate supply. Forage crops can remove a large amount of phosphorus from the field when harvested.

The availability of phosphorus is also tied closely to the soil pH. At low pH levels, phosphorus is bound up in aluminum compounds, and not available for uptake by plants. At extremely high pH levels (8 and above) phosphorus becomes bound in calcium compounds. The ideal pH range for phosphorus availability is 6.5-7. Maintaining the soil pH in this range helps to make sure you get the most out of your fertilizer dollar.

Potassium recommendations for forages are also based on a soil test. Potassium levels can build in soils, but many forages remove large quantities of this nutrient at harvest. If you need to apply a large quantity of potassium to reach your yield goal, it is best to make split applications. Some should be applied prior to the first cutting or grazing cycle, and the balance applied after later cuttings.

Many grass forages exhibit luxury consumption of potassium. When a large excess of potassium is available, forages take up much more potassium than is required. As a result, a large amount of potassium is removed from the field at harvest. Additionally, excessive uptake of potassium by forages can result in a reduced uptake of some other nutrients, like magnesium. This can result in animal health implications following the feeding of that forage.

Nitrogen recommendations for forages are based on the expected yield of the pasture or hayfield. Because cool-season grasses grow most rapidly in the spring, your largest application of nitrogen should be applied early in the year. As a rule of thumb, you should apply about 50 lbs of nitrogen for each ton of dry hay you expect to produce. Manure applications, including those from grazing livestock, should be considered in that calculation. Manure applications on a field within the past 5 years can supply residual nitrogen that will become available to your crop over time.

In pastures, the ideal timing of nitrogen application should be based on your production goals. If you have a good method to collect that flush of spring growth (harvesting hay from some paddocks, grazing additional animals for a short period), then applying additional nitrogen to meet that maximum yield goal would be a good way to go. If you can’t collect that additional growth, your resources may be better spent in an avenue other than nitrogen fertilizer.

Early spring is an excellent time to fine-tune your forage fertility program. Consider taking the time to soil test those older stands to make sure you’re getting the most productivity out of that acre.

Quote Of The Week: “Never let the fear of striking out keep you from coming up to bat.” Babe Ruth