Degree Days Mounting for Alfalfa Weevil

As I write this column (May 21) the North Dakota Agricultural Weather Network reports there have been approximately 200 accumulated insect degree days in the Williston and Stanley area. This data has some implications as to when we can expect adult alfalfa weevil emergence which is around 250-300 degree days. Degree days are calculated by averaging the high and low temperatures of each day and subtracting the base of 48 degrees Fahrenheit. Depending on day time temperatures, it is possible adult emergence could occur by the time you read this.

Adult weevils do very little damage to alfalfa plants. It is the larvae which begin to hatch from the eggs they lay at about 300 accumulated degree days. During the initial egg hatch, the feeding of the little larvae is light but by the time 500 degree days are accumulated, the larvae are large enough to cause considerable damage to the alfalfa plant. The feeding continues until approximately 800 degree days have passed.

Scouting of alfalfa fields should begin immediately after egg hatch and fields should be scouted weekly for larvae up through the first cutting. Fields should be scouted in an “M” pattern or by selecting random sites within the field, with a minimum of five sampling sites per field. Large fields should have more sampling sites. Be sure your sampling pattern is representative of the entire field. Just don’t scout only along the edges or in small areas.

For sampling, you will need a sharp pruning shear, a white 5-gallon bucket and pencil, paper and calculator. At each sampling site, select a minimum of 30 plants and cut them off the base. Lower the cut plants into a 5-gallon pail and vigorously beat the plants in the pail to dislodge the larvae. Record the 1) number of plants sampled, 2) the total number of larvae, 3) estimate and record percent feeding damage (defoliation), and 4) the height of the alfalfa. Then, total the number of larvae and divide by the total number of plants sampled to calculate an average number per plant. Also, calculate percent feeding damage and plant height averages for the field.

The NDSU Entomology Department does not recommend the use of sweep nets for sampling because results are often highly variable and inaccurate. Sweep nets can be used to determine the presence of adults and larvae at or prior to the typical egg hatch of 300 degree days.

Several factors must be considered when making alfalfa weevil management decisions. Plant height, estimated yield, crop market value, management cost and plant injury based on the number of larvae per stem must be considered. North Dakota entomologists have established a recommended economic threshold based on plant height, crop value, treatment cost and number of larvae. This table can be found at our website: http://www.ag.ndsu.edu/williamscountyextension.

After the first cutting has been harvested, be sure to scout for larvae where the windrows were located. When the alfalfa is swathed, the larvae usually crawl below the windrow for shelter and remain there until pupating.

When the decision is made that insecticidal control is needed, pay attention to the pre-harvest interval. Most labels also have a pre-grazing interval.

Transplanting Trees – Match Spade to Tree Size

A tree’s size and potential for moving is determined by its trunk diameter or caliper which is measured about six inches off the ground for trees that are four inches or less in diameter. The caliper is measured about 12 inches off the ground for trees with a trunk diameter larger than four inches. As trees are selected for moving, it’s important to know the capacity of the tree spade. A common mistake is thinking that if the tree spade can dig and lift the tree, it is a large enough machine to do the job.

Although most tree spades can dig and lift much larger than they are designed for, this does not mean that it will remove enough soil and roots for successful transplanting. A high percentage of the tree’s roots are lost when the tree spade cuts into the soil to create the rootball. The percentage of roots that are lost during this process increase dramatically if the tree is larger than the rated capacity of the machine. Tree spade manufacturers rate the capacity of their machines differently. As a rule, the tree spade diameter should be 10 times the trunk caliper. There are exceptions though. Trees with wide, shallow root systems, such as poplars, will lose a higher percentage of roots than will a deep-rooted tree. Trees that have a deep taproot, such as oaks, are much harder to dig and their chances of survival will be less than that of a non-tap root tree.