The potato leafhopper is the worst pest of alfalfa and, if severe, can reduce yield and quality, especially by lowering protein content.
Now is the time to be on the lookout for these critters as they arrive in the area, according to multiple sources.
Potato leafhoppers are migratory and arrive here every spring on storm fronts. After potato leafhoppers have colonized alfalfa fields, the adults deposit their eggs in the stems and veins of the leaves. In hot weather, these eggs will develop into adults in about three weeks, so populations can increase rapidly.
Potato leafhoppers have straw-shaped mouthparts and extract juice from plants. Heavy feeding disrupts the flow of nutrients in plants, causing yellow triangles to form at the tips of leaflets (hopper burn) in about seven to 10 days. As feeding continues, the damage worsens and the chlorotic areas spread to the base of the leaflet. Once hopper burning is evident, economic losses occur.
Pyrethroids offer the best control when things get out of hand, but testing is also essential. Usually a single application of insecticide is needed to control the potato leafhopper all season, but only do so if the infestation warrants it and it is economical.
When to spot
For new spring seedlings, start sampling when the plants are 3 inches tall and resample weekly until the field is sprayed or 10 days before harvest.
For new summer seedlings, sample when plants are 2 inches tall and resample weekly until mid-September.
For the second and third cuttings, sample when the plant’s regrowth is 2-3 inches and resample weekly until the field is sprayed, or 10 days before harvest.
How to Scout
In square or rectangular fields, follow a “U” pattern. In narrow bands, an “I” pattern works best. Sample five sites in each, when the alfalfa is dry, and avoid sampling in cold or windy weather.
Perform 20 pendulum sweeps (100 sweeps in total) with the net at each site, sweeping 3 to 4 inches below the tops of the plants. Don’t stop swinging until you’ve completed 20 swipes. Collect samples from each site in a zigzag pattern and perform one or two steps between each scan.
When you have completed 20 sweeps, continue swinging the net several times to force the bugs into the small end of the bag. Take the bag about 10 inches from the small end of the bag.
Only count light green leafhoppers (nymphs are yellowish green) and ignore brown leafhoppers.
Be careful because adult leafhoppers are very active and can easily escape without being noticed. Slowly unfold the net and let the insects escape a few at a time, counting them as they appear. Be careful to check the inner walls of the net for nymphs. They cannot fly and will walk or cling to the fabric.
Count the total number of leafhoppers. Repeat the same procedure on the next four sites, completing your 100 scans of the field.
Calculate the average number of leafhoppers per sweep. For example, if you collected a total of 60 leafhoppers, find the average per scan by dividing 60 by 100, which equals 0.6 leafhoppers.
When the leafhopper population is high (40 or more in 20 scans) at the first site, spending time sampling the other four sites is of little value.
Make a sweeping net
A sweeping net is needed to monitor populations of potato leafhoppers in a field. Bug nets can be purchased for around $ 15, but they are not widely available. Materials for making a net can be found at most farms.
Use a piece of sturdy wood about three-quarters of an inch in diameter and 2.5 to 3 feet long for a handle. A broom handle cut to length is suitable.
The hoop can be a piece of heavy gauge wire or a thin steel rod. You need a piece that is 53 inches long. Tie the ends of the hoop securely to the wooden handle by wrapping them tightly with lightweight wire, such as bullet iron, or by slipping a metal sleeve over the handle and over the ends of the hoop.
To make the netting, you need two pieces of fabric measuring 24 by 36 inches. Heavy muslin or tightly woven nylon fabric will do the trick.
Prepare for next year
Resistant varieties of alfalfa are available. These varieties are covered with fine hairs (glandular trichomes) which reduce the feeding of leafhoppers.
Another option is to mix other forages with alfalfa. Stands of alfalfa or orchardgrass (or other combinations) tolerate leafhopper damage better than pure alfalfa stands.
Spiders and other natural enemies kill potato leafhoppers. Therefore, using integrated pest management and insecticide spraying only when economic populations are growing will help keep these allies in pest management.