With the recent rainy weather, harvest of the first cutting of alfalfa has been disrupted, so we have some areas of the state that are still waiting to harvest first cutting, while others have regrowth of what will be the second cutting. Entomologist John Tooker says this situation presents two different insect challenges that need to be kept in mind, though the bottom line remains: scout your fields to see what insect pests are active and whether insect populations are problematic.
For folks that have not harvested their first cutting, remember that alfalfa weevil is active across the state, but tends to be limited to first cutting. For the southern part of the Pennsylvania, larvae are approaching the fourth and final instar, after which they will drop to the soil to pupate.
In the northern part of the state, larvae are mostly in the first or second instar. Recall that alfalfa weevil larvae cause pin hole-sized damage to leaves near the tips of plants often on unfolded leaves, while older larvae consume leaves that are more open, typically leaving jagged edges.
Most early season feeding does not result in economic loss because introduced parasitoid wasps that attack the feeding larvae effectively control their populations. To know whether your fields are hosting economically damaging populations, scout them! If you are close enough to harvest, cutting your alfalfa is a reasonable alternative to applying an insecticide.
Do not blindly spray them with insecticides, because such treatments may not be needed and may flare pest populations later in the season by reducing populations of predators. As a reminder, 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. See our Alfalfa weevil fact sheet for thresholds, a sample protocol, and more details on alfalfa weevil biology.
For folks that have already harvested their first cutting, second cutting is when potato leafhopper tends to show. This pest species migrates into Pennsylvania each spring on storm fronts, arriving in late May and early June; we have yet to hear of any reports of their arrival. Nevertheless, I am mentioning them now to encourage growers to get their sweep nets ready and scout for them in the coming weeks as the second cutting really starts to grow.
Once potato leafhoppers colonize alfalfa fields, adults deposit eggs into stems and leaf veins. In warm weather, these eggs will develop into adults in about three weeks, so populations can increase quickly.
Potato leafhoppers have straw like mouthparts and extract plant juices. Heavy feeding disrupts nutrient flow within plants, causing yellow triangles to form at the leaflet tips ("hopper burn"), but this evidence of damage does not develop until 7-10 days after feeding begins. As feeding continues, damage gets worse and the chlorotic areas spread toward the base of the leaflet.
Once hopper burn is evident, economic loss has occurred. To target leafhoppers most effectively, populations should be sampled and treatment applied only when economic thresholds are exceeded.
Again, harvesting is a good alternative to spraying. In my experience, regular scouting and use of economic threshold can limit the need to insecticides to once a summer, if they are needed at all. Scouting details and economic thresholds can be found in our potato leaf hopper fact sheet.