Some alfalfa fields have been harvested for the first time and now is the time to scout the regrowth in those fields for potato leafhoppers (PLH). At our Western Agricultural Research Center near South Charleston, OH the PLH numbers last week in early alfalfa regrowth were generally about half the number needed to reach economic treatment thresholds. However, PLH populations can change quickly, and second crop regrowth should be scouted at least on a weekly basis. The forage this year is valuable because of low forage supplies, so protecting the crop from this insect is especially important and we might want to consider treatment at lower than normal population counts.
Videos are available at describing scouting for PLH in alfalfa and how to identify the insect and its feeding damage.
In addition, a factsheet is available at https://ohioline.osu.edu/factsheet/ENT-33. That factsheet includes the economic action thresholds for this pest in alfalfa under low, normal, or high tolerance to stress. Because of the value of alfalfa this year and the waterlogging stress many surviving stands have experienced, the low tolerance threshold could be considered for many fields in Ohio.
We have observed the classic hopper burn symptoms on first crop alfalfa that has not yet been harvested. Those stands have achieved their full yield potential, so insecticide treatment is not warranted on those uncut stands. Cutting those stands once the weather allows and then watching the regrowth for PLH re-infestations is the recommended approach. While the leafhopper feeding may result in slightly lower crude protein levels in those uncut stands, other factors such as foliar diseases and lodging are likely having a larger impact on leaf loss, forage quality and yield than is the leafhopper feeding.
Red clover stands can also suffer damage from PLH. Varieties that are less pubescent (less “hairy”) are particularly susceptible to this pest, and we have seen significant stunting and loss of yield in those types of varieties. There are no established economic action thresholds for red clover, so perhaps using the normal to high tolerance thresholds for alfalfa would be a guideline to consider for red clover this year.
The PLH is a small bright green wedge-shaped insect that arrives in our area each year on storm fronts from the Gulf Coast region. PLH is a sucking insect and its feeding causes stunting of alfalfa plants resulting in yield loss. Excessive stress on plants by heavy PLH feeding can result in yield reductions in the current as well as subsequent cuttings. A common symptom of PLH feeding is a wedge-shaped yellowing of leaf tips. If you are noticing these symptoms, some damage has been done. Regular scouting can help to detect PLH earlier and determine if there is a need for a rescue treatment.