As I write on May 24, there have been (to my knowledge) no incursions of potato leafhoppers in the Northeast. Still, thinking about this tiny six-legger is brain power well spent. The protracted chilly spring in the Northeast may be what kept these southern-based bugs from departing Dixie. Although most of our region saw 90º temperatures this past weekend, summery conditions were banished when cold fronts triggered thunderstorms at many locations. It’s hard to say whether leafhopper (Empoasca faba) hordes stayed south or they migrated north and are lurking in the shadows, waiting for things to warm up. At dawn this morning our thermometer in Hartwick read 48º.
Entomologists generally agree that these tiny, yellowish-green wedge-shaped insects blow into our region from the Southeast in late spring and early to mid-summer. E. faba wounds turn alfalfa leaflets yellow as well as pink and light purple – and the same symptoms show up on potato foliage. The biggest problem with leafhopper injury isn’t the discoloration – it’s the way that the plant no longer grows. The standard field recommendation is to harvest the discolored, stunted alfalfa forage and hope for an unstunted, untainted second or third cutting regrowth; most of the time, hope wins out. The phrase “blow into” refers to jet stream activity where adult insects hitch rides on thermal updrafts, then are ferried by jet stream toward the country’s northern tier of states, where they drop out on vegetation like alfalfa and potatoes.
An online Penn State Extension bulletin sheds more light on the subject. I’ll hit its high spots. Potato leafhoppers are migratory insects that overwinter primarily in the Deep South because cold northern winters will kill them. During early to late May, storm fronts develop in the Gulf of Mexico, move northward into the Midwest and then eastward into most of the Northeast. These storms transport leafhoppers. Low-pressure systems stimulate the adults to fly into updrafts that draw them into clouds, where they move with the direction of the storm. These storms can transport such tiny pests from the Gulf to the Northeast in five days. Downdrafts in front of these storms drop adult leafhoppers onto fields.
Because of this movement, scouting for this pest after each storm system passes during late spring is advisable. Only adults migrate northward. Immature leafhoppers lack wings. During spring migration north, about 70% of adults are pregnant females. They deposit their eggs in alfalfa stems and leaf veins as soon as they reach a field. In warm weather, insects will mature in about three weeks, and very large populations of leafhoppers can build up in a short time. Adults are very active and will fly above the plants when disturbed. Nymphs are also active and will move quickly when bothered.
Both adults and nymphs feed on alfalfa. They insert their mouthparts into the plant tissue to extract the juices. This feeding process distorts and blocks the tiny tubes that distribute nutrients within the plant, which causes yellow triangles on the leaves – “hopper burn” – and stunted plants. The alfalfa may also turn a pinkish or purplish color. Stunted plants do not recover until the stems have been harvested, after the leafhoppers are eliminated. Buds will then begin to grow. Although potato leafhoppers can be found from late May until frost, the bulk of damage is done from mid-June to mid-August during the second, third and fourth cuttings. Normally, the adults stop reproducing and populations drop off sharply after mid-August.
The alfalfa leaflet discoloration associated with hopper burn can be confused with a boron (B) deficiency symptom. According to University of Wisconsin agronomy professor Emmet Schulte, if a plant is B-deficient, its growing points shut down; if the B deficiency persists, the affected plant usually dies. B deficiency symptoms include death of the growing tip, bushy appearance of the plant and yellowing of the top leaves (this last item vaguely similar to hopper burn). Schulte also pointed out that recent B shortages in northern climates are much more widespread than other trace element deficiencies. He said that organic matter (OM) is the storehouse for most of the B in soils. B, in borate form, is negatively charged and thus likes to hang out in the soil’s OM fraction – along with nitrates, phosphates and sulfates, also negatively charged.
My Canadian cropping confidant, retired Guelph University agronomy professor E. Ann Clark, commented on this element: “Boron is particularly important in nitrogen fixation, so it would be of particular benefit to legumes. Like all micronutrients, the window of ‘enough’ is very narrow. So in addition to being needed in micro-quantities, it is difficult to apply enough without over-applying and resulting in toxicity. This is something to look for before applying, to avoid potential harmful effects.” One might consider this another plug for soil testing.