When Do Spotted Lanternflies Hatch? A Monthly Guide

In Pennsylvania, spotted lanternfly eggs hatch from late April through June. In New York and Massachusetts, most hatching happens in May and June. Hatching is spread over weeks, not one day, so a single date will always be wrong for some of the eggs.

The month-by-month picture

The table below uses dates published by extension programmes in three states. It is a guide, not a promise. The sources themselves say local temperatures can move the timing.

The Pennsylvania dates come from Penn State Extension, which says eggs are laid from September to December and hatch from late April to June. Cornell’s New York page gives May and June for hatch. UMass Extension gives May for Massachusetts and notes that its timing is based on Pennsylvania observations and may shift locally.

An older New York Invasive Species Information page gives April and May for hatch. That is earlier than Cornell’s current page. The likely reason is the same one UMass gives: insect timing varies with local temperature, and an older summary may have used a different year or region. Treat the newer state extension page as your local guide.

What each stage looks like

The sources do not agree on every measurement. Penn State Extension says newly hatched nymphs are about 1/8 inch and fourth instars about 1/2 inch. Cornell says first instars are about 1/4 inch and fourth instars can reach 3/4 inch. The difference may come from measuring insects of different ages within a stage. The pattern is the same: the first three stages are black and white, and the fourth turns red.

Egg counts also vary. APHIS says 30 to 50 eggs per mass. Cornell says 30 to 60. Penn State gives an average of about 35. Use the range, not a single number.

Why hatching is spread out

Spotted lanternflies have one generation per year in Pennsylvania. Females lay eggs from September into December, and the eggs overwinter. Hatching begins in spring and continues for weeks.

Cornell describes the first instar as active from May into July, with the second and third instars following. The fourth instar is active from July into mid-August, then molts to the adult stage. Adults appear in July and can be found through December.

Weather matters at both ends of the season. Spring temperatures affect when eggs hatch. Penn State Extension says adults remain active until hard freezes kill them, so the end of the season depends on the weather.

What to look for each month

• January to March: Check smooth outdoor surfaces for old egg masses. They look like dried mud and may be high on a tree.
• April: Finish your egg-mass checks. Hatching may start late in the month in Pennsylvania.
• May and June: Look for small black nymphs with white spots on new growth, herbaceous plants, and vines. This is the easiest stage to miss.
• July: Watch for red fourth-instar nymphs and the first adults. Adults often gather on tree of heaven trunks.
• August and September: Adults are large and active. Check tree of heaven, maples, willows, and grapevines.
• October and November: Adults and fresh egg masses are both present. Check vehicles, trailers, furniture, and firewood before moving them.
• December: Adults are mostly gone after hard freezes, but egg masses remain.

What to do if you find them

Spotted lanternflies do not bite or sting people. They feed on plant sap and excrete a sticky, sugary liquid called honeydew, which can coat surfaces and grow sooty mold.

If you are in an area with a known population, check your outdoor items before you travel. APHIS recommends inspecting vehicles, trailers, grills, furniture, and firewood for eggs or insects. If you find eggs, smash them and scrape them into a bag with hand sanitizer or rubbing alcohol, then seal the bag and put it in the trash. For a step-by-step egg-mass check, including where to look and what to do with masses you cannot reach, see spotted lanternfly eggs.

If you are outside the known range, report the find. In Pennsylvania, report online through the Department of Agriculture or call 1-888-4BAD-FLY. Outside Pennsylvania, APHIS directs residents to their state plant regulatory official.

Where the answer comes from

This page does not give a single national hatch date, because the official sources do not give one. The dates differ by state and by year. Use the table for planning, then check your state’s extension page or agriculture department for the current local window.

For the next step, see the spotted lanternfly species profile, the look-alike guide, or the Pennsylvania state page. If you are still learning to read life stages, start with the identification guides.

For the rest of the spring workload, including lesser celandine, garlic mustard and stiltgrass, see the spring invasive plants checklist.