Kevin Roth, director of education and outreach at the Pennypack Ecological Restoration Trust in Huntingdon Valley, Pa., said the decline has been noticeable.
"There's a lot fewer of them this year," Roth said.
While many residents may be wondering why the insect population appears lower, Roth said there is no single confirmed explanation.
"The numbers usually vary by neighborhood and from year to year," Roth said. "So weather, local management, and natural population pressures, they can influence what people see year to year."
Roth said weather may be one factor. The region experienced multiple snowstorms and an extended stretch of cold weather during the winter.
He also pointed to native insects, including praying mantises, as a possible reason for the reduced numbers.
In addition, the Pennypack Ecological Restoration Trust has worked to remove Tree of Heaven, the spotted lanternfly's primary host plant, from its preserve.
"I think that's part of the reason we've seen less here at the Trust," Roth said.
Roth said residents can help reduce dense swarms of spotted lanternflies by killing the insects on sight.