Spotted lanternflies seem scarce in parts of the Philly area. But they’re still out there.

Some people living through spotted lanternfly invasions in past years have noticed the pests are now gone or much fewer in number.

So where did they go?

Turns out, the spotted lanternfly is a complicated bug. Though they are still widely present — and spreading — their numbers can fluctuate greatly from neighborhood to neighborhood. Yet it’s clear they are here to stay.

“We have observed that areas can see a decline in population after several years of increase,” said Brian Walsh, of Penn State Extension, and experts are not sure why. The university has become a go-to place for information on the spotted lanternfly since it first appeared in North America in Berks County in 2014.

The spotted lanternfly is highly mobile, as anyone who has seen a nymph deftly hop out of the way of a squashing foot can attest. The nymphs, which go through four stages known as instars, grow into winged creatures as adults.

“We only have theories on the reason for reduced population presence, and they range from plant host fitness decline to weather to predator activities,” Walsh said.

» READ MORE: Spotted lanternflies are here to stay in the Philly region

Experts are not sure how far an individual spotted lanternfly can move in a single season, but they expect it can be miles. That complicates not only tracking them, but also how to define a typical geographic radius for the pest. Their eggs have also proved hardy, able to endure bitter winters.

Researchers are still learning about the quirks of the invasive insect from Southeast Asia, and when and why they might leave an area.

The bugs are notorious hitchhikers, able to latch onto the wheel wells or wiper blades of cars and ride rail cars. That way, they can spread dozens, or hundreds of miles.

Pennsylvania has declared 34 counties under quarantine, with eight of those added this year. The bugs now span the breadth of the commonwealth. Spotted lanternflies are present in much of New Jersey, as well as in Delaware, Maryland, New York, Connecticut, Ohio, Virginia, and West Virginia.

Though they might not have been as destructive to ornamental trees and plants as once feared, they still pose significant threats to vineyards and fruit crops. So keeping their spread down is a top priority, and a number of insecticides are being used to control them.

» READ MORE: How to kill spotted lanternflies

“We have seen populations fluctuate on individual properties within a season as well, especially on individual host plants,” Walsh said. “This is where data is so important and more definitive than anecdotal observations. We’ve recorded populations on individual properties being present in the spring, largely absent for a portion of the summer but then adults rapidly returning in high numbers late in the season [September] in time to lay eggs and start the cycle over.”

Walsh said that some plant hosts, especially grapes, “seem to keep consistently high populations season after season.” Ailanthus altissima, or Tree of Heaven, has been a preferred host.

Penn State Extension is hoping to refine its knowledge of the pest’s lifecycle. Just last month, scientists in the university’s College of Agricultural Sciences created an online tool that can help predict seasonal egg hatchings for spotted lanternflies.

Cornell also has an online map showing the present locations of the bug, with the bulk of the infestation in Pennsylvania and New Jersey. And Rutgers is hosting a free Zoom informational event for the public on July 16.

» READ MORE: How to find and destroy spotted lanternfly eggs

“Spotted lanternfly is very challenging in that it has a very long individual lifecycle for an insect,” Walsh said.

That lifecycle runs from May to freezes in November. It remains highly mobile the entire lifecycle except as eggs. Each life stage seems to lead to different feeding preferences and behaviors.

It feeds on many different plants, with about 70 documented. So treating a single tree won’t significantly affect a local population.

Scientists hope to use the data they gather to better understand the bug’s patterns, and be able to offer more targeted approaches to controlling a population.

“Trying to identify definitive behavior patterns is a challenge because the behavior can vary greatly depending on the plant hosts that it encounters in a given area, the time of the year, the life stage, etc.,” Walsh said.