Cicada killer wasps keep population in check

Delaware isn’t due for a brood of periodical cicadas until 2021, but dog-day cicadas are making their presence known this month. With the cicadas comes another species many humans find frightening – cicada killer wasps.

Cicadas

According to the U.S. Geological Survey, there are two types of cicadas: dog-day and periodical. Both types are found here. Dog-day cicadas live underground for a varying period of time and some adults emerge every summer, while periodical cicadas emerge once every 13-17 years in much larger numbers known as broods.

“Delaware has annual cicadas that emerge as adults every year,” said the Delaware Department of Agriculture’s Stacey Hofmann. “Their emergence is really dependent on how good the season was when the females laid their eggs.”

The term dog-day may relate to the fact that these cicadas generally appear during the dog days of late summer. Dog-day cicadas are usually about 1.5 inches long and green with black eyes.

At this time of year, Delawareans may notice loud cicada sounds outdoors. The males have tymbal organs, which can vibrate rapidly and are used for mating calls, and when in distress. The mating call has a distinct buzzing drone, whereas distress signals can sound like squawking.

Cicadas aren’t harmful to humans, and dog-day cicadas rarely cause significant plant damage. However, cicadas lay their eggs by cutting slits in tree branches, and when you have periodical cicadas in large numbers, they can cause noticeable “flagging,” or broken branches.

When the eggs hatch, the young cicada will feed on the branch until it drops to the ground. The juvenile burrows and for several years. Underground, cicadas feed on roots. When they emerge, they’re referred to as “nymphs,” and will immediately find a place to molt, shedding their exoskeleton and developing their adult body.

Right now, the outdoors is littered with cicada exoskeletons.

Adult cicadas only live for about a month and their only purpose is to mate. They typically live in trees, buzzing until a mate is found.

Cicada killer wasps

While cicadas make an interesting addition to any backyard ecosystem, Delaware property owners may be more concerned about the predatory cicada killer wasp. However, unlike other wasps, cicada killers are not aggressive toward humans. They won’t sting unless provoked, by, say, being stepped on.

Cicadas and cicada killer wasps are big insects, and sometimes loud. They can be frightening upon first encounter.

“You wouldn’t want to harass it or threaten it because it does have a stinger, but it is more worried about getting its prey,” Hofmann said.

Cicada killers are mostly reddish-brown with black and yellow stripes and, like cicadas themselves, are typically about 1.5 inches long. Females tend to be a bit bigger than males and live in burrows in areas of dry dirt. Female cicada killers will paralyze a cicada by stinging it and then carrying it back to its burrow -- an impressive feat since the cicada weighs twice as much as the wasp.

The female will then lay an egg on the cicada and close off the burrow, leaving the growing larvae to eat the cicada alive. Females grow to be larger, so female larvae require more food.

“The female is actually able to predetermine the sex of the larvae and will provide more cicadas for the larvae to eat depending on its sex,” Hofmann said.

Property owners can eliminate cicada killers with pesticides, but they are solitary wasps. They don’t live in nests, so there’s never a guarantee you’ve sprayed them all, only the burrows you’ve found. In addition, the wasps will likely return the next year no matter how many you kill, and that’s something an exterminator may not mention.

While dog-day cicadas aren’t numerous enough to do significant plant damage, cicada killer wasps do help manage plant damage by keeping the population in check. Many scientists with expertise in these wasps, like Chuck Holliday, formerly of the Lafayette College, believe pesticides pose more harm to humans.

Rather than eliminating them from your backyard, it may be more beneficial to try to live in harmony with them.