I promised we’d dig deeper into the biology and ecology of a handsome little beetle introduced last week, Tricrania sanguinipennis, so we need to head down to the ground, and beneath, where the beetle spends 99% of its yearlong life.
First, let’s pick a name that’s not such a mouthful – Beautiful Underground Gate-crashers, or BUGs, for short. The BUGs spotted a couple of weeks ago at Delnor Woods Park in St. Charles are part of a larger family of beetles called the Meloidae, or blister beetles. Besides producing a fluid that can cause skin irritation, members are famous for parasitizing other insects – sometimes grasshoppers but often native bees. The Delnor BUGS specifically target Colletes cellophane bees.
Our BUGs currently are engaged solely in reproduction. No feeding, so far as anyone has been able to tell; no flights, as their wings are nonfunctional; no attempts at nest making. What’s the use? Their adult stage is final and fleeting, and as for provisioning their offspring – well, that’s what Colletes is for.
Adult BUGs emerge from the ground about the same time Colletes bees do. After mating and laying hundreds of eggs, the adult BUGs fade away and their brood takes the main stage.
At this point, the young BUGs are known as triungula, and unlike many beetle larvae are quite mobile. Their mission at this first phase, or instar, is to hitch a ride to a burrow that will become home sweet home for the next 11.5 months or so.
Their quarry is male Colletes bees. The ground-nesting bees look to be living in colonies, but actually are solitary in their habits. Each female digs her own burrow and nest chambers, provisions them with eggs, and tries to not disturb her Colletes neighbors. Since they have no hive or colony to defend, their nature is gentle; stings are extremely rare.
Cellophane bees, also known as plasterer bees, are named for the transparent material a female will produce to line and stabilize the soil that forms the interior of her nest chambers, or brood cells. These excavations, only a few inches below ground, are masterfully dug off to the side of a main tunnel to minimize the risk of offspring drowning in a heavy rain.
The female attaches an egg to the wall of each brood cell, then delivers a nutritious mixture of nectar and pollen upon which the little bee larva will feed.
That is, unless a BUG larva beats the baby bee to it. That’s right, as mama Colletes is busily preparing for her future family, those BUG triungula (tri = three, ungula = claws) are doing their best to crash the party. Using their tri-clawed little feet, they grab hold of the hairs on papa Colletes, who is doing little else besides crawling around and looking for a mama to mate with.
Once he meets with success, the triungula transfer from hairs on papa’s underside to hairs on mama’s topside, then hitch a ride to the brood chambers. Once there, they first consume the Colletes eggs – to eliminate competition for food – and spend a happy year.
They feed, molt and pupate in a water-resistant chamber, amid the pollen-and-nectar “bee bread” feast, before emerging as an adult to start the process all over again.
Blister beetles fall under the classification of parasitoid, a parasite that kills its host. In nature, there is no good or bad, there just … is.
The beetles may reduce the number of Colletes bees in a localized area, but it would do the beetles no good at all to eliminate all Colletes, for then they would have no future either. Plus, if Colletes were to multiply unchecked, they could reach a point where they overtax their resources, also not good in the greater web of life.
And remember, mama BUGs lay hundreds of eggs. They have to. There’s a very good chance that many triungula will perish long before their ride comes along.
There’s also a chance that the Colletes bees that go unparasitized may exhibit a feature that is inhospitable to Tricrania triungula. Maybe their hairs are too slick or too spiky. Maybe the nest chamber is a different size or shape. Whatever the variation, it could turn into something over time – like thousands of years of time – that could make Colletes resistant to Tricrania.
Then there’s the contribution to the food chain that BUGs make. Adults and larvae, alike, feed plenty of hungry insectivores, especially spiders, that are out and active in early spring.
The Colletes-Tricrania drama is playing out right now at Delnor Woods Park, and countless other areas where grass and ground covers are sparse and the bare soil catches plenty of sunshine. Head out and observe the spectacle. It’s one bug you’ll be glad you caught.
Pam Otto is the manager of nature programs and interpretive services at the Hickory Knolls Discovery Center, a facility of the St. Charles Park District. She can be reached at 630-513-4346 or firstname.lastname@example.org. Feedback on this column can be sent to email@example.com.