Murder rates rise around the country during warm months, but the biggest spike may be in St. Paul. Specifically, on stage at Park Square Theatre.

They (fictitiously) bump off at least a couple of people every summer at the theater, where mystery plays have been a warm-weather staple 10 of the past 11 years. Agatha Christie’s “Rule of Thumb,” a trio of puzzles inspired by her short stories, opens Friday, the latest in a murderers’ row of clue-filled shows that have featured Sherlock Holmes, Nero Wolfe, Hercule Poirot and other top detectives.

Actor Bob Davis has been involved in many of those mysteries, playing Holmes’ Dr. Watson several times and summoning a Belgian accent to tackle Poirot this year in “Rule of Thumb.” Davis thinks one reason 80,000 people have attended Park Square puzzlers is that audiences know what they’re in for.

“There’s a familiarity in style. I think they feel like they’re in good hands with these classic stories. Audiences know how they work, what the pieces are,” said Davis. “There are things being juggled in the air and the fun trick is to keep the audience thinking the wrong way.”

C. Michael-jon Pease, the theater’s executive director, says familiarity also provides a marketing boost.

“There are so many die-hard mystery fans out there, even if they’re not particularly theater fans,” said Pease. “And, when you go with Sherlock Holmes or Agatha Christie, there’s great name recognition there. The more media hits you have, the more access there is into a story.”

He notes that both Christie and Holmes creator Arthur Conan Doyle are constantly gaining new fans with adaptations of their work such as the PBS “Sherlock” series with Benedict Cumberbatch and several recent Christie movies, including Kenneth Branagh’s “Murder on the Orient Express.”

‘Cozies’ make better summer plays

Carole Barrowman, a crime novelist who often reviews mystery books for the Star Tribune, says they strike her as more of a cozy, read-by-the-fire activity for winter. But she thinks thoughtful choices make Park Square’s mysteries appropriate for lemonade season.

“The most popular new mysteries, based on the 10 or 15 [advance copies of books] I get every day, are a lot more in what we call the ‘Girl on the Train/Window/Horse/Swimming Pool’ genre, which is huge right nowr,” said Barrowman. “And serial killer books are big right now, especially with female protagonists. But it’s hard to imagine theater audiences wanting that kind of thing in the summer. I think what we call ‘cozies’ make for better summer plays than serial killers.”

Barrowman also noted that the interactive, puzzle element of many classic mysteries — readers/audiences are given the clues and invited to guess the solution before the detective, but there’s no shame if they don’t get there first — creates a lighthearted vibe. That fits summer, when escapist fare rules movie theaters, bestseller lists and stages.

One secret to why Park Square mysteries sell so well may be demographics. According to a Nielsen study, mystery readers are disproportionately female and older, which is also true of theatergoers. Pease and Barrowman see that as both opportunity and challenge: Can young people be interested in mysteries?

“If you spend any time on Twitter’s crime fiction feed, you’ll see that it has become a bit of a concern within the genre. And if you look at who comes out to a mystery book signing, it tends to be people my age, which could nicely be put at ‘middle age,’ ” said Barrowman, who also teaches English and creative writing at Milwaukee’s Alverno College. “I’ve seen this among my students, too: They’re much more likely to pick up a memoir or a fantasy novel than a mystery.”

Finding new fans

Park Square is trying a couple of things to attract new audiences. Forty households are involved in its Mystery Writers’ Producers Club, which helps raise funds, select scripts and get the word out when murder takes center stage. And Park Square is seeking creative solutions to the lack of diversity in classic mysteries, which tend to be about two middle-aged white dudes — Holmes and Watson, Poirot and Col. Hastings, Nero Wolfe and Archie Goodwin — solving the killing of one white dude by another white dude.

“Where are the stories about women? Where are the voices of color? How can we embrace the modern world?” Pease asks.

The answer, he says, has to do with the way shows are adapted and cast, as exemplified by “Rule of Thumb” director Austene Van, who is African-American. “The first thing Austene said when we talked about casting was that she didn’t care about gender or race,” said Pease. “She just wanted people who could look guilty.”

Like most theaters, Park Square wants to create more opportunities for people of color and women. Last summer’s “Baskerville” was the boldest example yet.

“We liked that ‘Baskerville’ was a comic script, and that led us to Theo Langason as the director. Richard [former Park Square artistic director Richard Cook] said, ‘I want a younger person’s take, a different generation of artists taking a look at this.’ We wound up with women playing Sherlock and Watson, which we never would have guessed at the outset, but it was totally fresh and it reached a lot of younger audience members as well as dyed-in-the-wool fans,” said Pease.

Park Square also is commissioning new mystery scripts, including one that features a female detective, who appears in a series of books by a female author.

That sounds great to mystery expert Barrowman, hinting that the summer theater diet of William-Shakespeare-in-the-park in her Milwaukee home could use more variety. Says Barrowman, “If they figure this out, it’ll be great to see Agatha give Will a run for his money.”