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I Found Something Strange in my Garden

I recently received a question from one of our local gardeners about a strange thing they found in their garden, and thought the inquiry would appeal to a wider audience. It looked like a puffy little brown sack a little larger than a quarter, and attached to the dead stem of an herbaceous perennial from last year. Besides the obvious, “What is it?” the gardener wished to know if it should be kept – or, is it friend or foe?

The short answer is that it should be kept. Like this gardener, what you may also have in your garden are egg cases from a praying mantid. Praying mantids are voracious consumers of garden pests. In fact, they are so predatory in nature that they will eat each other. In fact, after mating, the female will eat the male for a rich supply of protein for egg-laying. They are called “generalist predators” because they aren’t picky about what insect they eat. The downside is that they also eat beneficial insects.

The foam-like egg case, called an ootheca, overwinters in plant debris, usually attached to a stem or twig. Some may be attached to Christmas trees that are cut and brought indoors. Your Christmas tree grower will usually put the tree in a shaker to remove dead needles and things like oothecae. Otherwise, you may get quite a surprise near Christmas.

Each case may contain dozens to hundreds of eggs which hatch with warming spring temperatures into nymphs, smaller versions of the adult.

Dr. Timothy Gibb, Purdue Extension entomologist, wrote that he had an unfortunate experience with oothecas earlier in his professional career:

“One year I found several praying mantis egg cases attached to weeds and tree limbs during the late fall and early winter, and then did what any other red-blooded, insect-loving, entomologist would do. I collected them, put them in a cereal bowl and stashed them in the back of my wife’s cupboard thinking that they would be tucked away out of her sight and that I would take care of them before she ever noticed. Well, as you probably guessed, I forgot about them.

“Once inside the warm house, the eggs reached the proper temperature threshold that restarted their development clock. They broke diapause and after the required number of warm days, bingo, the eggs began to hatch.

“The bottom line is that thousands of hungry, baby mantids began pouring out of the cupboard all at once. They were everywhere.”

Dr. Gibb related a humbling exchange with his wife to conclude his tale, but suffice it to say, he never brought praying mantis egg cases into the house again.

He used the term diapause, which he described as “a physiologically induced delay in development or a dormancy.” It’s different than hibernation, and many insects use diapause to overwinter. They may use it in any stage of development. Gibb said that insects produce a substance called glycerine in their blood that prevents them from freezing. It’s like insect antifreeze.

Similar to Gibb’s experience, I also brought an ootheca into my office earlier in my career, but fortunately, it was in a sealed container. On Monday morning, hundreds of baby mantids were crawling around within the container. I released them outside.

What about the name: praying mantid, or praying mantis? Although most people refer to any insect of this type as a ‘praying mantis,' not all mantids are mantises. Mantid refers to the entire group. So, all mantises are mantids, but not all mantids are mantises.

The two most common mantids in Indiana include the Chinese mantis (a.k.a. Chinese praying mantis), and the Carolina mantis (a.k.a. Carolina praying mantis). The Carolina mantis’s ootheca is longer and slenderer than the Chinese mantis’s ootheca, which resembles the shape of a toasted marshmallow.

So, the moral of this story is to leave them in the garden. They’ll hatch when the weather warms up and gobble up hundreds of garden pests for you.

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