Q. This isn't really a gardening question, but I'm curious about dragonflies. I know they're very beneficial so I'm always glad to see them in my yard down the shore. (I call them all dragonflies, even though I believe that some of them are actually called 'darners'.) Anyway, my question is: do the larger ones catch and eat the smaller ones?"
---Jim in Villas, New Jersey
A. Excellent! Another opportunity for me to play 'Amateur Entomologist'! And while Jim is right about this not being a specific garden question, it does fall under another of our specialties—natural control of truly troublesome pests; in this case, mosquitoes!
That's right—in addition to being very cool-looking, this specific group of insects is very important to our well-being, as they prey on the dangerous day-flying disease-carrying mosquitoes that are one of our newest imported threats. (Hey—and since these Typhoid Maries bite during the day—when we're outside innocently sowing our salad greens—maybe this IS a 'garden question' after all!)
Anyway, dragonflies are one of the biggest predators of mosquitoes. Many birds are also effective (and can be attracted to eat your skeeters with nesting boxes designed for swallows and martins and such), but a large adult dragonfly can eat twice its own weight in mosquitoes in a single hour—that's why one of their common names is "Mosquito Hawk". They curl their legs into the shape of a basket and scoop up mosquitoes, flies and biting midges by the dozens. One of my favorite real entomologists, Dr. Linda Gilkeson, says that researchers have found up to 100 mosquitoes in the 'basket' of a captured dragonfly.
And their larval forms keep mosquitoes down in advance. In one of the most astounding examples of insect metamorphous, dragonfly and damselfly eggs hatch into voracious little underwater predators whose diet includes lots and lots of mosquito larvae. And some species are so big at this stage that they eat small fish!
Now: what's the difference between a dragonfly and a damselfly? And what about 'darners'?
In general, 'dragonflies' are larger and much better fliers than the smaller damselfly. And again, in general, the eyes of dragonflies are much closer together than those of damselflies. Together, they make up the entire order of insects known as "Odonata", which comes from the Greek word for 'tooth'. 'Darner' refers to a 'family' of several species that includes some of the biggest, most abundant dragonflies in North America. The best known is probably the 'Green Darner' (who sounds a little like a DC Super-Hero: Green Lantern. Green Arrow; and now—the Green Darner!)
Although these are likely the ones people first called 'the Devil's Darning Needle', I think that term has become generic to any big dragonfly; it refers to the endless folk tales about the fearsome-looking insects sewing things together and attacking humans and livestock. The 'darning' part almost certainly came about because of their sewing needle shape. And the 'attack stories' probably came about because their size scared people—and especially because people realized that Grimm Fairy Tale-style stories about them would really scare children. My German grandmother used to terrify us with warnings that darners would try and sew our lips together.
Anyway: Back to the actual question—'DO the big ones eat the little ones'?
Yes. My "Field Guide to Insects of North America" (which calls damselflies "the kid sisters of the dragonfly") says that the "Dragon hunter", a dragonfly in what they call the 'clubtail' family is "a fearsome beast that specializes in eating other dragonflies." (The Field Guide breaks the dragonfly 'families' down into darners, clubtails, petaltails, emeralds, cruisers, spiketails, skimmers, pondhawks and meadowhawks. It considers damselflies to belong to their own 'suborder' of around 130 species.)
But the most important fact here is that all—or mostly all—of these insects eat large numbers of mosquitoes, flies and other pests. As you may have guessed, that's why I picked this question—to entice you to attract these beneficial "mosquito hawks" to your property. Heck—pull in lots of skeeter eating birds and dragonflies and set out traps of standing water baited with BTI and you may never donate blood unwantedly again!
And of course, the most important aspect of attracting either birds or dragonflies is to not kill them with pesticides or chemical fertilizers. As with equally beneficial frogs and toads, larval dragonflies breed in water (some dragonfly 'babies' spend five years as underwater predators before becoming flying adults!) and the use of senseless and unnecessary lawn and garden chemicals is a dagger through their collective hearts. In fact, the presence or absence of the underwater stage of some species is used to judge how polluted that specific body of water is.
Ah, but there are also ways to specifically attract dragonflies to eat YOUR mosquitoes. Dr. Gilkeson says that you can entice them with perches—narrow upright stakes that extend well above any plants in the area. She has found that bamboo poles placed in a kind of zig-zag pattern about three to four feet high are the best attractors.
Position your perches where they'll be in full sun at the middle of the day, when dragonflies are most active. And the more stakes the merrier. Dr. Gilkeson says that there's a dragonfly at the top of every one of her perches on most summer days, each one "looking like a bright jewel on the head of a pin."