Bug of the Week

Colorado Potato Beetle (Family Chrysomelidae)


Greetings, BugFans,

This is the story of The Little Beetle That Could, of a half-inch beetle that’s much too good at what it does. So good, in fact, that it’s on Cooperative Extension Most Wanted Posters in most states and counties in the U.S., and its emigration to foreign countries was not cause for celebration (but did inspire some great postage stamps and educational/propaganda posters http://www.potatobeetle.org/memorabilia.html).

Colorado Potato Beetle

One source says that the Colorado Potato Beetle (aka the Potato bug) (Leptinotarsa decemlineata) (“ten lines”) is among the 10 insects that everyone should know. Gardeners from California to Florida to Nova Scotia to British Columbia certainly know it; it’s said to make itself at home in 8 million km2 of North America (that’s about one-third of the land mass) and 6 million km2 of Europe and Asia.

At the end of a BOTW about the dogbane leaf beetle (August, 2010) the BugLady threw in this “by the way” paragraph about the DBs famous fellow leaf beetle (Family Chrysomelidae), the CPB:

“The Colorado Potato beetle’s is an amazing story of a home-grown pest. A native leaf beetle, its “pre-settlement” range was restricted to the Upper Missouri River Basin [an alternate theory is that it originated south of the Rio Grande] , where it fed unobtrusively on a few members of the Potato family in harmony with its environment , and where it was first “discovered” in the early part of the 19th century. Then the frontier pushed West, bringing with it the kitchen gardens and “Irish potatoes” of the settlers (potatoes are native to the Andes and had been brought to the Old World a few centuries earlier, had conquered Europe, had come back across the Atlantic to New England in the 1700’s, and then headed West with the settlers). The CPB liked potatoes, and with its food source increasing logarithmically, it left the Plains and headed East in 1854, hop-scotching from garden plot to garden plot and arriving on the Atlantic shores twenty years later, a reverse pioneer.” That CPB’s had quietly switched their main host plant from buffalo bur and ground cherry to potatoes was noticed when the first CPB outbreak occurred in 1859, about 100 miles west of Omaha.

Adults and larvae feed on the leaves of plants in the Potato family, which includes potatoes as well as tomatoes, nightshade, peppers, eggplant and other secondary targets. The people who measure these things tell us that during its larval stage, a CPB may eat about 40 cm2 of leaves and that an adult dines on an additional 9.6 cm2 of leaves per day. The larvae feed in groups, compounding the destruction.

No BOTW is complete without a vocabulary challenge – the humpbacked CPB larvae/grubs are cyphosomatic – their dorsal and ventral surfaces are decidedly not parallel.

CPBs are in “survival mode” 24/7. They lay a lot of eggs on the undersides of potato leaves (as many as 800 in a lifetime), and multiple encounters with multiple partners result in genetic diversity. They grow fast, have several generations per year, and overwinter in the soil as adults so they can come out swinging when the sun warms the earth in spring. Adults, including gravid females, may move to nearby fields, so all of their eggs are not in the same basket. Add to these external/behavioral attributes the CPBs’ enviable ability to develop an inheritable resistance to almost any of the chemicals we’ve thrown at them (possibly because they’ve “practiced” by rendering harmless the naturally occurring toxic glycoalkaloids of their host plants), and you’ve truly got a force to be reckoned with.

CPBs and their eggs and larvae have a number of natural predators including some species of ladybugs, stinkbugs, and ground beetles, and CPB larvae that hatch first may cannibalize the nearby unhatched eggs of their siblings. Home and organic gardeners can control them by hand-picking the adults, larvae, and eggs, by mulching or row covering, and by removing related weeds that feed spring adults, but our reactions to their presence have typically been chemical.

The BugLady tries not to get embroiled in debates about pest control measures, but she’s weighing in on this one. Here it is in a nutshell: the BugLady doesn’t like "collateral damage.” The neonicotinoid insecticides (nicotine-like neurotoxins), which have been successful against CPBs and other agricultural pests since the mid-‘90’s (although some CPB resistance is now being seen), have been implicated in Colony Collapse Disorder in bees. The chemicals spread through treated plants, appearing in pollen and other plant parts, and they are transported back to the hive. They disturb the central nervous system, disorienting homeward bound bees, and if they don’t actually cause Colony Collapse Disorder, they make bees (wild and domestic, solitary and colonial) more susceptible to lethal fungal and viral infections. Thirty-five per cent of agricultural crops rely on pollinators; honey bees are important pollinators, but native bees are, too.

No one can blame farmers for wanting a single product that is effective across the board, and heaven knows we all want to be able to afford to eat, but the complete answer to “How much does it cost?” may take years to calculate. This is not Either-Or; not Potatoes vs. Butterflies. Bees are the canary in the mineshaft. Generalized pesticides lop off strands of food webs indiscriminately, killing, as a Texas exterminator once said, “your good bugs” along with “your bad bugs,” and that is fundamentally unsustainable. The result can be an oversimplified world where imbalances favor outbreaks of pests whose predators perished in the chemical or bacterial barrage (and so require more chemicals). And then there’s the domino effect – fewer insects means fewer insect-eaters, including birds, bats, spiders, and the BugLady’s beloved dragonflies and more. A very different world.

The real questions are “Why would we think that we can eliminate organisms, disrupt eons-old connections, and lower biodiversity, and still expect the subtle systems that keep us alive and healthy to keep functioning?” What can we do about it?

If you shake the BugFan tree, an awful lot of scientists and educators would fall out. So, how about it? How do we bridge this “disconnect” between actions and their ecological effects, and how do we educate people about keeping our life-support systems healthy?

Meanwhile, do you want fries with that?

The BugLady