Potato Bugs

Picked 30 July on two rows of potato plantsBesides black flies, Windswept Farm is plagued with Colorado Potato Beetles (CPB – Leptinotarsa decemlineata), whose larvae will chew a potato plant like a dog chews a smoked pigs ear, and almost that fast. They don’t touch the tubers underground, but if there’s no plant to feed the tubers, you don’t dig potatoes in October. They are a scourge not just here in Monroe Maine, but around the world despite originating in Colorado. The Wikipedia article mentions that they may have been used as a crude form of biological warfare against Germany in WWII, and the Soviet Union in the 1950s.

What makes them so insidious is their high tolerance to many pesticides, as well as their ability to develop resistance to specific pesticides very quickly, probably because of their short lifecycle and high number of offspring (thousands per female).

One pesticide that has been very effective against CPB is called “Bt” which are the initials of a soil bacteria that produces a substance that causes CPB and only CPB larvae to stop eating, and thus die in a day or two. Because it’s targeted to CPB (i.e. doesn’t kill ALL insects it touches like many pesticides), and it’s derived from a “natural” source, Bt has been approved for use in organic production. However, because it’s so effective against CPB, the bacterial genes that produce the effective substance were one of the first that were genetically engineered to become part of the GE versions of potatoes, most famously called “New Leaf” variety/brand. The difference there is that when you spray Bt on a leaf, where it is eaten by the larvae, the Bt eventually washes off the leaf and/or degrades in the environment, never to touch the edible part of the crop. When it’s engineered to appear in every cell of the plant, we then eat Bt with every french fry.

Anyway, if I can’t pick the bugs and larvae off the plants fast enough (which is often) I will use Bt spray on my potatoes, as well as rotenone (another organic pesticide that is not targeted to CPB, so I use sparingly) when they get too old to be affected by the Bt. Today is Sunday, which is when I do my once-a-week weeding of my garden, and I also brought a plastic cup down with me to pick CPB (which I have to do almost daily, actually). Pictured is my harvest. It is interesting to note that the CPB seem to prefer certain varieties of potato over others — I picked more than half of these beetles off of the Green Mountain potato plants. (Green Mountain is an heirloom Vermont variety that makes good dry baking type potatoes without a thick russet skin.)


3 thoughts on “Potato Bugs

  1. Entomological notes:

    The CPB’s origins are thot to be in Mexico, rather than Colorado. Their pre-Columbian range likely extended up the Rockies to Colorado, where their first encounters with non-native American farmers occurred.

    “Thousands [of offspring] per female” is a bit overstating things — it’s like saying “major league baseball players hit lots of homeruns (more than 60 per year), although the pitchers strike out lots of opposing batters (20 per game)” — average female fecundity is likely in the hundreds rather than the thousands but that’s still a lot.

    You might be surprised how much defoliation (especially early-season) a potato plant can sustain and still produce a decent crop. When I was an IPM scout on the Cape, one of the farms where I scouted potatoes was the Plymouth Co. Jail (hence “county farm”), where the prisoners grew a lot of their own veggies. The farmer there was a genial Irish guy called Frank with a ruddy, doughy face. He was a 40ish local farmer who decided that farming for the county jail was a much better option than trying to turn a profit on his own land (steady paycheck, less worry about droughts & plagues, no need to mortgage the next three generations for farm equipment), even if it meant doubling as a sometime C.O. I taught him how to time his Bt sprays (for potatoes and sweet corn) and how to hold off from spraying the flea beetles on his cabbages and rutabegas until the populations reached the “economic threshold.” I would make my rounds once or twice a fortnight and see how things were going. He was more than happy to learn about IPM but dealing with the prisoners sometimes got in the way. (Other guys I scouted, especially the guys that were big sweetcorn suppliers to the local supermarkets, didn’t give a rat’s ass about the IPM program they had signed on to with us — they sprayed Lorsban or Lannate whenever their muse inspired them. They really didn’t have much option since the Stop & Shop produce manager would reject an entire load if more than 2% of the ears had earworms.) One day that July I swung by the jail after not having been there in a couple weeks (prolly skipped it over the 4th). The potato field was nothing but stems. “I’m sorry Brian,” Frank said, as though he had to apologize to me for losing his own spud crop, “we had a few rough days and had to lock the guys down, so I didn’t have any labor (of course, he said “lay-buh”) to help me in the garden (which is what he called the jail’s 20-acre farm).” “Looks like you might have to buy your potatoes this year,” I said. “I dunno,” he said, “the stems are still green and if we keep them watered I think they’ll grow back okay. I’ve seen worse.” Looking at his field, that was hard to believe but I began to get a sense for why he traded in his own farm for the county’s. Sure enough, he didn’t abandon the potato field and after a couple weeks the foliage had more or less returned. The inmates calmed down and he was able to resume his Bt program against the potato beetles (whose population may well have temporarily crashed when they ate themselves out of their local food supply). When fall rolled around, Frank had over 80% of his normal yield, which was pretty darned good even without the massive early defoliation, considering how hot a year it was.

    Doc B sez yer best bet is to seek out those varieties that are most resistant to CPB and a couple that you like. If you can’t live without Green Mtn. or other more susceptible varieties, plant them in a separate patch that you can manage separately. Another option is trap-cropping (see the following websites for examples: http://www.mainepotatoipm.com/ipmfactsheets/cpb.pdf
    http://www.uvm.edu/vtvegandberry/factsheets/PerimeterTC.html). (N.B. Ruth Hazzard, who is mentioned in the UVM article, was my boss when I did my IPM scouting.) Also, if you really want to satisfy yer curiosity about how much you really need to worry about the little suckers, plant matching potato plots next year and only manage one for CPB. You may find that yer time is better served worrying about something else.


  2. According to Wikipedia:

    The female beetle can lay up to 800 eggs at a time, and up to three times per year.

    2400 eggs sounded like “thousands” to me…and when I look at the devastation on my plants, it’s hard not to say “millions”!

    Green Mountain is a good variety (good flavor, good “dry flesh” alternative to russets which don’t tend to do well in Maine), and worth a bit of trouble. As I see it, no matter what variety I plant, CPB will find it and eat it, so I just plant what I like, and then work with the CPB. Most years I get a decent crop, but it’s heartbreaking to see your beautiful plant reduced to twigs by horror movie type grubs the color of rotting flesh.


  3. According to Wikipedia, Tom DeLay (a former exterminator) is prolly a fine, upstanding man and Ken Lay, a “good man” and “straight arrow” (have these asssessments from his funeral made it to Wikipedia yet?) is prolly on his way to sainthood.

    According to the U. of Maine Cooperative Extension Service website, “Over a 4-5 week period, each female [CPB] lays eggs in batches of 20-35. A total of 400-600 eggs may be deposited by one female.” Of course, even this must be taken with a grain of salt since everyone who’s ever tried to rear any insect knows that “each female” doesn’t do what “every” female would do. Some don’t lay anything, whether because of problems in development or other hazards of being a wild creature, and whenever you see the qualifier “may” in a scientific text, it basically means that they’re talking about the record-holder of egg-laying or whatever else they’re studying. It’s like anything else. You could say that “each taxpayer will benefit from DB’s tax cuts and MAY receive over $1,000,000 back on their returns compared to previous years.” Anyway, the bottom line is still that under favorable conditions (such as huge, unnatural monocultures of a good foodplant) CPB populations can definitely explode before you can say “Rumplestilsken” (altho I don’t have precise data on this).

    It’s definitely dispiriting for a farmer to see his crop, that he’s worked hard to establish and is counting on to feed his family (whether directly or indirectly), being attacked by something and that’s the whole point behind IPM and economic thresholds: to give them hard numbers to rely on so that they can feel confident about when it’s worth it to spray. Of course, it’s easy for an ivory-tower type to pull up in his beemer (actually, I was pulling up in the UMass Cranberry Station’s ’74 Dart — a great car, btw) and tell the farmer not to spray yet but not all of them were cold-blooded enough to sit back and watch the bugs going to town on his livelihood.


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