From UNH Cooperative Extension, this is Over-Informed on Vegetable IPM
Anna: Ohhhhh. This week’s pest is a good one. It’s notorious. It’s diabolical. It’s resilient. It’s kind of adorable. It’s the Colorado potato beetle, or CPB. I think CPB are very cute. They play dead if you scare them. You pick them up and they roll over on their backs and throw up on themselves! It’s a defense mechanism and, I don’t know why, but I find this very endearing.
Colorado potato beetle, of course a pest of potato, and it is native to North America but was not a pest insect until we introduced cultivated potato to its native range. It’s also got an important place in our modern history, which we’ll get into, but first The basics:
CPB overwinters in the adult stage, in the soil. When they emerge in the spring they walk search for food plants, where they eat potato leaves and lay lots and lots of eggs. Those eggs hatch and the larvae eat lots and lots of potato leaves, which reduces the yield of potatoes belowground. The fact that they WALK into potato fields during cool spring temperatures is important. Best management practices call for rotating fields out of potatoes or other solanaceous crops year to year and, if you can, keep potatoes some distance from previous crops – that’s at least 200 yard from where potatoes, tomatoes, eggplants where last year. Even better if there is a physical barrier between new and old fields, like roads or ditches, but trenches or straw mulch can be put in place as a barrier as well.
Once CPB do colonize a potato field, it is important to remember that potato plants can endure quite a bit of aboveground damage without negatively impacting belowground yield. After plants are mature – that’s after flowering and fruiting – you can lose almost all of the above ground tissue without impacting yield. So during the vegetative phase, scout fields by selecting 50 potato stalks selected from various parts of the field and counting the adults small larvae and large larvae. Treatment is not necessary unless you observe more than 25 adults, or more than 200 small larvae, or more than 75 large larvae.
Consult your management guide to select your protection approach and, remember, pesticides must be applied only as directed on the label to be in compliance with the law, so read those labels!
Alright so a little history: Like I said CPB is a native species and was first described by the American entomologist Thomas Say in 1824 as a thoroughly un-important species found feeding on buffalo bur, a wild solanaceous plant. Not until 1859 was this insect associated with crop losses, when it began destroying potato fields in Nebraska. After this, it spread eastward and was first reported in New Hampshire in 1876. For those of you who are tired of hearing about invasive insects coming here from other countries you should remember that other countries get them from us too. In the 1870’s, CPB was accidentally introduced to Europe where it was later given the name Yankee Beetle.
Of course CPB spread eastward through Europe and, by the 1950s, Soviet occupied areas of Europe were so heavily infested with Yankee beetle that propagandist claims that the United States had actually released these insects as a biological weapon didn’t seem so far-fetched. You can see old posters depicting the U.S. airplanes dropping these beetles in our show notes. I had a professor in grad school who grew up eastern Europe around this time and, although he denied believing this story, he kind of talked about it like he did, like the Americans held some blame. It’s a really devastating pest and it would make a mark on anyone’s psyche.
To really understand why CPB was and remains such an important pest, let me introduce you to the concept of the pesticide treadmill, a term was coined to describe how quickly pest insects can develop resistance to chemical pesticides. With CPB, resistance can develop so quickly that the pesticide industry must constantly develop new pesticides with new modes of action, new ways of killing the insect. By the time a new product is ready for use, we’ve already lost the old one, as though we’re on a treadmill. In natural settings, this is sometimes called an evolutionary arms race, or Leigh Van Valen’s Red Queen hypothesis, which is a reference to a line from a Lewis Carroll novel when the red queen explains to Alice that in the land beyond the looking glass “it takes all the running you can do, to stay in the same place”. Here all the “running” is the co-evolution necessary to avoid predation – this could be actual running, like being good at running away from a predator, but in plants and insects this could be chemical defenses. A plant might produce noxious chemicals to deter insects feeding on them but the insect might develop a way to tolerate those noxious chemicals. Obviously a co-evolutionary arms race takes place over a much longer timeframe than pesticide resistance. Heavy reliance on one class of insecticide really speeds up the process. But there are ways to slow this down. I spoke with someone who knows this story very well:
Andrei Alyohkin, University of Maine: To a degree, it’s a charismatic insect. It’s easy to spot. It’s kind of cute. Yet another interesting thing about it is it’s a reminder that we should not become particularly cocky…as in, I’m a human being, I have a good brain – I mean I have a big brain not necessarily a good brain – so I am invincible. That’s very much not the case because, for years and years, everything and the kitchen sink has been thrown at Colorado potato beetle. Hundreds of millions of dollars spent developing products and testing them. Some pretty smart people worked on the pest – I’m not saying I’m one of them for the record – but the problem is still there. While the problem is relatively under control, that may change any time. We’re a few product classes away from failure. Neonics, which essentially rescued the industry in the 90’s, they’re already kind of a pale shadow of themselves in many areas.
Anna: So you said that neonics came in to replace older chemistries and now we’re seeing the diamides and a couple other active ingredients, or classes, come into replace them. Is there anything being done now to prevent losing these classes?
Andrei: I believe there is a better awareness of the issue right now. The main thing people are doing is rotating chemical classes. That’s a good thing but it’s only one of the approaches. It would be nice if people used economic thresholds more and put less neonicotinoids at planting. It would be nice if we had better biological control options but, with Colorado potato beetle, it hasn’t been found to be very successful. They just outbreed natural enemies very quickly. I wish there were more soil management techniques – using cover crops, mulches, organic amendments. The main goal is to reduce soil erosion and increase soil fertility, but those techniques have been shown to create a less favorable environment for beetles and, to some extent, aphids. All these things I wish would be done more but there are advances here as well.
Anna: When you’re talking about measuring sensitivity of beetle populations, there’s a lot of talk about regional populations. Can you give me a sense of how big these regional populations are? Obviously there would be big differences between populations in New England versus the mid-Atlantic or out west, but how fine of a difference are they? Like are there differences between the beetles in Orono versus in Aroostook county or can they be really different from farm to farm?
Andrei: Yes. They can be different from field to field, literally. Beetles can be very mobile – they’ve been found 100s of miles out to sea – but if they don’t have to fly, they don’t like to fly. If they have food and they have mates, they will just stay put. As a result, we may have local populations that are separate from other populations, not because of big mountain ranges or anything, but because they aren’t very motivated to move. So there could be big differences [in chemical susceptibility] even on one farm from field to field.
Anna: Again I find this insect so adorable. Its really amazing that they are such a devastating pest because they are such wimps. They play dead and throw up on themselves when they’re frightened and to top it all off, they’re lazy. This is good news for us – or at least those of us that are able to rotate out of potato and other solanaceous crops. – the potato production Andrei is talking about are large areas, dominated by potato. If you can rotate crops, this the number one management strategy. Andrei also mentioned rotating chemical classes. For those of you familiar with the Insecticide Resistance Action Committee classifications, or IRAC classifications, the numbers you see on insecticide labels. According to Andrei those were first put there by chemical companies to help people slow the rate of insecticide resistance specifically for CPB. This classification scheme has grown in use, with recommendations for avoiding chemical resistance in other insect groups, as well as for fungicides and herbicides.
I asked Andrei about what’s new in Colorado potato beetle research:
Andrei: Right now the big thing is beetle genetics. The beetle genome has been sequenced. Now that its been sequenced, people start thinking about what they can do with those sequences. One of those things is trying to track down the movement pathways, where different populations came from, where they are going. Almost archeological interest in trying to retrace ancient caravan routes for Colorado potato beetles, except they’re not necessarily that ancient.
Another area where people are working is in trying to identify particular genes responsible for certain traits, including pesticide resistance. The big question is why resistance is such a problem in the eastern United States, particularly the northeastern United states and Atlantic Canada, but is not much of a problem in potato growing areas of the west coast. If we can find out why, perhaps we can find out what to do about it. At this point, we don’t really know why. It could be genetic differences. It could be growing practices. It could be environment. It could be interactions of all of that.
Anna: So a tough pest but lots of ways to slow chemical resistance. Stay tuned because we will be returning to the topic of pesticide resistance as well as pesticide resistance in CPB. So with that, my thanks to Andrei Alokhin from the University of Maine. I would suggest going to his website, there’s lots of great stuff there. And a special thanks to Brentwood’s favorite son, Jason Lightbown, who wrote and performed our theme music.