Sunday, June 14, 2009

Fighting Colorado Potato Beetles in the Adirondacks

Last year it seems my yard was the staging ground for every potato beetle in Newcomb. No other gardeners I’ve spoken with seem to have had any, yet my potatoes were covered and defoliated faster than my patrols could keep up. This spring I resolved that I would not fall victim to these insects. I ordered an organic-certified insecticide, and read that by planting my ‘taters later (say, mid-June), I could avoid an infestation.

Well, I went out in the garden the other day (after planting my potatoes…I just couldn’t wait another week), and found a Colorado Potato Beetle on a potato plant that had just emerged, a sprout from an overlooked potato from 2008. I crushed it beneath my boot. Then I found another…and another. A couple days ago I went back out and found that not only did I have adult beetles grazing on these resurrected plants (they are sprouting up all over the place – I must work on my potato digging skills), but they were mating and laying eggs. I smooshed several clusters of the brilliant orange eggs before I went inside and mixed up a batch of spray. Thus armed, I commenced my attack. Then it rained.
Today I decided to read up on CPB to see what the common wisdom is on the subject. What I discovered is not promising.

It seems that the CPB is actually a native insect (for a change). However, its native land is the eastern Rockies, where it fed happily on Buffalo Burr (Solanum rostrum), a native plant that is a relative of the potato. In the early 1800s, settlers moving westward brought potatoes with them to plant, harvest and eat. It took only 30 years for the beetle to discover potatoes and claim them as a new host plant. From then on it was all downhill for the potato. By 1859 CPB was officially a pest. It moved eastward at a rate of about 85 miles a year, and now it is a pest not only here, but also in Europe and it’s heading for Turkey.

Farmers started to attack the CPB with lead arsenate, a popular pesticide in the late 1800s. I don’t know about you, but “lead arsenate” doesn’t sound good to me. Times have changed, but chemical pesticides are still very toxic. And, just to make things worse, CPBs have become one of those “Super Bugs” – insects that have developed resistance to all sorts of pesticides. What is a gardener to do?

First, one must learn the life cycle of this pest. Let’s start with the adult. It is winter, and snug underground a potato beetle is pupating and turns into an adult. Spring rolls around; the ground warms up. Potatoes that were accidentally left in the ground (or those planted early by eager potato growers) are now sending up their first green shoots. The adult beetle emerges, finds its favorite host plant, and commences to eat the leaves. Soon, it looks for a mate. The female then lays clutches of 10-30 bright orange eggs on the underside of the potato leaves. Within her lifetime, she can lay up to 350 eggs (and she only lives for a few weeks).

The eggs hatch and the larvae start to feed. As they grow, the larvae molt, each successive molt producing a new “instar.” The fourth instar eventually drops to the ground, where it burrows to pupate. If the summer is in full swing, the adult will emerge to start the cycle again. If it is fall, the adult will stay underground until the following spring.

How long is it from egg to adult? That depends on the temperature. If it is cool, say averaging about 60 degrees Fahrenheit, it may take a month. If it is warm (mid-80s), it could happen in 8 to 10 days! Yikes!

Both larvae and adults eat the leaves of the plants. The potato plant can tolerate up to 30% defoliation prior to the blossoming of its flowers. However, a loss of more than 10% of its leaves once the tubers have started to form will result in a decrease in tuber production, both in number and size. The infestation I had last year ate up to 90% of the leaves.

Now, if you are smart, you start crushing beetles and eggs as soon as you see them. If you only have a few, this is easy, but if you are invaded in numbers beyond counting, you may have to take other steps.

Some sources suggest that you plant early. Not going to happen here – the ground would be frozen. Other sources say to plant late; if you delay planting, you might miss the emergence of the first adults and they may just starve before your plants sprout, thus breaking the life cycle. Still, other CPBs may move in from the surrounding countryside later on. Problem not solved.

What about crop rotation? My research suggests that this can work for large commercial operations, but the backyard gardener isn’t going to benefit from this.

Natural predators? Nope. Those that do exist seem to have little impact on the overall population.

Supposedly companion planting with bush beans helps, but you couldn’t have proved it by me last year. Eggplants are said to be good sacrificial plants, attracting CPB away from your potatoes, but I’ve never had any luck growing these. (Keep an eye on your tomatoes as well, for, like eggplants, tomatoes are related to potatoes and can serve as an alternate host.)

Insecticides seem to be the only option left. There are some organic pesticides out there, such as those made from botanicals like neem and rotenone. Because they are botanicals, they are short-lived and must be applied often.

Application of pesticides seems to be most effective if applied to the larvae in the first and second instars. You should start spraying when you see 5-10% of your plants with larvae at this stage. If you wait until the third or fourth instars, spraying won’t be very effective. Adults and eggs are not impacted at all (so much for the spraying I did this week). If you can get 15-30% of the larvae sprayed, you will be doing well. Good luck.

Bonide has another organically approved pesticide I read about, and it is supposedly the only thing out there that will nail the adults (besides your foot). I looked it up. This product contains spinosad, which is a bacterium that was found in the soil at an abandoned rum distillery in the Caribbean in 1982 (and hasn’t been found since). It acts through ingestion, where nervous system of the insect that consumes it becomes “over-excited” and death follows in 1-2 days. Insects that suck nourishment from plants (like aphids), and predatory insects (like damsel bugs) are not impacted, but bees are. Bees are hugely impacted by this pesticide, and beneficial wasps like Braconids and Trichogramma are also affected. Lacewings and ladybugs, two more beneficial insects, are also known to be adversely affected, but to a lesser degree.

Hm. I think I’m having second thoughts on using this stuff.

Many years ago I spent three summers working at Camp Treetops. I seem to recall that the farm manager at the time had a unique method for controlling CPB: he pureed the adult beetles and sprayed the resultant schmezz on the potato plants. He claimed it worked very well to repel further infestations. I have yet to find any corroborating evidence for this. If anyone out there has tried it with success, please let me know!

In the meantime, I think I will resort to the hand and foot method, patrolling my potatoes every evening for adults and eggs, squashing any I find. Maybe this year I can keep the problem under control.

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3 Responses

  1. Ellen Rathbone says:
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