Understanding, Forward Thinking About CPB

What's been done and what may come

Published in the March 2009 Issue Published online: Mar 30, 2009 Andrei Alyokin, University of Maine
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The Colorado potato beetle (Leptinotarsa decemlineata) needs little introduction to growers. As soon as plants come out of the ground, the beetles seem to appear out of nowhere. Both adults and larvae feed on the foliage, and if no control action is taken, they can completely destroy plants.

Compared to some other insect pests (e.g., the Biblical plague of locusts), the Colorado potato beetle is a relatively recent problem. Native to the southwestern United States and Mexico, it was first collected in 1811 by famous American naturalist Thomas Nuttall. It did not even become known to science for another 13 years, when it was finally described as a new species by Thomas Say.

For more than four decades, wild populations did not cause any trouble, peacefully feeding on buffalobur and several related species in the family Solanacea. However, the situation changed dramatically after the beetle's native range was settled by farmers of European descent.

The settlers brought with them potatoes and started to cultivate them over considerable acreages. Unfortunately, the potato proved to be a highly suitable host plant for at least some of the wild Colorado potato beetles present in the area.

They exploded in numbers and spread throughout potato-growing areas like a wild fire. Without any real means for beetle control, the growers were largely defenseless against the new pestilence and suffered heavy losses.

STUDYING CPB

Despite all the technological advances of the past 150 years, the Colorado potato beetle still represents an ominous threat to potato crops.

A considerable amount of research effort has gone into studying this pest. I remember being overwhelmed with all the literature on the subject when I first started graduate school at the University of Massachusetts in 1994. It seemed like every possible experiment has been already conducted, and there was nothing I could possibly do for my dissertation.

When I shared my concerns with my advisor, Dr. Dave Ferro, his immediate response was "Well, but the problem is still here, isn't it? So, obviously there is more work to be done. Now go and help Andy wash the cages."

Interestingly, 10 years later, I was approached by my own graduate student with exactly the same concern, and gave him exactly the same response. That sort of made me feel old .

VARIABLES

So, does this make agricultural entomologists a bunch of losers, good for nothing except wasting taxpayer money? Well, not necessarily.

First, although the problem has not been solved once and for all, it is largely contained. If appropriate control measures are taken, the beetle populations could be suppressed well below damaging levels.

Secondly, despite its timid appearance, the Colorado potato beetle is actually a very formidable nemesis. It is very prolific, with one female laying 300-800 eggs. Should need arise, the beetles can easily fly for several miles. This species also has a complicated and diverse life history, distributing its offspring in both space (within and between fields) and time (within and between years).

As a result, nailing it down with whatever control technique we are trying to use is rather complicated. On top of this, the beetle has a very impressive ability to evolve resistance to our control efforts. In other words, managing it is no walk in the park.

LIFECYCLE

Colorado potato beetles overwinter in the soil as adults, often aggregating by flight and by walking in woody areas adjacent to fields where they have spent the previous summer.

The state of hibernation is induced by decreasing day length in the autumn and lasts for at least three months (some beetles are known to skip the following season and remain in the state of hibernation for two or more years).

After that, the beetles respond to elevation of temperature by emerging from the soil and colonize potato fields both by flight and by walking. If the fields are rotated, the beetles are able to fly up to several miles to find a new host habitat. Mating starts before beetles leave for the host habitat, with over half of the population mating within the overwintering sites. Most overwintered adults usually die during the second summer of their lives.

Eggs laid by the overwintered adults begin to hatch within one week. Newly emerged larvae disperse over short distances and almost immediately start feeding on potato foliage. The larvae spend most of their time on plants.

They undergo three molts, with the last (fourth) instars burrowing into the soil to pupate. Larval development usually takes 10-20 days, and pupal stage lasts for another 10-15 days.

The first-summer-generation adults dig out of the soil and climb back on potato plants. It takes them about 7-9 days to develop a reproductive system and flight muscles. After development has been completed, the beetles mate and start laying eggs. The reproduction continues until shortening days tell the beetles that it is time to prepare for winter.

MANAGEMENT OPTIONS

Currently, synthetic insecticides remain the foundation of Colorado potato beetle control on commercial farms. Although it is not politically correct to say this in the academic community, but the situation is unlikely to change in the near future. The only other readily available effective technique is crop rotation.

Although the beetles can eventually find rotated fields, it usually takes them a while (beetle senses do not appear to be particularly efficient in finding host habitats) and many perish on the way.

Unfortunately, adequate separation between rotated fields is often difficult to achieve for operational reasons. Still, beetle colonization is often reduced and delayed even when rotated fields are located close to each other.

Other management approaches still largely remain on the experimental stages of development. Biological control, which is often promoted as a safe alternative to chemicals, is rather challenging because natural enemies cannot kill fast enough to keep up with the highly prolific Colorado potato beetle.

Artificial augmentation of natural enemy populations is usually prohibitively expensive. The only natural enemy used with some success in a commercial or semi-commercial setting is a pathogenic fungus Beauveria bassiana that can be bought from a number of suppliers and applied with a regular pesticide sprayer.

Recent identification of chemicals that attract the Colorado potato beetles to host plants and mates raised a possibility of luring beetles to certain areas of the field and killing them there instead of treating the whole fields. However, field testing of these compounds has so far produced somewhat inconclusive results. The same applies to attempts to develop a beetle repellent.

Host plant resistance is a promising, yet currently underutilized, approach to the Colorado potato beetle management. Potatoes were among the first plants to be genetically engineered to resist insect herbivores, but for economic and political reasons transgenic potatoes were withdrawn from market after only five years of use.

Traditional breeding for Colorado potato beetle resistance proved to be a big challenge. However, the recent release of traditionally bred "King Harry" cultivar with broad insect resistance (including the Colorado potato beetle) gives hope for further advances in this area.

While insecticides provide a valuable tool for beetle control, they are not exactly the ideal solution. First, while sometimes exaggerated, their environmental and health hazards are very real and potentially extremely serious.

Secondly, the beetles have an impressive ability to develop insecticide resistance. Since the middle of the last century, the beetle has developed resistance to 52 different compounds belonging to all major insecticide classes. Resistance levels vary greatly among different populations and between beetle life stages, but in some cases can be very high (up to 2,000-fold).

I remember being amazed when I first saw in 1994 an absolutely intact imidacloprid-treated potato plant surrounded by a circle of dead beetles. Well, nine years later I was equally amazed seeing perfectly content beetles chewing away on a plant treated with a higher dose of the same compound.

Finally, insecticides cost money, and good insecticides cost good money.

GOAL SETTING

I believe that many of the common problems in pest control arise because people set wrong goals for themselves. The major objective is not killing all the pests; it is to avoid yield losses. Well, it is not necessarily the same.

Potatoes can tolerate 30-40 percent defoliation during early growth stages, 10-60 pecent defoliation during middle growth stages, and up to 100 percent defoliation late in the season without noticeable yield reduction.

Not only does chasing every single beetle with a sprayer result in a waste of time and money, but also contributes to rapid resistance development.

Trying to kill all the beetles with insecticides usually results in killing all susceptible beetles. Only resistant beetles survive (that's why they are called resistant in the first place). When resistant beetles mate with each other, all their progeny are resistant. When resistant beetles mate with susceptible beetles, their progeny are less resistant, and usually can be killed by the full label rate of insecticide.

Growers who abuse insecticides trying to make their fields spotlessly clean sometimes remind me of bodybuilders who use steroids. Sure, it might look nice to some, but it is rather pointless in the short run (isn't working out supposed to be about being healthy and attractive to the opposite sex?) and extremely damaging in the long run.

SEEKING SUCCESS

The secret of Colorado potato beetle's success as a pest is its diverse and flexible life history coupled with a remarkable adaptability. Therefore, to be successful in our control efforts, we also need to be diverse and flexible in our approaches, as well as adaptable to ever-changing circumstances.

Mindless reliance on a single tactic is doomed to fail, no matter how fundamentally sound this tactic is. The only sustainable way to manage this insect is integration of multiple control techniques based on a scientifically sound understanding of its biology.

Using a variety of different insecticides with different chemistries, applying them only when necessary according to the economic threshold recommended by the local cooperative extension, selecting chemicals least damaging to natural enemies and rotating fields as far away from the previous year's crop as possible are likely to keep beetles in check for the years to come.

For more information, visit www.potatobeetle.org, a new website dedicated to the Colorado potato beetle.

Editor's note: Alyokhin has established www.potatobeetle.org, which includes a variety of information including beetle biology and management options.

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