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| Department of Agriculture and Aquaculture |
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I am amazed these days, when I walk through the isles of the supermarket, to find so many products that are certified organic. You can find everything from organic cake mixes and cookies to organic olive oil and organic baby foods. The fresh produce aisles have sections devoted to organic fruits and vegetables, including potatoes.
With this growing interest in organic foods, many producers are thinking about the possibilities of growing organic products on their farms. Despite the fact that potatoes are one of the most difficult crop to grow organically, the presence of organically grown potatoes on the store shelf demonstrates that it can be done.
As with the production of most organic products, excellent management is a must. Since neither traditional pesticides nor genetically modified varieties are allowed, pest control involves an array of options not commonly associated with traditional potato production.
One of the most significant pests affecting the potato crop in the Maritimes is the Colorado potato beetle. This is an insect with a phenomenal capacity for adaptation and was able to develop resistance to DDT in as little as 6 years. Similar resistance has developed to other pesticides.
Rotation is the first method of control that should be considered, when dealing with the Colorado potato beetle. The goal is to arrange your rotation in such a way that this year's potato fields are as far away as possible from last year's potato fields, including your neighbour's. Adults usually over-winter in the ground, in hedgerows or protected areas near the potato field where they fed the previous summer. When they emerge in the spring, they like to be able to walk or fly a short distance to their food source. If the new field of potatoes is nearby, they will find it immediately, start eating and lay their eggs. The farther away it is, the fewer the number of beetles that will be able to find it, and the later they will arrive. According to studies, only 20% of the beetles will find a new field at 800 metres and 50% at 300 metres. Any distance provides some protection, even if it is only the width of another field, planted in cereals or other crops.
If you cannot place your field at a sufficient distance from last year's field, you will probably notice that infestations will start on the edge of the field closest to where the beetles over-wintered. It is possible to plant a few rows of potatoes early as a trap crop, and to destroy the beetles on them. In general, this will not be sufficient. It has been shown that 70% of the adults emerging in the spring spend less than 24 hours on the first 3 trap rows. If you regularly experience massive migrations to your fields in early spring, you can resort to the use of plastic lined trenches. Shortly after it is applied, the plastic will conform to the contour of the trench. The beetles that walk to the field will fall in and will not be able to escape as the electrically charged particles of dust on the plastic stick to their legs and prevent escape. A 50% reduction in adults and in egg masses has been demonstrated with the use of this technique.
Floating row covers can also be used, but you must be absolutely certain that beetles are not over-wintering in the field, or you will have major problems.
Other methods have also been tried. A machine called the "Beetle Eater" was manufactured in New Brunswick for a while. The machine used a forced air current to dislodge the beetles and at the same time used powerful suction to catch them in mid-air and pulverize them. One of the problems with this equipment was that beetles have a natural defense mechanism that makes them fold their legs and drop to the ground if they perceive that a danger is imminent. You can imagine that the passage of a big tractor overhead did worry them a bit. To avoid this, the beetle-eater was front-mounted. But it takes a lot of power to dislodge large larvae and the results were not as good as was hoped for. Although 48% of the adults were removed, only 39% of the small larvae and 27% of the large larvae were removed. The larvae are usually responsible for most of the damage in potato fields, as they hurt the plant during the critical growth period, which generally corresponds to flowering.
The propane flamer is another method of control. It was most popular with conventional producers in the 1990s, when beetles were difficult to control with available chemicals. It is still manufactured in New Brunswick. The flamer will kill 80% of the adults in the spring in a single pass, if it is well timed. As a consequence, there will also be 80% less egg masses and larvae in the field during the summer. Success with this machine is very dependent on proper timing. It must be used early, on warm sunny days when the adults are seen on the plants in great numbers and when the plants are smaller than 6 inches in height. The initial effects on the potato crop can be alarming. The plants will turn gray almost immediately and in the days to follow, they will react as if there had been a terrible drought. After a week, they will have recovered and it would be hard to detect any damage. Yield and quality are not affected with a single pass when plants are less than 6 inches in height. Tuber set is increased after a single flaming while tuber size is reduced slightly.
Natural predators are not sufficient to reduce beetle populations in the field, but biological control is nevertheless possible. It has been shown that multiple releases of stinkbugs at the beginning of egg laying, can be as effective as chemical pesticides. In trials conducted recently in New Brunswick, egg masses were reduced by 50% and larvae by 90% with multiple releases of stinkbugs. Unfortunately, it is difficult to locate supplies of stinkbugs for field application and costs are high.
It is also possible to favour the spread of naturally occurring diseases. Beauvaria is a fungal disease that attacks beetle adults and larvae. This product is registered in the US under the name of Mycotrol. In experiments on potatoes, it provides some protection, but the level of control is often not adequate.
Bt products such as Novodor, ( Bacillus thuringiensis subsp tenebrionis ) are very effective at controlling beetle larvae. If timed properly, with the 1st application at 10% egg hatch and 2 others at 5-7 days interval, effectiveness can be equal to that of conventional insecticides.
One of the preferred pest control methods in organic agriculture is the use of resistant varieties. Unfortunately, all non-GMO commercial potato varieties are considered susceptible to the beetle. To breed resistance into the potato, researchers breed it with wild solanum species. Several mechanisms of resistance are involved in these species. One of them is the presence of glandular trichomes. These are small hairs with sticky ends that contain natural chemicals that deter feeding and affect the digestive physiology of the insect.
It takes time to obtain a variety that has both a high level of resistance and acceptable quality, but already there are selections being tested that have improved resistance to the Colorado potato beetle with acceptable yields and quality. In tests, some of these selections have shown only 50% of the damage suffered by Russet Burbank in the same plot. This, of course, is far from complete resistance but when combined with other methods of beetle reduction, the use of resistant varieties may be enough to reduce damage to acceptable levels.
Various other products have been tried over the years to control the beetles. Rotenone, a natural product extracted from the Derris plant is registered for use in Canada against many potato insects, but I cannot attest to it's efficacy. We have used Neem, a product derived from a tropical tree and Trounce, an insecticidal soap containing 20% potassium salts of fatty acids and found both to reduce larvae numbers quite effectively. Trounce, unfortunately was quite phytotoxic at the rate used. Neem is not registered in Canada.
We have also tried a concentrated garlic extract made by Guardian and have found that it had little more effect than pure water on beetle larvae and adults.
The potato flea beetle is another insect that could give organic potato producers a bit of trouble. Flea beetles lay their eggs at the base of plants in the spring and the small larvae that emerge feed on tiny root hairs for a while. They pupate in the soil and emerge as adults around the 1st of August. Damage is not usually important in the spring, because the plant is vigourous, but it can be severe in the fall, especially if the plant is already stressed by a lack of moisture.
Populations can be extremely high in the fall, and we have seen fields go down in one week. To prevent this, organic producers could use the flamer in the spring. This will reduce flea beetle numbers by 80% for the length of the season.
Aphids are another common insect in potato fields. In conventional potato production, they often become a problem after the use of insecticides, aimed at other pests, have eliminated natural predators. In organic production, where predators are abundant and fungi, that attack aphids, are not destroyed by chemicals, one would not expect aphids to be a huge problem on potatoes. If there were localized infestations, the use of soaps or biological control agents such as the ladybug could be used effectively to control them.
When it comes to seed production, where aphids are major vectors of virus diseases, it would seem that organic seed potato production would be very difficult, however such is not the case. Viral diseases are best controlled by the use of clean seed, careful roguing, and early top-killing, rather than by the use of pesticides.
There is cause for concern however about another insect, the potato leafhopper. This is an insect that flies in every year from southern locations. Both the adults and the nymphs are able to transmit a toxin to potato plants. The flow of sap is restricted and the leaves gradually die off. Yield reductions can be quite important. A combination of rotenone and natural pyrethrum has been suggested for its control. Fortunately, heavy infestations of leafhoppers do not happen every year.
While there is no fail-safe method available for organic control of the insects that affect potato production, there are numerous options available that offer some degree of protection. On-going research and on-farm experience will continue to improve and increase the options available and make organic control more effective and more economical.
NOTE: It is important to note that some of the pest control products listed in this document may not be permitted for use by all organic certification agencies. It is therefore recommended to verify with your certifier prior to using any of these products.
March 2002