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If you see many little holes under the leaves of your alfalfa plants or if you notice tiny tunnels being bored inside the leaves which become bigger and bigger and gradually curve to form an inverted comma, you have discovered signs of the presence of the alfalfa blotch leafminer.
Originally from central Europe, this insect first appeared in Massachusetts in 1968 and since then has made rapid progress toward the north. It can cover a distance of 50 miles or 80 kilometers in a generation. Since there are an average of three generations per year, except in the Lac Saint-Jean and Gaspé regions where there are only two, it is easy to see why this insect can now be found everywhere from the Maritimes to southern Ontario. It is unknown in the western United States and natural barriers seem to have spared northwest Québec, at least for the time begin.
The alfalfa blotch leafminer is a tiny black two-millimeter long fly with a reddish head and little white spots (balanced) behind the wings. After spending the winter in the ground in the pupal stage, it emerges as an adult. The females then pierce holes in the undersides of leaves to extract food and to deposit their eggs by sliding them inside. These eggs will hatch larvae after five to ten days. The larvae then start travailing by digging a tunnel up toward the upper layers of the leaf. These tunnels get progressively longer and wider and curve to form the characteristic shape of a large comma. Depending on the temperature, the various stages of the larva's development last three to ten days after which it makes an incision in the leaf epidermis and falls to the ground where it becomes a pupa. This cycle repeats itself in each generation of the insect.
Visible damage caused by the leafminer is considerable, but right now it is difficult to assess real damage. The perforations, which may amount to 250 per leaflet in cases of severe infestation, cause parching in dry climates, encourage some diseases in humid climates and lead to the loss of nutrients. It has been shown that perforated leaves lose amino acids potassium and protein nitrogen which are washed away during rains
Protein loss is especially important. Crude protein may be reduced from 9,4% to 6%, digestibility from 9% to 7% and dry weight from 7% to 5%. However, if varying conditions such as the time during the season and certain other factors are taken into account, crude protein loss may be as high as 9,45% and the dry matter digestibility rate may be reduced to 1 0,2%.
The leafminer has very few natural enemies and therefore has been able to settle wherever climatic conditions permitted. Infestation varies from region to region according to the season, the age of the alfalfa field and even the cutting plan. In Saint Hyacinthe, it was observed that in 1979, 28% of alfalfa leaflets had one or more leafminer tunnels, in 1980, 50% did; in Amqui, the rate was 23,7% while in Normandin, it was only 2,1%. The losses incurred under such conditions are certainly more significant than the assessments previously referred to. The effects of this insect are also reflected in the plant's ability to survive the winter. No proof has yet been furnished to support or refute this hypothesis but experiments are under way to assess the influence of repeated leafminer attacks.
The fight against insects which are harmful to crops must be unrelenting. Researchers have developed several strategies designed to put these pests out of commission. The nature and value of leguminous plants such as alfalfa, however, call for the development of methods which respect the natural balance governing plant/insect relations and which are as inexpensive as possible.
Chemical means In cases of severe infestation, chemical weapons must certainly be resorted to, but insecticide use is a double-edged weapon. Besides attacking the insect pests, these products also destroy their natural enemies. Moreover they are pollutants. From 1975 to 1978, tests were conducted on certain insecticides at Saint-Hyacinthe and some of these can be recommended.
The most effective means of combating the insect is to attack it during the most vulnerable periods of its life or to try to break up the insect's life cycle. Among these methods is early reaping which sometimes reduces insect populations and at least cuts down on losses. While it is true that the leafminer's cycle is well-adapted to the plant's development, a study also conducted at Saint-Hyacinthe showed that considerable population reduction was possible during the normal season by reaping about ten days before the budding stage. This technique prevents optimal harvesting. But if reaping takes place during the budding stage or at the beginning of flowering, this permits losses due to drying, leaching, disease and leaflet loss to be considerably reduced after leafminer attacks. In some cases, cutting at the 10% flowering stage gets rid of 40% to 50% of the leafminers in the field. However, the harvest should not be left in the field, since recent observations have shown that waiting five days may prolong the survival period of the pupa by 30% to 50%. This is due to the fact that the pupa is more sensitive to heat at the beginning of the pupal stage.
Leafminers are relative newcomers to Québec, and when they first appeared, their natural enemies were few and poorly synchronized with their insect targets. A program involving the introduction of European Parasites was carried out and now parasites was carried out and now two tiny insects called Dacnusas dryas and chrysocharis punctifacies are recognized as the most effective natural enemies. According to the work of Dr. B. Hendrickson of the United States, these two parasites are capable of reducing leafminer larval populations by 63% in 3 years.
In the near future specialists hope to perfect an integrated control program which would make possible an appropriate combination of parasite activity, cutting dates, the perfection of strains of alfalfa which are resistant to leafminer attacks, and the application of effective insecticides which respect the various components of the system. In order to achieve this balanced program, however, several points must first be cleared up. For example, we must increase our knowledge of the environment called the agro-ecosystem which includes these insect populations. The ability of the parasites to survive the winter must also be assessed. There are also studies to be done on leafminer biology in various regions of Québec and the data required to predict leafminer appearance and development dates to be collected and computerized.
At Université Laval, researchers are studying the factors which influence the insect's chances of survival. They are concerned with the leafminer's life during the vulnerable pupal stage as well as cutting plan techniques.
Copyright © 1982 Québec Agricultural Newsletter (Vol 6, no 5, June 1982)