THE AIRPLANE IN FOREST-PEST CONTROL

J. S. YUILL, C. B. EATON.

The airplane has become a new weapon in the never-ending battle against destructive forest insects. As in military operations, it is bringing about radical changes in strategy. Aircraft are serving two purposes in this phase of forest protection: For detection surveys to locate serious insect outbreaks and for the application of insecticides to control dangerous infestations.

The extent to which those operations can be carried on from the ground is seriously limited because the areas involved are often large and remote, and because the cost of ground operations in forests is high, even under the most favorable conditions. Many outbreaks of insects in the past consequently have had to be allowed to run their natural course until eventually they were checked by exhaustion of the food supply, changes in weather conditions, increase in the abundance of natural enemies, or other factors. But, in contrast to ground equipment, airplanes can cover large and isolated areas quickly and in most cases at a reasonable cost. Although improvements must be made in equipment and procedures to develop aerial methods for extensive general use, the progress since the Second World War has been encouraging.

Finding the enemy, estimating the numbers, and determining the rate of movement are as essential in combating insect outbreaks as in conducting a successful military campaign. The Bureau of Entomology and Plant Quarantine, in cooperation with various Federal, State, and private agencies, carries on extensive surveys each year to obtain such information for planning control operations. The work commonly includes cruising representative sample plots; reconnaissance inspections by truck, horseback, or foot; and visual examination from mountaintops or other vantage points. Obviously, the surveys are limited by the relatively small proportion of total forested area that can be covered in a season. In the search for better and faster methods, the idea was advanced that if the observer could use a moving observation point an airplane--instead of a mountaintop he could cover much more territory in a day.

THE FIRST AIR SURVEYS of defoliating insects were conducted in Canada in 1922 and 1923. In a week, air-borne observers mapped several thousand square miles seriously defoliated by the spruce budworm; by ground methods, that work would have taken 3 to 4 months. In following years, limited air surveys were made in both Eastern and Western States to detect and map several other insect outbreaks. The disadvantage in all these attempts was that only the severe infestations could be detected; the lightly infested areas could not be distinguished, with the equipment of that time, from the areas that were uninfested.

From 1925 until the outbreak of the Second World War, periodic attempts were also made to use air surveys in connection with bark beetle control. Flights made over western forests were disappointing. Dying trees could be seen from the air, but equipment had not been developed for mapping accurately their location or determining the type and size of trees attacked. In the Eastern States, air surveys were most successful for locating trees infected with Dutch elm; disease, which is transmitted by elm bark beetles. Observers flying at slow speeds in an autogiro could easily detect trees showing symptoms of the disease and could pin-point their location on a base map.

MORE RECENTLY, the depletion of forest resources during the war, the greater need for more adequate control of forest insects, and the wartime improvement in the aerial observation methods gave further impetus to surveys from the air.

Three methods are used : Sketch mapping, ocular estimating or strip counting, and photographic sampling.

The first is a "look-see" method similar to that employed in the early defoliator surveys. The area is covered in a systematic pattern and observers sketch in the boundaries of infested areas on previously prepared maps. Estimates of the extent of damage are made as the mapping progresses. In the Pacific Northwest an outbreak covering more than 700,000 acres was surveyed in this manner in 1947 at a cost of about one-tenth of a cent an acre. The method is still inadequate for detecting very light defoliator infestations, but recent improvements have made it a good way to get a quick, rough estimate of the insect conditions over a large area.

Ocular estimating is being used primarily for the bark beetle surveys in western forests. In this method the plane is flown along predetermined lines over the forest. The observer watches the ground through a port in the bottom of the fuselage and counts the number of dying trees in the sample strip traversed. The estimates obtained are then checked by limited ground surveys at various points within the forest area covered from the air.

In photographic sampling, representative localities within the forest are photographed with a special aerial camera. By taking pictures that overlap, stereoscopic methods can be used to pick out the dying trees and to estimate their size and crown characters.

These new improvements have already widened the scope of forest-insect surveys. Although the air surveys still supplement rather than replace ground methods, we expect that further improvements, particularly those in aerial photography, will make it possible to do more and more of the work from above the forest instead of in it.

INSECTICIDES were first applied from the air in 1921, when a small infestation of catalpa sphinx in Ohio was controlled by dusting lead arsenate from an open-cockpit biplane.

Soon afterwards, dust applications were made in the United States and Canada against the hemlock looper, spruce budworm, and gypsy moth, and in Europe against the nun moth, pine looper, cockchafer, and other pests. The results varied. The insecticide dusts killed most of the different insects, but the method of application had three shortcomings : The dust was frequently carried away by air currents after release from the plane; the dust particles did not stick to the tree foliage and were quickly removed by strong wind or rain; and, with any of the insecticides known at that time, the quantity of dust required made treating costs high about $7 an acre.

Later, concentrated arsenical and fluorine sprays were developed to re-place dusts. The sprays were less affected by wind and adhered to foliage much better, but the quantity of insecticide needed was still too high for economical application by airplane in this country. Aerial distribution of both sprays and dusts continued to be used to some extent in European forests because of the higher values at stake.

THE DISCOVERY of the astounding insecticidal properties of DDT in 1943 revolutionized airplane spraying. Here was a chemical that by previous standards was unbelievably toxic to many insects and was therefore just what was needed to make aerial application practical. Soon the military forces were spraying entire islands in the Pacific to kill mosquitoes and other disease-bearing insects, and when DDT and other new organic insecticides became available for civilian use, airplane applications were tried on crops and forests. The results of the trials in forest spraying were so encouraging that the insecticides have been put to a wider use each succeeding year.

Much of the forest spraying has been limited to applications covering fewer than 1,000 acres, although in 1947 413,000 acres of western forest land were successfully treated for control of the Douglas-fir tussock moth and in 1948 more than 200,000 acres of eastern woodland were likewise treated for gypsy moth. DDT sprays have been so effective against those insects that airplane spraying has become the standard method of control.

In experimental tests, good results also have been obtained in controlling the spruce budworm, hemlock looper, pine sawflies, and the Saratoga spittle-bug, but with bark beetles and certain other insects control has been unsatisfactory. Thus, airplane spraying does not solve all forest-insect problems, primarily because of the difficulty of obtaining a uniform deposit on all trees and on all parts of a tree.

The method is most effective for the foliage-feeding species that actively move about in the tree crowns; with them, a uniform deposit is not necessary, because their normal activities eventually bring them in contact with a lethal dose of insecticide. The less active defoliators and those in protected, situations can probably be controlled, but higher dosages or multiple applications may be required to compensate for the uneven distribution of the insecticide.

It has not been possible to obtain an efficient deposit of DDT sprays on tree trunks or other vertical surfaces with aerial application.

THE SPRAY MIXTURE most commonly used in forests is a solution of DDT in No. 2 fuel oil. The DDT is first dissolved in a naphthenic hydrocarbon solvent and then diluted to the desired volume with the fuel oil. The usual dosage rate is 1 pound of DDT in 1 gallon of liquid an acre, although under favorable conditions dosages as low as 1/4 pound in 1 gallon or less an acre have been effective for some insects. In spraying watersheds where fuel oil might impart an objectionable taste or odor to domestic water supplies, xylene is used as a solvent and kerosene substituted for fuel oil.

Emulsions and suspensions have been used to a limited degree in experimental work. The former are prepared by first dissolving the DDT in a solvent as in preparing oil solutions, then adding an emulsifying agent and diluting with water. Emulsions have two serious disadvantages : They cannot be exposed to freezing temperatures and they are more toxic to fish and other aquatic animals. Suspensions are made by dispersing wettable powders or so-called colloidal preparations in water. Wettable powders have not been satisfactory, because the suspended material tends to settle rapidly after mixing, clogging the equipment. The colloidal dispersions have not been adequately evaluated.

THE AIRPLANES most commonly used in 1948 to apply sprays were military biplane trainer-type aircraft (N3N and Stearman PT-17) , which can carry about 80 gallons of spray and operate at 75 to 90 miles an hour. They are not ideal for the purpose, but they perform reasonably well, and, being war-surplus items, their initial cost is much less than that of many nonmilitary models.

Several other types of planes also have been employed; the light, high-wing monoplanes that fly at 60 miles an hour and carry only 25 gallons of spray, up to multiengine transports that fly at 150 miles an hour and carry 1,000 gallons of spray.

Helicopters, tested in experimental work, may prove useful in specialized operations because of their ability to fly low and slowly and to maneuver in small areas.