56 



THE AGRICULTURAL NEWS. 



February 24, 1917. 



INSECT NOTES. 



WIND AS A FACTOR IN THE DISPERSION 



OF CERTAIN INSECTS. 



In the insect world when the competition for existence 

 in a given locality becomes too intense owing to overcrowding 

 and consequent lack of food, the pressure can usually be 

 relieved by dispersion to new localities. Some insects, such as 

 locusts, are very active at all stages of their lives and never 

 have any difficulty in moving wherever they will by their own 

 efforts. Others, for example some moths or beetles, are 

 comparatively helpless in the larviil stage and can only spread 

 as winged adults. Both these groups of insects are practically 

 independent of the wind, as are most of the larger and stronger 

 insects, although at times a strong wind does help them in 

 their long flights. There is, however, a large group of minute 

 and comparatively helpless animals which can only travel short 

 distances by their own powers of locomotion, and, but for 

 the fortuitous help of certain artificial or natural means of 

 dispersal, might, under certain conditions have great difficulty 

 in obtaining sufficient food. 



To this group belong such forms as scale insects, red 

 spiders and other mites (classed with insects for convenience), 

 and similar microscopic animals. 



It has been shown that the spread of these minute forms, 

 many of which (as in the case of scale insects), remain tixed to 

 one spot for the greater part of their lives, is often due to man 

 in his cultural operations or other activities, to birds, and to 

 other insects. These agencies may cause their dispersion to 

 considerable distances, but spread by this means can in the 

 case of man be controlled to a great extent, by rigid inspection. 



Recent experiments have shown that red spiders, scale 

 insects, etc., may under certain conditions be transported 

 through the agency of wind much farther than was formerly 

 thought possible, and this definite information has an impor- 

 tant bearing on the control of these forms. A brief account 

 of some of the experiments made in this connexion may be of 

 interest to readers of the Agricultural Xews . 



It has been known for .some time that red spiders, or 

 more correctly spider-mites, may be blown from one tree to 

 another in an orchard, but it was generally considered 

 that an open piece of ground between two orchards, or 

 a similar barrier, was sufticient to prevent a spread of 

 these pests from one orchard to another. Experiments made 

 in California in 1913 (Cilifornin. Monthly Bulletin, ■<, \i, 

 p. 777) by E.E. Munger, and later in the same yejr by H.P. 

 Stabler in connexion with the almond mite [Bryohia pruiends) 

 showed that these small animals may be blown by the wind 

 to a considerable distance. Sheets of sticky paper were 

 placed at varying distances from a 10-acre orchard of large 

 almond trees, and at ditierent elevations so as to face the 

 prevailing winds. Numerous spider-mites were found on the 

 sheets placed at a distance of 6.")0 feet, and at an elevation 

 .of 50 feet from the infested orchard, and on sheets placed at 

 the shorter distances. 'These experiments were repeated 



with precautions to preclude all possibility of the red spiders 

 having reached the paper in any other way than by wind 

 carriage. The results were again the same, and it is regarded 

 as established that red spiders are blown sufficiently far to 

 make infested orchards a menace to other orchards within 

 reasonable distances.' 



In California where citrus orchards infested with scale 

 insects are fumigated in such a way that practically all of 

 the insects are killed, this question of the reinfesting of treat- 

 ed groves from neighbouring infested groves is a matter of 

 no small importance. 



In the Journal of Economic Entomology for October 

 1916, we have an account of some experiments made by 

 Professor H. -J. Quayle, of the University of California, on 

 the 'Dispersion of Scale Insects by the Wind'. Previous 

 experiments made in 1912 by Professor Quayle on the 

 locomotion of young scale insects showed that it is hardly 

 possible for these small animals to travel over the surface 

 of the soil from one tree to another by their own powers of 

 locomotion. As mentioned above, it is known that man, 

 birds and even other insects may be responsible for the 

 spread of scale insects to considerable distances, but recent 

 experiments have shown that wind plays a very important 

 part in the dispersion of young scales throughout an orchard, 

 and to adjoining orchards. 



The results of the first series of these experiments showed 

 that young black scale {Saissetia oleat) were carried by the 

 wind for varying distances up to 4.'")0 feet from infested trees. 

 Tanglefoot sheets were placed in difterent situations and at 

 different distances from infested trees. The prevailing 

 wind was from the west and south-west, and many more 

 scales were caught on the sheets facing west and south than 

 on those having a nnrth and an east exposure. These ex- 

 periments were carried out simply to determine to what; ex- 

 tent the young of the black scale miaht be caught on tangle- 

 foot fly paper, and only lasted over a period of two or three 

 days, as it was found that in that time the sticky material on 

 the sheets became hardened in the sun. It may be mentioned 

 that the.se experiments and those given below were conducted 

 towards the end (pf .June when the definite annual production 

 of young scales was in progress, and it is evident that the dis- 

 persion by wind takes place mainly during the period of 

 young production from April to September in California. 

 Further experiments along the .same lines as the above were 

 made in September after the hatching of the scales was 

 over, and very few scales were caught on the tanglefoot 

 sheets. 



Another .series of experiments was made by Professor 

 Quayle to determine to what extent a grove freed from scale 

 by fumigation can be reinfested from neighbouring orchards 

 which were covered with young scales. 



A 4-acre block of citrus trees was chosen which had 

 been thoroughly fumigated in 1914 so that practically 100 

 per cent, of the black scale was killed. An examination of 

 this block in April 191-5, before the annual hatching of young 

 scales began, showed that no scales were then present. This 

 block of clean trees was surrounded by severely infested 

 orante trees on the v.est, south and east sides, while on the 

 north side was an abrupt slope of barren ground. The trees 

 were examined again in August 191-"), and from tests made it 

 was found that yrpunir black scales were distributed through- 

 out the entire 4 acres of trees, without question chiefly by 

 the wind. The tests also showed that the spread was 

 mainly in the direction of the prevailing south west wind. 



"These experiments are only of a preliminary nature, but, 

 so far as they go, establish the fact that, under certain 



