3U 



THE AGRICULTURAL NEWS. 



October 6, 1917 



INSECT NOTES. 



WARBLE FLIES. 



Cattle in Europe nnd North Aji;erica are subject to the 

 attacks of warble fly maggots or 'bots', which cause 

 swellings or 'warbles' to form under the skin of the back. 

 Subsequently the hide in thi- region bec.iines perforated with 

 holes through which the thick fleshy maggnts emerge, 

 dropping to the ground to pupate, and a few weeks later 

 appearing as adult flies. The two common species of 

 ■warble flies attacking cattle are Hypoderma lineata and 

 H. bovis; both species are common in Europe, while 

 M. lineata is *he common species in the United States 

 and Canada, although the other occurs. The life-history 

 of a warble fly (the two species may be treated as one) 

 pre.sents several features of interest,- for, although the 

 development of the later stages of this fly has long 

 been known, there are certain points in its life which for 

 many years have been a puzzle to workers in Europe and 

 North America. It is only quite recently that the mystery 

 has been cleared, and the complete life-history of this pest 

 established with some degree of certainty. 



An interesting account has recently been given by 

 Mr. Warbnrton* of the way in which our present knowledge 

 of the life-history of the warble fly has been reached. In 

 preparing the following short sketch the above report has 

 been drawn on, and the more recent literature on the subject 

 of warble flies has been consulted, as summarized in the 

 Ej-perinunt Station Record, Washington, and in the Review 

 oj Applied Entomology, London, since in most cases the 

 original leportsand articles have not been available. 



About thirty-five years ago the late Miss Ormerod 

 issued a popular leaflet on the warble fly after years of 

 study on this pest in England. The main |)oints of this 

 leaflet, as given in the above report, are as follows: — ■ 



'Warble flies lay eggs in the backs of cattle during the 

 summer. The grubs live in the hide and set up irritation 

 on the products of which they live. In early spring the 

 swellings on the back develop openings to the exterior into 

 which the grub introduces its tail, which bears the breathing 

 pores, and here it remains till it is '"ripe", when it emerges 

 and changes to a chrysalis, which presently gives rise to the 

 fly. Therefore, there are two things to be done, namely to 

 destroy as many grubs as possible before they escape, and, 

 by preventive smears, to warn off the fly from laying 

 its eggs on the animal.s' backs during the hot weather.' 

 Miss Ormerod described the later stages of the warble fly 

 accurately, but was mistaken about the egg-laying process. 

 tShe was however able to modify her views before her death 



This simple and straightforward account was altogether 

 upset a few years later by the discovery that the eggs are 

 not laid in the hide, but are fastened to the animal's hairs 

 Moreover, the hairs to which eggs are attached are not in 

 that part of the body where the warbles appear, but are 

 almost as far away fixpin it as possible, the favourite 

 position for egg-layini; being on the hairs of the legs, or of 

 the flanks. And now comes the problem, how does an egg 

 laid on a hair of the leg give rise to a warble on the back? 



The details of the life history as modified by this 

 discovery were given by C. V.Riley in 1892 (Insect Life, 

 Vol. IV, Nos. 9 and 10, 1892). With the discovery that the 



*J(nirnul Roytd Agricultiiral Society, England, 

 LXXVTI, I). 227. 'Report of Zoologist for 1916. 



Vol. 



eggs are not laid on the back but upon the legs and flanks, 

 it was found that the young larvae first appear in the wall 

 of the gullet, and later migrate to the back. How they 

 reached the back from the gullet, wall was not then known, 

 and it remained for recent investigators, &s will be shown 

 below, t3 trace out at least one definite route taken by larvae 

 or maggots. 



To account for the presence of the larva? in the 

 gullet wall, it was quite rea.sonably suggested by 

 Riley and others that the young maggots are taken 

 on to the tongue by cattle when they lick themselves, 

 and are passed to the gullet, into the wall of which 

 they bore their way. (Jtber theories were also put for- 

 ward, among which was one that the tiry maggots on 

 hatching bored their way through the skin, and gradually 

 wandered in a general upward direction under the skin until 

 they reached the back. The 'licking' theory, being the 

 simpler, was generally accepted, and has appeared in accounts 

 of the life-history of this pest for many years. In view of 

 recent investigations, however, it would appear that this 

 theory will have to be discarded. 



THE LIKE-HISTORY OK A WARBLE KLY AS NOW KNOWN. 



As mentioned above the two common species of warble 

 flies are Hypodernui lineata and H. bonis. 'These are rather 

 stout flies, somewhat larger than a blue bottle, with 

 a large head, but with mouth parts unfitted either for biting' 

 or sucking. These flies usually appear in sunny weather 

 during the summer months, but have been noticed in British 

 Co'umbia by Had wen as early as April, and by Carpenter in 

 Ireland as late as September. The eggs are attached to the 

 hairs of cattle, chiefly on the hind limbs just below the heel 

 joint or hock; they are rarely laid on the belly, flanks, or 

 breast, and never, under natural conditions, on the back. 

 The two species of Hypoderma differ in the egg-laying 

 process; H. hovis attaches a single egg to a hair near its 

 base, while H. lineata lays its eggs in rows of seven or more 

 half way up the hair. 



It has been suggested by Hadwen that the extreme 

 uneasiness and often terror inspired in cattle by H. hovis 

 is due to its persistence in attack, and to the fact that it is 

 able to lay its eggs regardless of the movements of the 

 animal, since it only lays one at a time. 



The egg, in the case of //. hovis hatches about the 

 fourth day and the minute maggot, about one-thirtieth 

 of an inch long, penetrates the skin near the ba.se of 

 the hair, causing a watery exudation of serutn. In the 

 case of //. lineata it is suggested by Hadwen that, since a 

 number of eggs are attached lo the same hair the larvae may 

 follow one another through the ^ame opening, thus causing 

 a larger opening and bigger flow of serum. The.se observations 

 show definitely that the maggots enter the animal's body 

 through the skin, and not through the mouth or gullet. They 

 were first made by Carpenter and Hewitt in Ireland from 

 1910 to 1914, ami confirmed by Hadwen in liritish Columbia 

 in 191.5. 



After entering through the hide the maggots disappear 

 for a time, only to reappear in the wall of the gullet, where 

 they are to be found all through the autumn and winter. 

 It is suggested by Warburton that the microscopic larvae 

 may enter the blood stream, and be carried by it to the gullet 

 but nothing definite is known as yet. The maggots disappear 

 from the gullet in March, and from there until they reach 

 the back they have been followed with more certainty. One 

 dfinite route has been traced by Hadwen and Rruce, and an 

 account is given in Bulletin 22, Department of Agriculture. 

 Canada, 1916. This is summarized by Warburton as 

 follows: 'According to this {^Bulletin I') the maggots 



