202 



THE AGRICQLTURAL NEWS. 



June 24. 1911. 



INSECT NOTES. 



PARASITISM OF THE BLACK SCALE. 



During the past two or three years, a considerable 

 amount of attention has been attracted to various forms of 

 insect parasitism in the West Indies, chiefly on account of the 

 beneficial effects which arise from the relation of parasite and 

 host among insects. It is likely that all the species of scale 

 insects which occur in the West Indies are attacked by other 

 insects, which live within their tissues or under their protec- 

 tive covering. 



The parasitized insect, or host, as it is called, is generally 

 killed as the result of the; feeding of the immature parasite. 

 In many instances, the parasite is fo small as to be observed 

 only with great difficulty; but in the case of the black scale, 

 it is .sufficiently large to be seen by the ordinary observer by 

 means of a pocket lens, or even with the unaided eye. 



The black scale t)f cotton is very abundant in many 

 localities at the present time, both on cotton plants and on 

 Hil>iscus. If an infested liranch of one of these plants is 

 carefully examined, it will be seen that the scale insects are 

 present, in nearly all stages of development: the large, very 

 convex, blackish-brown individuals are the adult females. 

 The younger stages may be distinguished by their smaller 

 size, flatter surface and lighter colour. 



The parasite, Zalophothrix mirum, is a small hymenop- 

 terous insect. The adult is about |-inch in length and at the 

 first glance appears to be very much like an ant; but it is 

 easily seen to possess small wings which lie Hat upon the 

 back. These are very delicate, nearly transparent at the base, 

 and are crossed by a broad transverse darker band, the 

 tip of the wing being transparent like the base. The general 

 colour of the in.sect is dark-brown, almost black: the head is 

 reddish brown, and provided with dark-coloured eyes. 



This parasite attacks only the older scales, that is the 

 mature females which are about to begin, or have already 

 begun, egg-laying. 



If one of the young, very fiat scales is raised from the 

 bark by means of a needle or knife blade, the ventral surface 

 of the insect will be found to be very Hat; but if an older one 

 is examined in the same way, it will be .seen that this 

 surface is very much arched or concave so that a considerable 

 space is left between the insect and the liark of the plant. 

 This space or cavity becomes filled with eggs, which hatch, 

 and the j'oung insects escape under the edge of the scale. 



The ])arasitc does not attack the Hat scales. The egg of 

 the parasite is deposited under the body of the mature female 

 scale insect. The grub of the parasite is whitish in colour. 

 The youngest form which can be seen with the unaided eye 

 is slightly tinged with pink. Examination of a considerable 

 number of scales will probably reveal the parasitic grub in 

 various sizes ui) to about ^-inch in length, when it is fully 

 grown and ready to transform to the pupa. 



The grub is very soft and fleshy in appearance, 

 and seems to have no legs and no well defined head. The 

 larger grubs will be seen to l>e lying in a curved or bent 

 position; probably because of the narrow limits of the space 

 occupied. The pupa is somewhat less than ^-incli in length, 

 motionless, without any cocoon or other covering, and is very 

 dark — almost black — in colour. Examination of it will, 

 however, reveal the well defined head ai\d developing 



antennae, legs and w-ings, which are characteristic of the 

 adult. 



It is perhaps in the matter of its feeding that this 

 parasitic insect presents points of the greatest interest. The 

 egg is deposited in the cavity under the body of the 

 female scale insect, and the grul> which hatches feeds 

 for the most of its life on the eggs, and probably 

 also on the young, of the scale insect. The tissues 

 of the adult scale are not attacked until the grub is 

 fully grown, or nearly so. If parasitized scales are exam- 

 ined, it will be found that the grub of the parasite lives 

 practically embedded in the mass of scale insect eggs, except 

 in the case when the fully grown, or nearly fully grown, grubs 

 are found. Then it will often be seen that \ery few eggs are 

 present, and that the adult scale is dead, .so that nothing 

 remains but a shell, which gives protection to the parasite. 



This seems to be a wise provision of Nature, which is 

 illustrated in the case of nearly all parasites. That is, the 

 host insect is not actually killed until the parasite has no 

 longer need of it as a source of food. In this case, the scale 

 insect is allowed to go on producing eggs as long as the grub 

 has need of them, and then the parent scale is fed upon by 

 the parasite. 



When the changes in the pupae have become complete, 

 and the adult, winged parasite is ready to emerge, escape from 

 the protecting scale is effected by means of a small round 

 hole, or puncture, and when scale insects are seen with these 

 small circular holes in them, it may safely be assumed that 

 those particular scales are dead, and that the parasites that 

 caused their death have emerged through the hole in the scale. 



The black scale will very rarely be found to contain more 

 than one parasitic grub in a single scale; but in a few instances 

 two have been observed. In such cases, it seems probable 

 that one grub will devour the other, and that only one para- 

 site will eventually emerge from that particular scale. 



In a recent examination of a large number of black scales 

 on cotton in one field in Antigua, it was found that nearly 

 every scale insect which had reached the egg-laying stage 

 harboured a grub of this jjarasite. There were very few 

 punctured scales to be seen, and it was very difficult to obtain 

 specimens of the adult parasite by means of the insect net. 



The infestation of cotton by black .scale, in this particu- 

 lar field, was most remarkable, nearly every plant was liter- 

 ally covered from base to tip with the insect. It was cotton 

 which had yielded its full crop and should have been pulled 

 up and destroyed some weeks pre\ iously. It may be that, as 

 far as the black scale is concerned, considerable benefit might 

 be derived from allowing this cotton to stand until the major- 

 ity of the parasites had completed their development and had 

 begun to escape. If the cotton bushes could be cut and left in 

 the field just at this time, the immature scales would be killed, 

 and the parasites, as they emerged, would Hy away into the 

 wild land in search of scale insects in which to deposit eggs; 

 the old cotton plants could then be burned and the parasites 

 from the scales in the wild land would be within convenient 

 distance to come back into the field and parasitize the black 

 scale on the succeeding crop of cotton. 



Every grower of cotton should make himself familiar 

 with the appearance of this parasite in its diflerent stages. 

 At this time of the year, the grulis may easily be discovered 

 by examination in the field, and the pupae are often to be 

 found in the same way. The adult parasites may be obtained 

 by placing scale-infested twigs in a glass covered with fine 

 muslin. In the course of four or five days, these small 

 winged insects may be seen on the walls of thegla.ss, or walk- 

 ing about on the scale-infested t^yigs. 



