Further Investigations regarding the economic Importance of the Gramang-ant.
AbstractThe life-history of the gramang ant, Plagiolepis longipes, is here described in detail, though the major portion of this paper deals with investigations as to its economic importance [see this Review, Ser. A, iii, p. 663; v, p. 143]. The egg-stage lasts for about 18-20 days, the larval stage 16-20 days, and the pupal stage usually 20 days, the total duration being thus between 50 and 65 days in the case of the workers. The pupal stage of the queen occupies from 30 to 34 days. The winged males and females only appear during the second half of the east monsoon, after a more or less prolonged drought.
P, longipes is a scavenger, feeding on the dead bodies of other insects and small animals and rarely, if ever, attacking living insects. Both the larvae and full-grown individuals of Helopeltis are unmolested if alive, though their dead bodies are carried off. The helpless larvae and pupae of the beetle infesting the pods of Tephrosia, Araecerus fasciculatus, were carried away in the laboratory, but the ant was never observed to penetrate into growing pods in order to attack the larvae. P. longipes is therefore unimportant as a destroyer of insect pests. It also feeds on the honey glands which are found in many plants and are common on Leguminosae. This explains its numerous visits to shade-trees (Caesalpinia, Deguelia, Albizzia, etc.) and Crotalaria striata, though these may be uninfested by Aphids or Coccids. The gramang ant also abounds in places where fallen leaves and other debris are plentiful and Aphids and Coccids are scarce. This was often noticed in the cacao plantations and, during the dry weather, in the teak woods. The presence of the dead bodies of numerous insects, on which the ant can feed after Aphids and Coccids have become scarce, may account for this. The highest altitude at which P. longipes occurs is about 3, 300 feet; at altitudes above about 3, 900 feet it is rarely met with, being replaced by a big brown ant, believed to be Myrmicaria brunnea. This restriction in range does not affect the plantations of Coffea robusta, which are mostly below 3, 300 feet. The lesser injury by Coccus (Lecanium) viridis (green scale) at and above this altitude is due, partly to the absence of P. longipes, but mainly to the development of the scale being retarded by the lower temperature. Mites are the chief enemies of P. longipes. Tyroglyphus avptralasiae, Oudms., feeds on the eggs while T.krameri, Berl, and Anoetus longipes, Oudms., live attached to the workers. A Tachinid fly, Bengalia laire, De Meij., was repeatedly observed robbing the nests of Dolichoderus bituberculatus (black cacao ant) and P. longipes and it probably attacks other ants living above ground. Poultry, small quails and other birds feed on the pupae and queens of P. longipes. In a previous paper [see this Review, Ser. A, iii, p. 663] the author was of opinion that P. longipes could not be said to protect Coccus viridis by building covered passagges, or carrying the scales about or destroying their natural enemies. The only way in which P. longipes favours the scale is by facilitating the evacuation of its excreta. A detailed account is given of experiments extending over four months which establish the fact that P. longipes has a peculiarly favourable influence on C. viridis; on ant-infested bushes, the death rate of the scales is considerably lower, they develop more rapidly, their parasitation by Ichneumonids is reduced, and their progeny is twenty times more numerous. The suggested explanation of these facts is that feeding is promoted in scale-insects in which excretion is accelerated by the stroking performed by P. longipes. It was noticed that feeding seems greatly to influence the development of the scale; on the same coffee bush the young scales on the shoots develop more rapidly than those on the leaves, which form a less suitable food.
In investigating whether other ants also benefit Coccus viridis, the choice naturally fell on the black cacao ant Dolichoderus, bituberculatus, which protects cacao against Helopeltis [see this Review, Ser. A, v, p. 143]. D. bituberculatus is seldom seen in plantations of Caffea robusta, but was frequently met with in those of Libertan coffee and, when this was replaced by cacao, the latter became infested. Experiments similar to those with the gramang ant showed that the death rate of the first generation of the scale was practically the same whether D. bituberculatus was present or absent; the development of C. viridis occupied on an average 83 days when no ants were present, 74 in the presence of the black cacao ant, and only 65 in that of the gramang ant; the average percentage of Ichneumonid parasitation was 39 per cent. when no ants were present, 3.5 per cent. in the presence of the black cacao ant, and 5.7 per cent, in that of the gramang ant; the average number of scales after 41/2 months was 70 on ant-free bushes, 403 on bushes infested with black cacao ants, and 1, 057 on those infested with gramang ants. It is suggested that this difference in the influence of the two species may be due to differences in their behaviour. Only a few individuals from a gramang colony perform the milking; they move quickly over the scales and stroke lightly and for a short time only. On the other hand many members of a black cacao ant colony are employed and stroking is both heavy and prolonged, so that the sensitiveness of the scales is blunted instead of being stimulated.
Other ants occurring on Javanese coffee estates include Oecophylla smaragdina, Pkeidologeton diver sus, Cremastogaster spp.5Camponotus camelinus, Myrmicaria brunnea, etc. Though no experiments were made, it was noticed that the advent of various ants among ant-free colonies of C. viridis always led to an increase of the latter. Some planters have attempted to drive away P. longipes by introducing other species, but, as C. viridis invariably thrives better when ants are present, not only P. longipes, but all other species should be kept away from coffee estates as much as possible.
The white cacao scale, Pseudococcws crotonis, Green, is abundant on, but apparently harmless to the pods and twigs of cacao, in the plantations of which it forms the chief attraction for D. bituberculatus. Under experimental conditions it was ascertained that P. crotonis died out on cacao trees uninfested by ants, while it nourished on ant-infested ones. Under natural conditions the scale usually disappeared, when the gramang ant was present, because this ant does not sufficiently protect it (as the black cacao ant does) against parasitation by Diplosis. That the presence of the gramang ant is not of itself antagonistic to the development of the scale was proved in experiments in which Diplosis was absent. The practice of refastening to the cacao trees pods which have been plucked and emptied is useful only in plantations where the black cacao ant occurs. These data refer to P. crotonis and do not necessarily apply to other mealy bugs. For instance, P. citri, Risso, appears to thrive equally well on both ant-free and ant-infested trees. The lamtoro Coccid (P. virgatus), which is not much visited by ants, and P. adonidum, which is usually ant-free, also thrive in their absence.
The injuriousness of the gramang ant depends on the plant involved. In coffee plantations it is a most dangerous pest owing to its influence on C. viridis. On many estates this scale is present in such small numbers as to be practically negligible and such infestation remains stationary until the gramang ant appears, when it rapidly increases. It has been stated by Keuchenius that the gramang ant kills many insect pests, hinders the development of sooty fungus on coffee bushes and is an important distributor of Cephalosporwm kcanii [see this Review, Ser. A, iii, p. 646]. The first of these conclusions has already been dealt with and data are adduced to prove that the other two are incorrect. In cacao plantations the gramang ant is also injurious, for it drives away the black cacao ant and does not itself efficiently protect Pseudococous crotonis against Helopeltis, In this connection it is suggested that Helopeltis is disturbed by the active, dense masses of the black cacao ant, an effect which the sparse and smaller numbers of the gramang ant cannot produce. Cacao planters have sometimes ascribed a lessened fruit production to the .gramang ant and the author states that in places where gramang infestation borders on black cacao ant infestation, the difference in production was very remarkable. This will be further investigated.
As regards the control of P. longipes [see this Review, Ser. A, iii, p. 664] it is pointed out that the removal of fallen leaves and other debris only causes the ants to enter the soil without leaving the plantation. The employment of other ants is inadvisable in coffee plantations and, though the black cacao ant may safely be introduced into cacao plantations, it has little chance of surviving in the presence of P. longipes. Attempts to poison the latter have failed, because the ants soon discover the poisons and avoid them. Traps made of sections of bamboo stuffed with dried leaves proved expensive and incapable of entirely ridding a plantation. For use in coffee and cacao plantations, when these ants abound, carbon bisulphide may be recommended, the minimum quantity required being 5 pints per acre. Trenches measuring about 5 feet long by 2 feet and 8-12 inches deep are dug at regular intervals, about 140 being required per acre. In cacao plantations this amounts to one trench per tree. Many small trenches are preferable to a few large ones. The plantation is thoroughly cleaned up and the fallen leaves are placed in the trenches, any surplus being burnt. At 20-inch intervals bamboo pipes are thrust into the mass of leaves which is then carefully covered with earth so that only the pipes stick out. Twenty cubic centimetres of carbon bisulphide are required per trench and this quantity is divided equally between the three pipes in each trench. If this method of fumigation is carefully carried out during the west monsoon, the complete destruction of the gramang ant is assured, and if neighbouring estates are also treated there will be no re-infestation. The carbon bisulphide may also be poured into holes made by thrusting a stick into the leaves, but the bamboo pipes increase the efficacy and economy of this method.
Burying the infested leaves in trenches was only successful when the trenches were 18 inches deep. At this depth the result was tolerably good, but still inferior to that with carbon bisulphide, and, as this method aims at suffocating the insects, it must be practised in the rainy season; the experiments were conducted during the rains and in clay soil. Gasoline and another by-product of petroleum refining, aerogine, proved much less efficient than carbon bisulphide. This also applies to potassium cyanide, even in a solution of 2 per cent. strength, and to formalin. Banding experiments were made during the rains and a dry period lasting a few weeks. Under those conditions a material obtained from Amsterdam retained its adhesiveness for more than two months. Banded coffee bushes were entirely free from ants unless they were prevented from going elsewhere, in which case some of the workers crossed the banding. Up to the time of writing, no injury had been caused by applying the adhesive direct on the trunk instead of on a band of paper. Banding is suggested as a likely means of dealing with ants which are not so susceptible as P. longipes to control by means of trap-trenches and it is recommended against Myrmicaria brunnea, the nests of which are deep under ground.
Some supplementary notes are given on Coccus viridis, chiefly dealing with the points in which the author compares his experience with information given by Keuchenius. The latter has stated that the developmental period (by which the present author means the time up to the production of the first progeny) occupies about 45 days, whereas observations have shown it to last at least 65 days, thus proving that considerable individual variations occur. Again, the green scale increases in four months as from 1 to 400 and, this increase, though great, is considerably lower than that stated by Keuchenius. The latter has pointed out that this scale attains a larger size on coffee than on Gardenia florida[Gardenia jasminoides],. It may be added that. on coffee alone does this insect really thrive. Keuchenius has also stated that new food-plants are gradually being attacked, but the view here taken is that the increase observed is due to the greater attention now given to the subject. The following plants, not recorded by Keuchenius, are general and important hosts of this scale in central Java: -Citrus aurantium (orange), Justicia betonica, Plumiera acutifolia and Psidium guajava (guava).
Indigenous insect enemies of the green scale include: -The Coccinellids, Chilocorus melanophthalmus, Muls., Orcus janthinus, Muls., and the Chalcids, Coccophagus bogoriensis, Koningsb., Myiocnema comperei, Ashm., Aneristus ceroplastae, How., Coccophagus sp., Cheiloneuromyia javensis, Gir., Cristatithorax latiscapus, Gir. These species are briefly described and accounts of their life-histories are given.