Injurious Animals (continued)
( Originally Published 1915 )
Aphides and their life-histories; preventive and remedial measures. The Apple Sucker. Scale Insects. Greenhouse Pests and Fumigation Methods.
The present lecture is devoted to a consideration of certain injurious Hemiptera. Members of this order of Insects are characterised by the presence of a jointed rostrum or beak, enclosing two pairs of stylets used for piercing the tissues of plants and imbibing sap therefrom.
Nearly always four wings are present, the young resemble the adults in general form, and a pupa stage is almost always absent. The family of the Aphididae is of great importance, including as it does the " Green Fly " or " Plant Lice." Aphides may draw the sap from all parts of plants, even the roots, and the injuries they cause are often great. They bear near the end of the body a pair of tubes, which secrete a substance commonly termed "honey dew." This accumulates on the leaves, blocking up their stomata, and also provides nutriment pon which various Fungi develop. Aphides undergo a remarkable life cycle. In the autumn we usually find the fertilised winged females. The eggs laid by them develop the following spring into wingless females.
These latter breed with great rapidity by parthenogenesis —i.e., without the agency of the males, none being present. Eggs are not laid but living young are brought forth, and this goes on for several generations until the summer. Winged females then appear but there are still no males, and living young continue to be produced.
After a variable number of generations of this kind, winged males and winged females become evident, and the eggs laid by the latter give rise to a similar cycle in the following spring. Some Aphides are restricted to a particular species of plant, while others have alternate hosts. Thus the Hop Aphis winters on damson and flies to hops in the spring. The Elm Aphis goes to the roots of Ribes, the Mealy Plum Aphis to rushes and aquatic grasses, and the Bean Aphis to mangolds, poppies, clock, etc. Some of the most destructive Aphides are the Bean Aphis (Aphis rumicis), Currant Aphides (Rhopalosiphon ribis Linn. and Myzurs ribis Linn.), Hop and Damson Aphis (Phoroclon hirmuli), 'Apple Aphides (Aphis pomi De Geer.; Aphis sorbi Kalt., and Aphis 7itchzi Sand.), Plum Aphis (Aphis prutzi Réaum.), Cabbage Aphis 'Aphis brassicae Linn.), Turnip Aphis (Aphis rapae Curt.), Rose Aphides Woolly Aphis (Schizoneura lanigera), and others. In dealing with these pests it is important to remember that when using insecticides contact insecticides only arc of use. Insecticides frequently do not destroy the eggs and, notwithstanding spraying, fresh broods appear from the unaffected ,eggs. When the leaves of the host plant are curled insecticides are of little value, as they do not reach the Aphides within. Frequently the undersides of leaves alone shelter the Aphides, The spraying must be adjusted so as to reach them. The earlier measures are applied after the appearance of Aphides, the better the chances are of success, before the latter become numerous.
The Bean Aphis (17) is usually black and very conspicuous; it especially attacks broad beans. It appears when the beans commence to bloom and attacks the heads. It breeds rapidly, covering the plants with a black sticky mass which gradually extends downwards over the stems and leaves. A .simple and effective measure is to pick off the infested tons as soon as any Aphides, are seen thereon, and drop tiem straight away into a pail of lime. In bad infestations spring spraying with a knapsack sprayer is necessary. Two applications are desirable and are usually successful. The Cabbage Aphis (17) may also infest swedes and turnips. It usually appears about May, but evidences of injury are not generally noticeable until June, when the leaves begin to show blister-like areas on the upper surface, while the Aphides are to be found in the corresponding hollows on the undersides of the leaves. The leaves become yellow and discoloured, and in late summer the plants may swarm with the Insects. In the early stages of attack benefit is derived by cutting off the blistered or yellowish leaves and destroying them. Later on dusting with soot is worthy -of a trial, and spraying is practicable on a small scale in garden plots. All methods of cultivation tending to produce vigorous growth are serviceable, and copious watering in dry weather, which favours Aphid multiplication, is of great value. Aphis pomi and A. sorbi (4) which attack the apple, puncture the leaves and cause them to curl and become discoloured, while A. fitchii attacks the blossoms and buds, but does' not usually cause leaf curling. For all three species it is important to spray about the middle of April, when the eggs have hatched and the young are most vulnerable. The Rose Aphides are familiar to every gardener, and there are three species commonly met with. Siphonophora dirhoda Wlk. is stated to migrate to grasses, Polygonum and wheat, S. rosae Reau. to teazles, while S. roswwm Wlk. appears to have no alternate host. If only the latter species be present one or two early sprayings are sufficient, but with the other two species additional applications are some-times necessary owing to fresh infections from their other plant hosts. Paraffin should never be used on rose trees, and only lb. of soft soap should be mixed to 10 gallons of water in the quassia wash referred to further on. When only affecting a few twigs here and there, Rose Aphides can be readily killed by momentary immersion in a vessel of water just too hot to keep the hand in, without injury to the plant. Of the Currant Aphides (4), Ribis produces reddish blister-like galls on the surface of the leaves, while M. ribis causes the leaves to curl up especially those on the terminal shoots. Both species as they become numerous are difficult to deal with, as they are protected in the hollows of the blisters on the undersides of the leaves, or within the curled up leaves. One of the effects of their presence is the frequent falling of the fruit before maturity. Both species deposit their eggs under the broken rind or upon it, chiefly on twigs of the previous year's growth. Both currants and gooseberries are attacked, and M. ribis especially frequents black currants. Early spraying in April is the best measure, and care must be exercised in order that the fluid reaches the undersides of the leaves.
The Plum Aphis (4) appears before the buds open, and the parent form may be found in March. They are dull fat purple Insects, while the young are olive green at first, becoming purple later on. The latter attack the young unfolding leaves and soon cause them to curl, not only those of plum and damson, but also allied fruit trees. Spraying should take place as soon as the Aphides are noticed and before the buds are open, if possible, as the Insects are then most readily destroyed. Further spraying late in September and in October helps to kill off the egg-laying females.
The Woolly Aphis, or American Blight (4) is a universal pest of apple in this country. By constant sucking of the sap it lessens the vitality of the trees ; their punctures in the bark and young wood cause abnormal growths of soft tissue which form characteristic rounded swellings. Later on these swellings split and from them arise large rugose deformities often ascribed to " canker." These wounds further predispose the trees to the attacks of fungus enemies. Under ground this Insect further causes gall-like swellings on the roots. As a result of Wooly Aphis attacks, young trees may die, stunted trees often result, and the fruit is deficient and of poor quality. The parent wingless Aphides are reddish or purplish-brown, and are invested with a white wooly substance. Living young are produced and become similarly invested with this white material, forming conspicuous objects on the branches of the trees. The wingless egg-laying females and winged males occur in autumn. Each female is very small and lays a single egg near the foot of the tree, and the .egg hatches the following spring. Winged parthenogenetic females appear to be rare and are seldom met with. In the winter the Insect lives in the adult state on the bark, or in the roots below ground, and also in the egg stage. The possibility of resistent varieties of apple is worthy of attention, especially as in Australia the roots of apples grafted on to the Northern Spy and Majetin (an English apple) are said to be proof against this Insect. Spraying with soft soap and quassia is an efficient summer treatment, but the solution must not be spared, and force is necessary or the wooly covering of the Aphide will not be wetted. The only way of getting rid of the root forms is by injecting bisulphide of carbon. For an average-sized tree four 1 oz. injections into the soil 2 feet away from the trunk are sufficient. The fluid must not reach any of the larger roots and, moreover, it is very poisonous and highly inflammable. If summer spraying is not sufficient, a winter wash, as detailed in Leaflet 34 of the Board of Agriculture, is advisable.
Banding the tree trunks in early spring is said to give encouraging results, and several cases have been reported of large numbers of Aphides migrating from the roots, being trapped on these bands as they were ascending the trunks. " Tree tanglefoot" is best for this purpose.
As regards preventive and remedial measures (4,15) against Aphides there are three seasons of application.
(a) In autumn to kill off the egg-laying females. The leaves are then of much less value, and it matters very little if they are injured. Thorough spraying with a mixture of 1 pint paraffin and 1 1/2lbs. soft soap, added to 10 galls. of soft water, which should be made up in the manner suggested in the previous lecture for dealing with the Onion Fly. In the case of Currant Aphides heavy pruning is valuable, as the eggs are present in large numbers on the shoots. All prunings should be taken away and burnt. (b) Winter measures : these consist of using sprays, which have the effect of sealing up the eggs with a coating through which the young Insect is unable to make its way. A good mixture recommended by Theobald consists of 1 cwt. of fresh lime, which should be gradually slaked, and mixed with 100 galls. of water in which 30lbs. of salt have been dissolved. The addition of 5lbs. of water glass is stated to be an advantage, though not essential.
Failure in obtaining satisfactory results are usually to be traced to want of care in starting with freshly-burnt lime, or in slaking this. It is best used as a late winter wash, as its effects wear off owing to weather action if it remains on the trees all through the winter. This wash is useful in sealing up the eggs of the Apple and Plum Aphides, and may also be used for the same purpose against the Currant Aphides if pruning has not been done. (c) Spring spraying, which should be done as early as possible. A useful mixture for most Aphides is made up by boiling for 2 hours 1lb. of quassia chips (which must be quite fresh) in just sufficient water to keep liquid. This solution should be strained, and then well mixed with 10 galls. of warm water, in which 1lb. of soft soap has been previously dissolved. Washes containing paraffin are liable to injure the delicate spring foliage, and if adopted should be used in weak strength. When the leaves commence to curl it is waste of most insecticides to use them ; nicotine and soft soap compounds are the only ones which offer any prospect of partial success. Summer spraying is not to be recommended except in the case of the Wooly Aphis.
Another family of Hemiptera, viz., the Psyllidae, or "jumpers," includes the well-known Apple Sucker 'Psyltamali) , which is a not distant relation of the Froghoppers, or "Cuckoo Spit" Insects. The adult Apple Suckers are small, greenish-yellow, four-winged Insects about ;in. long. They are to be found flying and leaping about apple leaves from May until the autumn. They lay their eggs from late September until early in November, usually on the bark of one-year shoots below buds or around leaf scars. The eggs are orange, darkening to orange-red, and hatch in April. The young larvae are very minute, flattened, dirty-yellow Insects with red eyes, and they secrete a waxy substance from the hind end of the body. As soon as the buds open they congregate within, while the older larvae and nymphs are to be found on the undersides of the leaves. Damage is caused by the larvae and nymphs piercing the young leaves, which become brown as if frost-bitten, and wither. In this way floral and leaf buds are destroyed wholesale. The adults cause a relatively small amount of injury. Undoubtedly the most vulnerable period in the life-history of the Insect is when the young larvae emerge from the egg; for various reasons, however, spraying at this time presents difficulties. Owing to the waxy substance which the larvae exude, sprays should contain a wax solvent which, however, is liable to damage the developing leaves. Furthermore, the larvae very soon enter the buds, and then spraying is of little value. The Iarvae, emerge from the eggs during several weeks, and the time appears to vary in different kinds of apple; for this reason several sprayings are necessary. The best period for dealing with this species is apparently February and March, 2-3 weeks before the buds open. A wash of lime and salt recommended by Theobald appears to be an effective measure. It should be used on dry days, and is made by taking 1-1 1/2 cwts. of best quality lime, slaking it gradually, and mixing it with 100 galls. of water in which 30-40lbs. of salt have been dissolved. The mixture should then be strained through sacking or other material before being used. Lime washes are useful in other ways and beneficial to the trees. This mixture coats the eggs and prevents them from hatching, and also seals up the buds protecting them from any larvae that may be hatched. In the autumn, spraying with paraffin is also valuable--it should be done as soon as the fruit have been gathered. so as to kill the females before the eggs have been laid. Paraffin 4pt s., soft soap 1 1/2 lb s., and water 10 galls., forms a suitable mixture, but a stronger proportion of paraffin can be used at this time of the year if desirable. Heavy spraying is necessary, not only on the leaves but also on the clouds of Apple Suckers which are disturbed and take to the wing.
The family of the Coccidae, or Scale Insects, include some highly injurious members. The females are degenerate, and spend their life hidden beneath a scale-like covering formed by the cast skins of the larvae, cuticular secretions, and other means. The males live under smaller but similar scales, and when mature issue as minute winged Insects. The Mussel Scale (Lepidosaphe ulmi = Mylilaspis pomorum) is the commonest and best known species, and is an abundant pest of apple trees. During the spring and summer it sucks the sap, and passes the winter in the eggs which are hidden beneath the paient female's scaley covering which still remains. One or other of the Woburn washes (4, 15) have given good results in destroying the eggs of this Insect. They can be used any time during the winter, and the lob formula given by Pickering and Theobald (15) is as good as any. The Brown Scale (Lecaniun persicae) often attacks currants and gooseberry, and the Woolly Currant Scale. (Pulvinaria vïtis) both currants and vines. For use against these two Scale Insects the wash lob referred to above, is generally recommended and should be well sprayed over the bushes during January. With regard to Scale Insects on vines different treatment is necessary, and the same applies to Mealy Bug and to the Green-house White Fly. The latter belongs to an allied family of the Hemiptera, viz., the Aleurodidae, and is a minute moth-like white Insect with four wings. Its larvae are green and resemble young Scale Insects, and are destructive to tomatoes and other greenhouse plants. For Insects in greenhouses which attack the leaves of plants, and do not live in the soil, fumigation with hydrocyanic acid gas is the best general remedy. It is made up as follows:-2 ozs. cyanide of potassium, 4 ozs. sulphuric acid, and 8 ozs. of water. Estimate cubic space of green-house and use this preparation for each ',coo cubic feet of space, doubling the quantity for 2,000 cubic feet, and so on. First close up all windows and doors and any holes should be securely filled up. One window should alone be left open for purpose of introducing the mixture, which is extremely poisonous to man and all forms of animal life, but does not inure plants. Use at dusk when the plants are dry, and the temperature of the greenhouse should not exceed 6o F.—the best temperature is 5o F. Do not use when there is a bright light (daylight). Pour the water into a jar first, and then add the acid slowly. Wrap the cyanide in a piece of blotting-paper and drop into the jar with a suitable instrument from outside through the window of the greenhouse. Then close the window and leave for at least 1 hour. Afterwards open all doors and windows from the outside, but do not enter the greenhouse until another hour has elapsed. Cost about 6d.