Injurious Animals Other Than Insects
( Originally Published 1915 )
Injurious and beneficial Animals. Birds and the need for scientific investigation with reference to their food at different seasons of the year. Eelworms and more especially the Stem Eelworm and the Knot Root Eelworm. Pulmonate Molluscs (Snails and Slugs). The Black Currant Gall Mite.
All who engage in agricultural or horticultural pursuits sooner or later have to concern themselves with some of the forms of animal life which are associated with their plants. Very frequently certain of these animals are directly injurious to the operations of man, but there are others which, on the contrary, are distinctly beneficial in their effects. It is, therefore, a matter of considerable importance to be able to discriminate between these two classes, for it is obviously bad policy to devote time and money in destroying organisms which are beneficial in their action. The animal kingdom comprises a vast assemblage of different forms, but fortunately for our present purpose we need only concern ourselves with a relatively small proportion of their number. These include certain birds, species of Nematodes or Eelworms, Oligochaeta or Earthworms, Pulmonate Molluscs or Slugs. and: Snails, land Isopods or Wood-lice, certain Acari or Mites, and the great class of the Insects.
Dealing first with merit some amount of attention, but the fact cannot be emphasised too strongly that we possess extraordinarily little reliable knowledge concerning the food of some of our very commonest birds. Both the British Association and the Board of Agriculture recognise that,, before, any effective legislation can be recommended, a very full scientific enquiry is needed. It is necessary, for instance, to examine and tabulate the contents of till crops of certain common birds in each month of the year so that an opinion may be formed of the benefits or injuries caused by them at the different seasons. It is further necessary that some estimate should be made of the available food in the district where the birds were. feeding when killed, in order to decide whether the foods discovered in the crops were selected from choice or necessity. Much information is also desirable as to the. food of nestling birds. Fortunately some progress is being made towards supplying this much needed information, and the Department of Economic Zoology in this University is performing a useful part in the work on behalf of the Board of Agriculture. Certain species of wild birds may be direct-injurious by feeding upon or injuring plants or parts of plants; others are indirectly harmful in that they may food upon forms, of animal life which are in themselves beneficial. Fortunately there are very few species of birds which we may declare to be wholly destructive and, of these, the House Sparrow and Wood Pigeon are the most important. The Black-bird also appears to have very little utility in the eyes. of man, and is a most persistent devourer of fruit. On the other hand, there is a large number of birds whose. rôle is doubtful; in many cases we lack adequate knowledge to judge fairly as to their feeding habits, but they all appear to have a good deal of utility in their favour. Among these may be cited the Song Thrush, Great and Blue Tit, Greenfinch, Chaffinch, Rook, Robin, Linnet, Yellow-hammer, Skylark, Starling, Woodpeckers and others.
Invertebrates. Consequently even the most destructive birds may perform a useful function during that stage in their life.
EELWORMS (4), (5), belong to the group of the Nematoda. They are always small in size and have thread-like bodies, the two ends being more or less pointed. They can be readily distinguished from the Oligocharta or Earthworms by the total absence of body rings or segments. Those which are plant parasites are microscopic, forms living free in damp soil or inside the tissues of plants. Others live in decaying vegetation, and both the parasitic and saprophytic forms can be recognised by the presence of a spine which can be, protruded through the mouth and serves to penetrate the cell-walls of' plants. The Eelworms spread from one plant to another by wandering through the soil, and when they leave the dead plants they lie near the surface of the ground. Frequently when these animals are numerous It Is useless to grow susceptible plants in the same patch of soil during successive seasons, and then as long an Interval as possible should elapse between the growing of two crops of the same plant.
The STEM EELWORM (Tylenchus devastatrix.)
The (hiring Kuhn) attacks a great variety of plants including strawberries, onions, beans and peas, hyacinths, and also field crops. It is an extremely slender species, about 1 -25th of an inch long, and the males and females closely resemble one another. Strawberries when ,attacked decay away at the level of the soil or just below it, and the crowns and roots rot away. A remedy is to pull up and burn the affected plants and dress the soil with either lime or sulphate of potash in the proportion of r cwt. to the acre. The KNOT ROOT EELWORM (Heterodera radicicola, Greef) differs from the previous species in the male, being thread-like, while the female is greatly swollen except at the head end. It also goes through a more complex life--history. This species renders its presence evident by forming knot-like swellings or galls upon the roots of the affected plants. It is a great enemy of cucumbers and tomatoes grown in glass houses, but also attacks vines, potatoes, lettuces, and many other plants in the open.
As a temporary measure to save a growing crop, one part of permanganate of potash to 200 of water applied at intervals of ten days is recommended in the Kew Bulletin. It does not harm the plants, but since it does not destroy the eggs of Eelworms, no permanent value can be ascribed to it. Treatment of the soil with one part of carbolic acid to twenty of water, with a dressing of sulphate of potash, 3 cwt. per acre, intimately mixing the soil with gas lime or naphthaline, are among the remedies that are recommended. When applying remedial measures the soil must remain unused for at least six weeks for any permanent benefit to be derived. This species, however, is extremely difficult to eradicate owing to the fact it produces vast numbers of eggs throughout the year, and the young Eelworms are thus constantly being liberated into the soil. Furthermore, most of the above methods are not lasting in their effects, owing to the fact that frequently a number of eggs remain over undestroyed, and serve to start the infection afresh. When a glass house is infested with Eelworms, it is often necessary where possible, to remove the soil bodily and treat it by ,one of the methods already mentioned. In the case of plants grown under glass the horticulturist soon finds that the conditions encourage a host of other living things. In addition to Eelworms, Woodlice, insects of various kinds, and fungi often enforce their presence, and under the warmth and moisture that is provided they are liable to multiply exceedingly. Experiments carried out at the Rotharnsted Station have shown that we can very considerably reduce this undesirable population by partial sterilization of the soil by means of steam. In cases of very bad infestations of Eelworm this method is said to be the only effective remedy at present available.
SLUGS and SNAILS (6), (7), belong to the class of the Mollusca, which is a large assemblage of animals including such divers forms as Oysters, Whelks, Scallops, Octopi, and the familiar fossils 'which are known as Ammonites and Belemnites. Both Slugs and Snails differ from other Molluscs in being land and not aquatic animals. They are always provided with a pulmonary chamber, which is a kind of lung enabling them to breathe in the air. In aquatic Molluscs this pulmonary chamber is almost always absent, respiration taking place by other means.
SNAILS or Helicidae are provided with an external spiral shell into which the animal can withdraw itself, and there are three species which are commonly met with. The Garden Snail (Helix aspersa, Mull.) is the largest and its shell measures about 1 1/2 inches in diameter. It is well enough known to need no description, being easily recognisable by its brown shell marked with pale irregular lines. The Strawberry Snail (H. rufescens, Pen.) has a shell which seldom exceeds half an inch in diameter and is more flattened in form. It also varies in colour from dirty grey to brown or reddish-brown, often with a number of transverse streaks of a darker tint. The Wood Snail (H. nemoralis) has an extremely variable shell being white, grey, pinkish, yellow or brown, and is marked with one to five or more conspicuous brown spiral bands. It is, moreover, considerably larger than the Strawberry Snail.
SLUGS or Limacidae are naked and only possess a vestigal shell, which is placed near the hinder end of the body or buried beneath the skin of the back ; all the injurious species have the shell in the latter position. The situation of the shell is clearly marked externally and the area of skin covering it is known as the shield or mantle. Closely related to the margin of the latter, on the right side of the body, is the respiratory pore--a well-defined aperture leading into the pulmonary chamber. Slugs secrete an abundance of mucous, which serves to lubricate the skim; it is very tenacious and capable of being drawn out into a thread which is used as a means of descent from trees and bushes. The most injurious species are : (1) The Black Slug (Arion ater, Linn.) not-withstanding its name this species varies greatly in colour and may be either black, grey, reddish, or reddish brown. When at rest the animal can be further recognised by its contracted and almost hemispherical form. (2) The Grey Field Slug (Agriolimax agrestis, Linn.) is perhaps the most injurious species we have in this country. It is ashy-grey in colour with a yellowish or reddish tinge, and occasionally specimens have a mottled appearance; longitudinal markings are entirely absent. (3) The, Black Striped Slug (Limax maximus, L.) is the largest of the three and may attain a length of over six inches. It is usually some shade of grey, with longitudinal markings of a darker colour, frequently black. Individuals inclining to brown or dull yellow are also not infrequently met with.
It is well known that both Slugs and Snails confine their operations to night, and are seldom evident during the day except after rain. It is consequently useless to apply remedial measures during the warm parts of the day, or in very dry weather, the evening and early morning being the most suitable times. The mucous secreted by Slugs enables them to resist the action of obnoxious substances in the powered condition, they have the faculty of crawling out of their mucous investment, and in that way leave the powdered material behind them. This mucous, however, cannot be secreted indefinitely, and if two or more dressings are applied with an interval of about fifteen minutes between each application the Slugs are usually killed. A mixture of lime and soot applied two or three times is an effective remedy, but the lime should be quite fresh and very finely powdered. According to Theobald the most effective substance is hydro-oxide of calcium, a 1 to 2 per cent. solition in water. Snails, on the other hand, are more difficult to destroy from the fact that they retract themselves into the shell and can close the mouth of the latter. In this condition they can remain completely dormant for several years. Dressings of soot is a well-known remedy against Snails, it acts as a deterrent making the plants and surface of the soil obnoxious to these animals. Nitrate of soda is an effective dressing for use on a large scale against both Slugs and Snails and, moreover, is beneficial to the plants.
Natural enemies arc also an important factor: thrushes, blackbirds, starlings, and also ducks and fowl render help in keeping down an excess of Slugs and Snails.
Among other Injurious animals WOODLICE (8) and MILLIPEDES (7) were also referred to but, owing to the limited space at my disposal, I must pass over these and deal with the Acari or MITES. They are classified as a group of the Arachnida, which also includes Spiders, Harvestmen, and Scorpions. All can be recognised by the presence of eight pairs of legs, the absence of feelers or antennae, and the fusion of the head and thorax into one compact region or cephalothorax. Acari are further distinguished by the abdomen not being definitely marked off from the rest of the body. The Red Spiders or Tromhididae belong to this group, but the most important for our purpose are the Eriophyidae or Gall Mites. Eriophyes ribis (4) or the BLACK CURRANT GALL MITE is responsible for the "Big-Bud" disease which has spread throughout the country. Its presence can be readily detected by the swollen and distorted appearance of the buds which harbour the Mite. Badly infested buds seldom develop into shoots, they remain unopened and, after retaining their green colour for a time, become brown and die off. The damage is caused by the jaws of the Mite cutting through the epidermis of the delicate young leaves, followed by the inserting of the sucking tube which extracts the sap. Throughout the winter the Mites feed and shelter in the galled buds. Migration takes place from the infected buds, which open from about the middle of April until well on in June. The Mites then crawl out in great numbers in order to find new and succulent buds to serve fir their future sustenance. This migration is aided by the habit the Mites possess of often attaching themselves to passing insects wandering over the twigs.
By this means they become distributed to other branches and to fresh bushes. Strong winds are also a factor aiding their dispersal. Having entered new buds the Mites commence laying their eggs and thereby multiply rapidly until the end of the summer. Shoots, examined during the end of August and in September, exhibit the "Big-Bud" appearance, and are filled with the new generation of the Mite, which will carry on infection for the next season. A certain number of eggs arc to be found all the year round but are most abundant in the summer. Our knowledge of the life-history of this Mite is incomplete, we still require definite information as to whether the species can pass the winter elsewhere than in the buds—whether it can survive under the bark, in the roots, or beneath the soil.
With regard to remedial measures, so far as I am aware, no completely successful methods of treatment have yet been devised. Instances are known where all diseased Gushes in a plantation have been cut down, the stumps and oot stocks subjected to treatment, and yet the young ;hoots came up infested with this Mite. It is of first mportance to cultivate from perfectly clean stock, and uttings taken for setting should also be selected from such plants. Hand picking of the infected buds at the end of winter is valuable, and all buds collected should ae burned as soon as possible ; with badly infected shoots xtensive pruning is necessary. When the bushes are very )adly infected there is no remedy beyond taking up and running them, followed by replanting with clean new tock. Spraying .or dusting with a mixture of lime and ulphur during the migratory period has been recommended, but often the results are unsatisfactory. An fficient spraying mixture still remains to be discovered. ionic varieties of currant are claimed to be less severely ,ttacked than others, and among them may be mentioned he Boskoop Giant, Lee's Prolific, and Edina. Varieties laimed to be immune have been placed on the market, but rhether they will remain so time alone will determine. 'here is a ;possibility that careful selection and interrossing of likely varieties along scientific linen may lead the production of resistant stock.