CHAPTER 6
PESTS AND DISEASES, THEIR EFFECTS AND
Plant Quarantine
The recognition of the importance of strict control of plant importation under specific conditions contained in official permits, and the quarantine of those allowed to come in is a comparatively recent innovation. The more slowly moving sea transport which until very recent times brought about conditions under which visible signs of disease or insect attack could appear, helped the work of those responsible for plant protection to de- tect readily any undesirable characteristic. Plant importation and quarantine measures have been progressively improved, until now some countries require that new sugar-cane plants imported under carefully specified conditions shall be grown in quarantine for a period of 2 years before being released ; and even then they are handed over to an officially recognized research institution for use either in connection with cane breeding or for field testing in regard to their commercial suitability.
Effect of Air Transport
The speed with which modern air transport makes it possible to convey new plant material from one country to another has removed the time factor of more slowly moving sea transport so that even stricter measures and much greater care are necessary to protect the planter against the risks of introduced pests and diseases. Insects which easily obtain access to passengers' baggage and other goods now have a good chance of survival under air transport conditions, while flowers, fruit and parts of plants picked up by the air passenger and often unintentionally imported are a greater menace than before. Seeds of undesirable plants may become attached to clothing. They collect in such places as trouser cuffs and to mud on shoes. There are now numerous ways in which pests and diseases of plants and undesirable plants can be distributed unintentionally and accidentally. Apart therefore from the protective measures taken at airports, regular and careful inspection of surrounding areas should be carried out. A further protection might be the prohibition of the growing of certain economic plants within specified distances of such places, as a protection against new insects, diseases and weeds becoming established and getting out of control.
The Planters' Interest
It is essential that the sugar-cane planter should appreciate fully the dangers of possible new introductions of enemies of the
cane, and should himself support the measures which are taken to protect the industry in which he is so closely interested. Unfor- tunately this attitude is not always displayed by those who are most vitally concerned. Recent cases of cane smuggling have occurred, the effects of which may not yet be fully evident. Such actions in the past have brought about the introduction of economic pests and diseases which have menaced various crops, including the sugar-cane. The sugar-cane leaf hopper, which transmits the gravely serious Fiji disease, was brought into Hawaii by a con- signment of sugar-cane cuttings from Australia. Thus separated from some of its natural enemies it became rapidly established, causing extensive damage to sugar-cane until it was brought under a satisfactory degree of control. It remains a potential danger to the sugar industry there because it would bring about the spread of Fiji disease if that should ever find its way into Hawaii. For- tunately for the Hawaiian industry, the cuttings associated with the introduction of this insect were not affected by that disease.
Mosaic disease is stated to have been introduced into Jamaica by infected cuttings smuggled into the island by a planter. That disease has been a constant anxiety to all connected with sugar- cane growing for more than 30 years.
Weed Introductions
The rapidity with which some plants establish themselves under new and favourable environmental conditions is remarkable.
Darwin in his Voyage of H.M.S. Beagle commented on this fact in connection with the guava in the Galapagos Islands. This plant is a serious problem in pastures and unused land in Fiji. Clidemia hirta, a common but not troublesome weed, in parts of the West Indies, was accidentally introduced into Fiji not many years ago and has latterly taken charge of extensive areas to the almost complete suppression of all other plants in lands cleared for pastures. The so-called " Seymour grass " Andropogon perpusus of Jamaica came to that island as seed in a military officer's baggage and has spread with great rapidity. For many years it was confined to places in and near to the garrison, but internal
road transport has brought about the recent rapid distribution of it to all parts where favourable conditions exist for its growth.
These examples in regard to pests and diseases could be multiplied.
They are quoted to emphasize the great importance of strict pro- tective measures against new introductions of plant enemies, and the necessity for all members of the public, and in particular the planting community, to appreciate the reasons for them and to co-operate in carrying them out.
PESTS AND DISEASES OF SUGAR-CANE
A Vigilant Outlook
The sugar-cane is attacked by an extensive variety of pests and diseases which affect different parts of the plant and bring about damage and loss ranging from negligible to gravely serious pro- portions. Each country offers its own peculiar problems of inci- dence, effect and methods of control of the agencies which bring about these conditions. Constant study by research workers, and vigilance and action on the part of planters are necessary to combat the various enemies of the cane, while protective measures against the introduction of new ones and the spread of those already present are the concern of territorial Governments on the one hand and agricultural industries on the other. Methods of control are generally specific for particular forms of pest or disease within one country, but they are not always of universal application. The study of local problems, while assisted by knowledge of conditions and practice in other countries, must be conducted within the particular ecological area in which they occur. Though pests and diseases differ so remarkably in their effect upon the cane, the view is taken that all should be regarded as potentially dangerous, though comparatively few may be of present economic importance.
Each ecological area should possess facilities for the continuing investigation of all creatures and organisms affecting sugar-cane, so that basic knowledge may be accumulated, changes in the extent of damage to the crop observed, and remedial or protective measures taken in good time to prevent the pest or disease from becoming a major danger. The importance of cane breeding is recognized as the principal approach to countering susceptibility to certain diseases, while some success has attended efforts to produce
new canes which possess a degree of resistance to attack by insect pests.
Discussion of the subject is necessarily limited to a few outstand- ing examples of pests and diseases with the object of illustrating the methods of control which may thus be summarized :—
(a) The use of poisons for rodents, and possibly insects.
(b) Selection of healthy plant material.
(c) Roguing—the removal and destruction of infected plants.
(d) Varietal breeding for resistance.
(e) Biological control.
The Rat Problem
The intensity of infestation by rats and the damage and loss caused by them to the cane varies considerably. In some countries these are negligible, and in others so serious that regular cam- paigns must be conducted to keep the pest in control. The extent to which rats are a problem in cane areas may to some extent be judged by references to rat control measures in various published reports, and the degree of attention devoted to it, from which it appears that no special measures are taken in many parts of the West Indies, while in Queensland, Hawaii and Florida, for example, the extensive distribution of poisoned baits through the cane areas forms a part of normal plantation routine. The presence of accessible and more attractive food in large quantities has some bearing on the incidence of rats in cane areas, and combined with hard rinded cane varieties which are less easily gnawed than soft cane, often relieves the planter from conducting frequent rat campaigns.
The mongoose, introduced from India to numerous cane growing countries in the 1870's and later, has found easier prey than rats and has failed to exercise more than a very limited and localized control.
The damage caused by rats to cane brings about direct loss by destruction of part of the stalk which may be followed by death and rotting, and indirect loss through organisms causing disease which obtain access to the cane through the damaged parts. An estab- lished rat population in a cane field is difficult to dislodge, and when this has been effected, re-infestation may soon occur if attention is not paid to near-by breeding grounds. Control measures applied to one part of an area, with neglect of the
Natural and artificial drainage systems are clearly seen.
[Courtesy of Hunting Aerosurzeys Ltd.
remainder, will bring about migration with no considerable reduc- tion in numbers. Rats are quick to react to conditions which are unfavourable and dangerous, and whether it be by instinct or intelligence, will defeat any but the most determined and well organized methods for their control. Complete extermination though occasionally possible for short periods seems unlikely to be permanently achieved anywhere. Rodents multiply rapidly, and the few which escape destruction are capable of bringing about the re-population of an area very quickly. Rat populations have been assessed from 10 to 25 with a maximum of 70 in Hawaii, and 30 to 100 per acre in Florida.
Methods of Control
There are several species of rat, each displaying particular habits of feeding and choice of environment. These characteristics should be studied in the planning of control measures, for which three methods are used :—
(a) biological; (b) trapping; (c) poisoning.
The biological method depends upon the infection of rats by viruses. This has from time to time been advocated as an effective means of destruction, but in practice it has been found that the initial virulence of disease caused by the organisms used rapidly decreases, as the rats appear to develop immunity. Various pre- parations of this nature are marketed, but none has demonstrated any superiority to mass poisoning methods, and it has not been definitely established that they are entirely free from danger to human beings and domestic animals. Indeed, cases of illness and death in human beings have been traced to the use of the Danysz and ratin viruses which are amongst those claimed to be lethal to rats. Neither the British Ministry of Agriculture and Fisheries nor the United States Department of Agriculture recommend the use of viruses. Some State Boards of Health in the U.S.A. have prohibited their sale within the States concerned. Preparations containing both a virus and a poison appear to be effective only because of the presence of the poison. For the virus method to be safely used the organism used should be pathogenic only to rats and mice, and so far as is known, no preparation of this nature is available.
' Trapping and poisoning both depend for success on the use of
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bait attractive to the rats both in nature and form. Carbohydrate material—grain, meal, flour, bread and sweet potato are among those successfully employed. Trapping is a temporarily effective measure in and near buildings and habitations when intensively carried out using large numbers of traps, but is costly in labour and quite unsuited to large-scale control as the rats soon learn to avoid the traps. Repeated at intervals of a few weeks, it will keep down the local rat population and possibly cause migration to less dangerous areas, where control will soon become necessary.
Poisoning Methods
Large-scale poisoning has proved the most effective method.
It depends for success on a knowledge of bait which will be readily taken, on the use of poisons which are lethal and if possible not harmful to domestic animals and birds and on the conduct of simultaneous campaigns throughout the infested area. In some cases, having ascertained the attractive type of, baits, pre-baiting with unpoisoned material is necessary so that the rats become accustomed to the food and its form. With certain poisons, notably zinc phosphide, the poisoned bait can be distributed at once.
The method used at Clewiston is the distribution by aeroplane of diced sweet potatoes in f-in. cubes poisoned with z\ grams of zinc phosphide per lb. of potato cubes (1 oz. to 11 ½lb.) at the rate of 14.7 lb. per acre. The sweet potatoes are diced and treated in a concrete mixer with the poison and a paraffin wax coating. The aeroplane hopper carries a charge of 700 lb. each trip and treats 420 acres in one hour at a cost of 6 cents an acre for labour, aircraft, and pilot, compared with 67 cents per acre for labour alone by hand preparation and distribution. Corn (Zea mats) is also used as bait, the applications being at the rate of 10.6 lb. per acre of the poisoned material. The usual procedure is to treat standing cane only in February, and to cover all cane fields in June and October.
Numerous other poisons are used and new ones are being dis- covered and studied. Many of them are general poisons and are subject to the same objections as viruses on that account. Thal- lium sulphate used with wheat in the proportion of about 1 part poison to 300 of grain is effective, with ¼ oz. (7 grams) per bait enclosed in small weather-proof packages and distributed at the
rate of a few hundreds per acre. In Australia it was found an advantage to spray the baits with linseed oil before wrapping.
Great care should be taken in handling Thallium sulphate, which is a depilatory. Thallium sulphate used with a bait of rolled oats has proved of service. Pre-baiting with unpoisoned bait is neces- sary. Among other poisons used with suitable baits are arsenical compounds strychnine, and phosphorus, but these are dangerous for anything beyond very local use under the most careful control as they are deadly to human beings and animals.
New and potent rat poisons tested in the U.S.A. include sodium fluoroacetate, " Antu " (a-alphanapthyl-thiourea) and " Castrix "
(2-chloro 4-dimethyl amino 6-methyl pyrimidine). The hydroxy coumarin compound " Warfarin " has been found useful where rats migrate into cane, and around buildings and houses. It is highly toxic to rats, but is reported to be otherwise safe in regard to accidental poisoning, when used at the concentration of 1 part to 4,000 of bait.
INSECT PESTS
Insect pests of the sugar-cane include those that cause damage by boring into and feeding on the different parts of plant, some of which transmit disease. For example the corn aphis is the vector of mosaic, and the cane leaf hopper carries Fiji disease. The commercial importance of all insects which attack, feed upon or in any way affect cane varies greatly among the sugar-cane countries in the world. Thus, the leaf hopper which carries the infection of the destructive Fiji disease in Fiji and other places, when acci- dentally introduced into Hawaii, developed rapidly and extensively damaged cane by its feeding habits. The absence of Fiji disease in Hawaii has undoubtedly saved the sugar industry of that island from a major catastrophe, while the application of biological methods has brought the insect under a satisfactory degree of commercial control.
The Moth Borer
Among the major insect pests of sugar-cane is the moth borer, Diatraea saccharalis, which is widely distributed and capable of causing extensive damage when out of control. In some countries, notably Barbados and Florida, special measures are necessary to
deal with the pest while in others, such as the D o m i n i c a n Republic and Jamaica, it is kept under satisfactory c o m m e r c i a l control by natural agencies, with no attention by the e n t o m o l o g i s t or the planter apart from an occasional inspection c a m p a i g n to ascertain the extent of incidence. The female moth deposits i t s e g g s on the young cane, and the larvae which hatch out bore i n t o t h e cane, eat the tissue and subsequently emerge as adults. T h e tunnels thus bored into the cane provide access for d i s e a s e organisms which often cause extensive secondary damage. T h e m e t h o d of attack in Barbados and Florida is one of biological c o n t r o l in the course of which the tiny insects, Trichogramma m i n u t u m , which are capable of parasitizing eggs of the moth borer a r e themselves bred in enormous numbers for liberation in the c a n e fields. In Jamaica the parasite, Lixophaga saccharalis is well distributed naturally and is the principal agency in keeping t h e m o t h borer under control. Where damaging outbreaks of t h e b o r e r have occurred in that island they have invariably been t r a c e d to the presence of conditions unfavourable to the n o r m a l g r o w t h of sugar-cane, such as poor drainage and bad cultivation.
T h e control of potentially injurious cane insects by p a r a s i t e s and predators which are naturally present in t h e s a m e t e r r i t o r y , Is related to the existence of favourable conditions. To continue the distinction between Barbados on the one h a n d a n d Jamaica and the Dominican Republic on the other, the l a t t e r a r e char- acterized by large expanses of land quite unsuitable f o r cultiva- tion which are covered with wild growths of trees, s h r u b s , grasses and herbs providing breeding grounds and h a r b o u r a g e for the enemies of some of the economic pests of the s u g a r - c a n e . In the intensely cultivated island of Barbados there are f e w , if any, areas where such conditions exist. In direct contrast to t h e com- parative situation in regard to insect enemies of t h e c a n e is the fact that mosaic disease has been completely e l i m i n a t e d from Barbados, while it continues to be a grave p r o b l e m in Jamaica and the Dominican Republic, a condition u n d o u b t e d l y d u e in part to re-infection from wild grasses which contract t h e disease.
R o o t E a t i n g G r u b s
T h e grubs of various hard-backed beetles are c a p a b l e of causing extensive damage by feeding on the roots, and in s o m e cases the
rhizomes of the cane. Important examples are the greyback beetle, Dermolepida albohirtum, indigenous to Australia, the white grub, Lachnostema jamaicensis of Jamaica, and Phytalus smithii which was formerly the cause of great damage and loss in Mauritius.
The Australian greyback beetle has been described as possibly the most destructive insect pest of sugar-cane in the world. It is capable of reducing yields by 15 tons of cane per acre yearly. The adult females of these beetles deposit their eggs at the base of tunnels in the soil. The eggs hatch after 2 weeks, and the grubs develop into pupae after passing through three instar stages. It is during the earlier half of the third instar stage of 6 to 7 months that the grubs, which then may exceed 2 in. in length, feed vora- ciously on the roots of the cane. One of the modern chemical insect poisons, Benzene hexachloride, or Gammexane, has pro- vided a remarkably efficient remedy, which has almost completely replaced soil fumigation with Carbon disulphide and Paradi- chlorobenzene. The present method is to use a 20% Benzene hexachloride, rock phosphate dust at the rate of 75 lb. per acre, the dust being applied in a band about 18 in. wide over the half- open drills after the cane has stooled, and just before the stool is to be worked towards the cane prior to the field being levelled, the insecticide is incorporated in the soil surrounding the very young cane so that the underground portions of the stalks are pro- tected by a layer of soil charged with the poisonous material which will kill any grubs that may come in contact with it. Various methods have been tried but none has been proved better than this so-called " half open drill " method.
A Catalogue of Cane Diseases
J. P. Martin has listed 90 diseases of sugar-cane, and their world distribution, with the cause of each. Most of them are the effects of known organisms. A few are of genetical origin, some caused by viruses, others brought about in various ways by mechanical, physiological and natural causes, by mineral defi- ciencies, and II undetermined, among which is the gravely serious Sereh disease occurring in Java and Formosa. T h e effect of hghtning injury, recorded in Australia, Hawaii, Mauritius, Puerto Rico, and South Africa, may easily be regarded as the result of attack by a disease producing organism, and though listed as a