366 



THE AGKICULTUKAL NEWS. 



November 17, 1917. 



BIOLOGICAL STUDIES APPLIED TO 

 AGRICULTURE 



In a most interebling and charming acco\int of a visit 

 paid to the United Stales in 19lf, M Paul Marchal, 

 Director of the Entomological Department of the French 

 Ministry of Agriculture, brings vividly before his readers in 

 Volume III of Annates du Service des Epiphyiies, published 

 in Paris in 1916, the wonderful organization of the United 

 States Department of Agriculture in biological studies applied 

 to agriculture. To this account we are indebted for the 

 following remarks which have been freely translated from 

 the French original, and which are likely to be of interest 

 to the readers of this Journal. 



The intensity of the struggle against the various enemies 

 of cultivated plants, which goes on in every agricultural 

 country, is especially noticeable in the United States. This 

 is explicable because of the extreme severity of the attacks 

 of parasitic plant pests in that country, where uniformity of 

 crops over immense areas furnish such pests with the richest 

 ancl most abundant food supply. 



There are three distinct methods employed in the 

 struggle against the various parasites which ravage the 

 crops; (1) cultural methods, the aim of which is to endow 

 the plants cultivated with more vigour and power of resist- 

 ance; (2) biological methods, which aim at the control and 

 destruction of the pests by means of natural enemies, insects 

 or fungi, themselves parasitic on the noxious insect plant 

 pests; and (3) technical methods, which aim at the destruc- 

 tion or control of the pests by chemical, physical, or 

 mechanical appliances. 



Interesting as is M. Marchal's description of the first 

 and third of these methods, it is not intended in this article 

 to refer to them. The story of how American genius and 

 application have employed the biological method is one of 

 the fairy tales of science of modern times. The struggle on 

 these lines may be strictly divided into two methods: (1) the 

 introduction and acclimatization of insects predatory upon the 

 noxious plant-eating species; and (2) the introduction and dis- 

 semination of fungi which are poisonous to and destructive of, 

 the insect plant pests. The first method in the hinds of the 

 American scientists has proved brilliantly successful in several 

 instances. One need only mention the successful introduc- 

 tion of a small Australian beetle {Novlus cardinal^) which 

 preys upon a scale insect of orange trees (Iccrya I'urchasi). 

 This scale insect seemed at one time to threaten the extinc- 

 tion of the large Californian orange industry, but in a short 

 time after its introduction, this predatory beetle had almost 

 annihilated the pest. 



Whenever one is confronted by the appearance of exotic 

 insect pests which do more damage in their new homo than 

 in their original habitat, there is reason to think that these 

 pe.'ts have been introduced without tbe parasites or natural 

 enemies which limit their inf-rcase in the country from which 

 they come. It is remarkable that there are found in America 

 many more noxious insects of exotic, and particularly of 

 European, origin, than are found in Europe of American 

 origin This is doubtle.ss explained by the fact that there 

 has been a much larger introduction of various plants into 

 America from Europe than vice versa. It is therefore not 

 astonishing that the method of combating the insect foes of 

 the agiiculturist by means of parasites has had its greatest 

 development in America. 



Predatory or para.sitic insects are not the only natural 

 aients which it has been sought to employ in the struggle 

 against harmful insects. Great hopes in fact have been 

 founded on the artificial dissemination of diseases caused by 

 /U'gi which live on insects, and by microbes. In America 



numerous researches have been mide particularly with respect 

 to fungi which live on insects. Certain fungi play a great 

 part in restraining the increase "f scale insects, as for instance, 

 Sphaerostilbe coccophila, which exercises in Florida such an 

 efficacious control over the purple scale {Mytilaspis Beckii) that 

 orange planters only employ fungicide sprays in extreme caseS; 

 because they know that their use only tends to destroy the 

 Sphaerostilbe at the very time that the scales begin to increase, 

 which consideration restrains the planters from treating 

 their orchards with chemical insecticides. These facts are 

 well known also in West Indian citrus orchards where 

 Sphaerostilbe is abundant. 



In Florida also it is well recognized what an important 

 part is played by parasitic fungi of the genera Asc/tersonia 

 and Aegtri/a in limiting the spread of Aleurodes cilri. one 

 of the most troublesome pests of citrus plantations. Tnose 

 who have studied the question agree that, thanks to the 

 presence of these fungi, orange and lemon orchards are 

 frequently practically cleared of the scales referred to. 



Unfortunately, it is impossible at present to lay down 

 the rules of procedure which would with certainty develop 

 epizootic maladies among insects, and so obtain by means of 

 the distribution of parasitic fungi sufficiently constant n^sults 

 to cause the employment of this method to be resorted to as 

 a regular practice. The development of these fungi in many 

 cases appears to depend too much, more or less, on exterior 

 conditions, such as moisture and temperature, over whichi 

 there is no means of control. 



It seems to be proved from tbe experiments in Florida 

 that it is quite possible to create new centres from which the 

 distribution of such parasitic fungi as Aschersonia and Aegerita 

 may take place. The question, however, as to whether these 

 fungi can be increased in orchards where they alread_y exist 

 seems to be far from being positively settled. A long series 

 of experiments made by competent mycologists as to whether 

 it was possible to augment in any profitable way by artificial 

 means the numbers of these beneficent fungi in orchards 

 where they are already to be found, has given only negative 

 results ; and the present position is, that in a locality where 

 these parasitic fungi already exist natura'ly, it is not profitable 

 to spend labour and time in endeavouring to multiply them 

 artificially. This view has been held fot some time by the 

 mycologists of the Imperial Department of Agriculture. 



As regards flying insects, such as locusts and grass- 

 hoppers, recent researches in America in the employment of 

 parasitic fungi as a method of control yield no more 

 positive results than with regard to scale insects. 

 If weather conditions are favourable, the spores already- 

 existing in the locality affected will c luse the fungus to 

 exercise its maximum eflfect. If, on the contrary, external 

 conditions are unf ivourable, the spores urtificially dissemin- 

 ated will germinate no more numerously than those present 

 naturally, and no added increase will result. 



With regard to burrowing insects, the question is just 

 as much undetei mined. New experiments are required in 

 the direction of ascertaining whether it will be possible to 

 combat them by means of parasitic fungi. Experiments 

 being conducted on species of Lachnoaterna in Illinois seem 

 to hold out some hope that the method will prove of practical 

 value in such cases. 



The same conclusions seem to hold good as to the control 

 of insect pests by iliseases caused by malignant bacteria, and 

 kindred organisms. It would seem that insect tnortality 

 caused by sich organisms only becomes serious and epidemic 

 when external conditions become unfavourable to the life of 

 the itisect, so that experiments in artificial dissemination of 



