"86 AGRICULTURAL RESEARCH AND THE FARMER : 



their life histories and habits and the conditions under which 

 they flourish ; for until we know this we cannot say what are the 

 best means of repression. Spraying, although the best available 

 remedy, cannot be regarded as reaUy effective. The first essential, 

 however, is a correct knowledge of the Aphid group, and in 

 order to get this, each important species within the group must 

 be critically examined. We need to know if any good will come 

 from eradicating the alternate hosts — that is, plants, other than 

 the injured crop, on which the aphids spend a portion of their 

 lives — and whether the enormous reproductive power of the 

 aphid is due to seasonal or plant conditions or to something 

 inherent in the insect which must be accepted and cannot be 

 modified. We need to obtain an index of the susceptibility of 

 different varieties of each plant to aphid attack, to see whether 

 it is possible by selection and breeding to produce aphid-resistant 

 varieties. We want to know whether cultural or manurial 

 conditions will affect the susceptibility of the plant to attack. 

 And as a preliminary to all such enquiries, it is essential to 

 obtain a complete knowledge of the life-history of each species, 

 and to find the most vulnerable stage at which to attack it. 



An investigation ha\dng these objects is now in progress at 

 Rothamsted, the first species selected for detailed study being 

 the Common Bean Aphis [Aphis rumicis, Linn). The results so 

 far obtained show clearly that this aphis, which utilises many 

 alternate hosts, is not able to reproduce itself with the same 

 facihty on aU of them. For example, dv/arf French beans are 

 only sHghtly infested. The aphis has two types of host, winter 

 (perennials) and summer (annuals). The range of hosts is so 

 wide that it is of no use attempting to eradicate them ; the most 

 that can be done is not to grow the hosts most favoured by the 

 insect in close proximity to one another. Sugar beet, for 

 instance, suffers severely from this aphis on the continent, and 

 if it is to be widely grown in this country it should not be grown 

 in close proximity to field beans, or to com fields containing 

 abundant poppies, which is another favoured host. It has also 

 been found that different varieties of beans exhibit a v/ide range 

 of susceptibihty to aphids. One variety, Vicia narborensis, is 

 only slightly susceptible, and an attempt will be made to cross 

 this with a highly susceptible variety, to ascertain whether 

 resistance is an inheritable character. 



Chemotropism : A method of trapping Insects. — One of the 

 great controlling factors in the lives of insects is their sense of 

 smell. By it, for instance, they find their food ; the male seeks 

 out the female ; the female discovers suitable places for depositing 



