Report on the Diseases of Wheat. 



23 



Section XIV. — On precautions to he taken against the 

 Wheat-Midge. 



Perhaps it may be thought unnecessary to take any precau- 

 tionary measures against so insignificant an enemy as the wheat- 

 midge ; and possibly the actual damage which it occasions may 

 generally be too trifling to make it worth the farmer's while to 

 trouble himself about it. Mr. Kirby calculated that in a certain 

 piece of wheat, of 15 acres, the loss would amount to about 

 5 coombs, or a twentieth part of the crop. Whether such a loss 

 may be worth thinking about I am not sufficiently aware ; but 

 there is a strange economy in the insect tribe, by which particular 

 species, in certain seasons favourable to their production, are 

 enormously multiplied, and are then capable of producing very 

 extensive havoc, though in most years their attacks are compara- 

 tively insignificant. I should therefore presume that any simple 

 method which might be devised for continually checking the in- 

 crease of the wheat-midge^ must be decidedly beneficial in the long 

 run. Since the chrysalides of this insect lie secreted during the 

 winter among the chaff, and the fly does not make its appearance 

 till June, multitudes of them might easily be destroyed by burning 

 or scalding the chaff after the grain has been threshed out. It 

 has however been asserted, and I have myself observed the fact, 

 that many of the caterpillars quit the ears and fall to the ground, 

 where (it has been supposed) they change to chrysalides, and re- 

 main buried till their final metamorphosis takes place. It seems 

 extraordinary that different individuals, of the same species of 

 insect, should have habits so distinct as these — that whilst some 

 spin a web within the chaff, others should bury themselves in 

 the ground. I suspect (but have not fully verified the fact) 

 that all those caterpillars which enter the ground have been ich- 

 neumonized, as entomologists term it. As it may be an object 

 of some importance to ascertain w'hether this is really the case, (if 

 ever agriculturists should think it worth while to attempt any 

 remedial measures against the attacks of the wheat-midge,) per- 

 haps I may be allowed to make a few remarks on this curious 

 department of the insect economy. There is an extensive group 

 of insects, collectively called ichneumons, though subdivided into 

 many genera, which lay their eggs in the bodies of other insects, 

 generally whilst these are in the caterpillar state. When these 

 eggs are hatched, the young maggots w^hich they produce (and which 

 are the caterpi liars of the ichneumons) feed upon the fleshy or mus- 

 cular parts of the caterpillar they are attacking, carefully avoiding 

 the vital parts. At length, the caterpillar they have thus been de- 

 vouring alive, dies ; or, as frequently happens, it changes to the state 

 of a chrysalis before it is destroyed. The ichneumon caterpillars 

 also pass to the chrysalis state^ and either remain within the body 



