[ 117 ] 



Migrations. 



The use of large numbers of larvae upon experimental plots or 

 open shallow cages brought to light the fact that scarcity of food in 

 spring due to over stocking results in the larvae migrating. The 

 first proof of this was obtained from a lot of 120 larvae contained 

 in a small box in soil in which clover had germinated. As the food 

 supply became exhausted the larvae left the box in large numbers 

 and were found crawling about the laboratory. Two other instances 

 of this occurred in connection with field plot experiments at Craibstone. 

 In these experiments, small plots of 2| feet by 6 feet were walled off 

 with wooden boards sunk six inches in the ground and standing about 

 three inches above the surface of the soil. It had been established 

 in experiments in a previous winter that these larvae even in very cold 

 weather rarely went below this depth of six or seven inches, and it is 

 not considered likely that they burrowed beneath the limiting boards 

 of the plots. The plots were stocked with large numbers of larvae; 

 in one instance three adjacent contained 900 larvae each and in other 

 three plots, there were 700 each. When it was discovered that the 

 crops in these had failed, at the end of June the soil in each was sifted 

 for larvae. The hatching season had just commenced, but only a 

 very few flies had been seen; of the 2100 larvae in the second group 

 of three plots only 295 were recovered, together with a small number 

 of pupae and two or three empty pupal skins. 



Another plot was not disturbed. It had stood exposed in the same 

 way as its neighbour, for some weeks, but later a large field cage was 

 fitted upon it. Here there had been placed originally 2700 larvae, 

 and now to these were added the 295 recovered from the other 

 plot. In due course flies hatched out and appeared in the cage, but 

 the numbers up to the end came very far short of those of the grubs 

 introduced. The soil was searched and no trace in the form of empty 

 pupal cases or dead larvae was found. 



In a duplicate experiment on Holm Farm, in Lewis, carried out at 

 the same time, designed to test the relative resisting or recovering 

 power of selected oat varieties, it is probable that a similar migration 

 took place. This experiment consisted of plots of Hamilton, Sandy 

 and Potato oats, into each of which 900 larvae were placed. There 

 was also a control plot of Hamilton oats, which received no larvae. 

 The oats were sown on 18th May, and "brairded" on 3rd June. 



