274 THE PRACTICAL APPLICATION OF ECOLOGY 



have been made by Holdaway (1927) and by Maclagan (1932). 

 It has been shown by these three observers that the soil reaction 

 has a marked effect upon the number of eggs laid. Soil moisture 

 is the dominant factor affecting oviposition, and this needs to be 

 considered in conjunction with pH values of the soil. Below 

 10 per cent, soil moisture the number of eggs laid decreases, and 

 where the moisture approaches saturation it also becomes 

 unfavourable. Davidson finds that above 12 per cent, moisture 

 normal egg batches are produced. The optimum pH value is 

 about 6-5, but the possible range for free oviposition extends 

 between about 5-5 to 7-0. Since the insects ingest considerable 

 quantities of soil, it seems that in passing through the gut the 

 soil would influence the normal ^^H of the digestive tract and so 

 affect general metabolism. In view of the economic relationship 

 between Sminthurus and leguminous crops, it may be pointed out 

 that lucerne is intolerant of acid conditions and prefers a pYL of 

 over 7-0, while clovers exhibit a greater tolerance to acidity than 

 lucerne. Whether the soil ^^H can be so altered from what appears 

 to be an optimum for the insect, and maintained within a range 

 favourable to the production of leguminous crops, appears to be 

 doubtful. Davidson remarks that, considering the lime require- 

 ment of the soils of many infested areas, it would be impracticable 

 to raise them to a pH value unfavourable for the Sminthurus. 

 Such a method, it appears, would have very restricted application 

 under Australian conditions. On the whole, it seems that pasture 

 management and the possibility of biological control by the 

 introduction of suitable predators offer better prospects for the 

 exploration of measures of control. 



6. French viticulturists have observed that infestation of 

 vines by the Phylloxera is accompanied by the most severe injury 

 where the plants are growing on certain types of soils. In other 

 types of soil, even in the same vine-growing district, injuries 

 are commonly much less severe. The correspondence between 

 the two series of phenomena appeared to be sufficiently close to 

 be regarded as an axiom. Some years back, the French vine- 

 growers came to the conclusion that light-textured or sandy soils, 

 containing" up to 60 per cent, of silicate sand, contributed to the 



