A STUDY OF THE FACTORS OF CONTROL. 81 



the headlands or near fields of sugar-cane often supplies this need, 

 through the honeydew, which is excreted by the eonnnon leafhopper pest 

 of this plant. 



These wasps Iniry their prey deep, so that the nunilxn- of cocoons 

 turned up by the plough gives little evidence of their abundance in the 

 field. I have found their cells with grubs or cocoons usually at depth^^ 

 of 2 feet or more — in one instance 42 inches. Hence the percentage of 

 parasitism can only be ascertained by digging deep in various parts of 

 the field. By this method I frequently found 25 per cent, of the grubs 

 had succumbed to- their work, and in one instance as high as 60 per cent. 



Evidently the numerous species of these wasps are considerably 

 hampered by natural enemies, else they would be more abundant. For 

 many years, in other countries, it has been known that Scoliids were 

 preyed upon by various species of Bombylid files and Rhipidophorid 

 beetles; hence it was not surprising when ]Mr. Dodd (91) bred these from 

 Seoliid cocoons. 



The habits of the Thynnid wasps are very little known, yet there 

 is scarcely any doubt that they are enemies of Lamellicorn grubs. Aus- 

 tralia appears to be the natural home of this family of wasps, for, as 

 Froggatt states, of about 400 described species 300 are peculiar to this 

 country. Some of the larger species in the Cairns district are commonly 

 taken while feeding on the honeydew of corn leafhoppers. The females 

 being smaller and wingless are very inconspicuous ; yet these may 

 frequently be collected while mating, for the male carries her about with 

 him while feeding. 



Australian Dexiid® are usually large, beautiful flies. Mr. Dodd 

 (91) has reared eight species from cane-grubs, though he came to the 

 conclusion that the percentage of grubs destroyed by them was very 

 small. The following species are rather common in cane areas at times : 

 — Rutilia inornata G. & M., B. S2)h'ndida Don., li. 2)cUiiccns Macq., and 

 Amenia imperialis Rob. Desv. 



These flies do not enter the soil after grubs as do the wasps, so their 

 larvae are probably usually deposited in loose ground, or dropped into 

 natural cracks in the soil, where they may themselves seek their prey. 



Though Tachinids have been reared from Lamellicorn grubs, as 

 Davis (104) records, we have only bred flies of this family from the 

 beetles. Two undetermined species have been frequently bred out, the 

 KK)st abundant being a small brown species, several maggots of which 

 occur in a single beetle. The other, a conspicuous large Tachinid, with 

 gold and black on the thorax, is peculiar in that never more than one 

 maggot has been found in a host, in which case the beetle may live for a 

 considerable period and even deposit eggs. 



Beports of Poisonous Plants. — There is some question as to whether 

 the leaves of certain trees and other plants are poisonous to beetles. 

 Considerable attention was given at the time to a report of a corres- 

 pondent of the i\Iackay Sugar Journal (27). This referred to a 

 persimmon tree in an orchard on the Pioneer River. ^Nlany dead beetles 

 were found under this tree for two successive years, and the leaves Avere 

 much eaten. It was said that beetles confined after feeding on this plant 

 soon died, while some from a fig lived for a considerable period. How- 

 ever, nothing apparently ever came of this report, for it was found that 

 ])ersimmon trees in other localities did not have this desired eft'ect uj)on 

 the beetles. 



