58 NATURAL CONTROL. OF WHITE FLIES IN FLOEIDA. 



RELATION BETWEEN ABUNDANCE OF WHITE FLIES AND RESULTS IN 

 SPREADING FUNGOUS INFECTIONS. 



Wliile theoretically introduction of fungi should begin as soon as 

 the presence of the white fly is discovered in a grove, the authors 

 have met with such poor success with all attempts at such introduc- 

 tions that they have recommended the waiting until the white fly 

 becomes abundant enough to cause a very slight blackening of foliage. 

 Attempted earlier introductions have proved practical failures. It 

 is contrary to natural laws governing the relation between host and 

 parasite to expect to keep the fungus abreast of the fly all the time, 

 and all experiments and observations during the past three years have 

 failed to bring out a single instance where the fungus has spread, 

 artifically or naturally, in a newly infested grove soon enough or fast 

 enough to prevent the blackening of foliage. One can reasonably 

 hope for success in holding down the fly in slightly infested groves 

 only by careful attention to the direct remedial measures. 



SUSCEPTIBILITY OF DIFFERENT STAGES OF HOST INSECTS. 



Experiments have shown that the presence of no one instar of 

 either species of white fly is essential to successful infections, or that 

 any one larval stage is more susceptible to fungous attack than 

 another, or than the pupal stage. Considering the large number of 

 larvaB that hatch and the high rate of mortality that greatly reduces 

 the number of forms in each successive instar, it is only natural that 

 such leaves sprayed with spore solutions when the larvae are very 

 young should develop a large percentage of pustules on young larvae. 

 It has been found equally true that a much larger percentage of 

 pustules develops on advanced larvae and pupa^ when introductions 

 are made when the fly is largely in these later stages. A count of 40 

 leaves of various ages, picked promiscuously and with the citrus 

 white fly in all stages, gave the percentages of rod and yellow Ascher- 

 sonia pustules developed on the first, second, and third larval, and 

 on the pupal stage as 33, 82.1, 22.2, and 12.7, respectively. Another 

 count following introduction of the fungus in experimental work gave 

 in percentages of the total number of pustules developed: Pupal stage 

 36.5 per cent, third larval stage 34.5 per cent, and first and second 

 larval stages 29 per cent. Examination showed that the various 

 stages of fly were present in about this proportion at the time of the 

 application of the fungous spores. 



COST OF INTRODUCING AND SPREADING PARASITIC FUNGI. 



The very low cost of introducing fungi into white-fly infested groves 

 has influenced many to resort to this method of control, hoping to 

 get much for little. Men who have taken up the matter in a commer- 

 cial way furnish the supply of fungus and spray trees with water 



