22 



SCIENTIFIC AGRICULTURE 



January, 1921. 



with the same fungus. Their results are summarized 



as follows: 



1 ''Iji fields where the natural presence ot the 



fungus is plainly evident, its effect on the bugs cannot 

 be accelerated to any appreciable degree by the arti- 

 ficial introduction of spores. 



2_"In fields where the fungus is not m evidence 

 spores introduced artificially have no measurable 



pf IGCt 



3 —'"Apparent absence of the fungus among chinch 

 bugs in a field is evidence of unfavorable conditions 

 rather than lack of fungous spores. 



4 "Laboratorv experiments can be made to prove 



that artificial infection accomplishes results upon bugs 

 confined in cramped quarters and without food, but m 

 the field, where fresh and usually drier air prevails 

 and food is abundant, an entirely different situation 

 is presented." . , , .^, ,, 



In 1912 Morrill and Back experimented with the ar- 

 tificial use of the wihite fly fungi. "Aegerita. webberi." 

 ''Aschersonia aleyrodis" and "A Chyocinta." They 

 summarized their conclusions in these words: 



1, "The fungus parasites thrive only under suit- 

 able weather conditions during a period of about three 

 months each vear; generally speaking the summer 

 months in the case of the two Aschersonias and the 

 fall months in the ease of the brown fungus. 



2. "Under natural conditions, without artificial as- 

 sistance in spreading, the fungi have ordinarily, in 

 favored localities, controlled the white fly to the ex- 

 tent of about one-third of a complete remedy through 

 a series of years. 



3. "The infections secured by artificial means ot 



introducing fungi, while successful in introducing the 

 fungi, have thus far proved of little or no avail in in- 

 creasing their efficacy after they have once become 

 generally established in a grove. 



4 __" Experiments by the authors, and by citrus 

 growers in co-operation with the authors, involving the 

 treatment of thousands of trees with suitable "checks" 

 or "controls" have shown that when fungus (red or 

 yellow Aschersonia) even in small quantities is pre- 

 sent in a grove, there is no certainty that from three 

 to six applications of fungus spores in water solution 

 will result in an increased abundance of the infection 

 on the treated blocks of trees by the end of the sea- 

 son. In some of the most important and carefully 

 planned and executed experiments, the fungus has in- 

 creased more rapidly in sections of the groves which 

 were not sprayed with spore solutions than in the ex- 

 perimental blocks." 



The Brown-tail caterpillar is attacked by the fungus 

 Entomophthora aulicae, and an attempt was made by 

 Speare and Colley in 1912 to use it in the control of 

 the pest. They state that considerable success has at- 

 tended their efforts, not that the fungus is a "cure-all" 

 but it is a powerful check. Under proper conditions of 

 introduction from 63 to 100 percent of the caterpillars 

 can be destroyed. 



Reference has already been made to the ''Wilt di- 

 sease" of Gypsy-moth caterpillars as a factor in the 

 control of these pests in New England. It made its 

 appearance about 1900 and is now distributed over 

 the territory infested with the Gypsy-moth, accord- 

 ing to Glaser. It is an infectious disease but epide- 

 mics occur only in localities infested heavily with the 

 Gypsy-moth. Infected caterpillars become flaccid and 

 later their tissues disintegrate completely, due to the 

 fermentative and toxic nature of the virus. The 



brown liquid of a dead caterpillar shows under the 

 high power of the microscope large numbers of poly- 

 hedral bodies of various sizes, but the exact nature of 

 the causal organism has not been determined. The 

 virus is filterable with difficulty. 



The success of wilt infection experiments is absolu- 

 tely dependent upon attention to seemingly insignifi- 

 details, but this much is known that infection takes 

 place through the mouth by means of the food. 



It is probable that further studies of the disease will 

 evolve some practical method of using the virus for 

 the destruction of larger numbers of the caterpillars. 

 Already, however, it has been ascertained that climatic 

 conditions appear to bear an important relation to 

 wilt ill tile field, and that temperature has an important 

 relation to the period of inciiliation of wilt. 



4. — The Relation of Insects to Inorganic Nature. 



This relation 'has also been investigated by many 

 workers for it has long been known that insects are 

 influenced profoundly by climatic conditions. The 

 effects of changing and unusual conditions of heat and 

 cold, siiOAvs and rains, humidity and drjaiess, and other 

 seasonal changes have long been known to be important 

 factors in regulating the number of insects. 



A variable winter is more fatal to most forms of 



Sphinx Caterpillar with cocoons of braconid parasites. 



insect life than a continuously severe or a continuously 

 mild winter. When insects go into hibernation they 

 become torpid and are able to resist quite low degrees 

 of cold, but if thaws occur they may become partially 

 active for a short time. With every change of this 

 nature the insect loses vitality and this loss may prev- 

 ent transformation in the spring. Moreover, the effects 

 of thaws in breaking up larval and jiupals cells in the 

 ground are often quite marked. 



Sudden changes of temperature of say -iO deg. 

 range within a few hours, whidh ai'e quite common in 

 our latitude, are very fatal to aphids and many 

 caterpillars during itJieir early stages. 



Another feature of the environment of insects is 

 the different degree of humidity demanded by each 

 species or genus. Some insects like t.iie thripsi, chinch 

 bug, wiheat midge and red spiders are more abundant 

 under dry conditions, while other insects like the 

 plant- lice and Hessian-fly develop bret under moist 

 conditions. 



Observations seem to show that meteorollogical 

 changes are often fatal to those insects that live on 

 the fringe of their distril)ul'ional range. Under fav- 

 orable conditions some insects may migrate north- 

 ward and even do mucji damage, but such move- 

 menits may be termed incursions as they are tem- 

 porary and spasmodic in t'heir character. 



The range of distribution of different insects has 

 been mapped, and this hais shown that insects tend 

 to conform to the same zonal disitribution as plants, 



