62 



If parasitism has begun early cnougli in a caterpillar or other host, 

 the victim is unable to progress to a further stage. This is the rule 

 with Ernestia rudis, Erycia aurulenta, and Ptychqmyia. Some species 

 emerge from the pupa as well as from the larva of the host. Erynnia 

 nitida has been said sometimes to emerge from adults of Galeruca. 

 Pupation usually takes place just beneath the surface of the ground. 

 Among the species that do not pupate in the ground are Sturmia 

 nidicola and Eudoromvia mafi^nicornis, which do not leave the host 

 caterpillars. The pupal stage may be as short as a week ; Arrhinomyia 

 innoxia, Meig., usually requires 3 weeks, but at 94° F. (35° C.) 1 week 

 suffices. 



Most Tachinids are polyphagous ; Compsilura concinnata has 67 

 hosts ; Bucentes cristata attacks Tipnla maxima, living in water, and 

 various caterpillars. Ceromasia inchisa, Diplostichus, TricJioparia, 

 and Sesiophaga are the only monophagous Tachinids known. 



There are from one to three generations a year. Compsilura and 

 Lydella nigripcs have a complete third generation, at least. Different 

 generations often have different hosts. In spring Phryxe vulgaris 

 attacks Hihernia and Cheimatobia , and its second generation attacks 

 Orgyia. 



The efficiency of Tachinids may reach 100 per cent, in outbreaks 

 of insect pests, their increase becoming very marked when the hosts 

 become abundant. Among the causes that check their increase are 

 unfavourable conditions for finding a host ; even eggs that are laid 

 on a host may be lost with the latter's cast skin. Fungi, such as 

 EmpHsa and Spicaria, and Chalcid hyperparasites also check Tachinids. 

 Still more important are flies of the genus Anthrax. During recent 

 nun-moth outbreaks the extraordinary increase of Hemipenthes 

 {Anthrax) morio, L., gave reason to fear a decrease of Tachinid 

 parasitism. Another check is the unsuitability of the available host, 

 which raises the question as to whether polyphagous or monophagous 

 species are the more beneficial. The former exercise a preventive 

 action that uniformly checks the host, whereas the latter exercise a 

 curative action by appearing in large numbers following on a rapid 

 increase of their specific host. 



Notes are given on methods of investigating the extent of parasitism, 

 on the avoidance of combative measures disadvantageous to the 

 parasites, and on cultural methods favourable to them. The best- 

 known example of such cultural methods is the growing in vineyards 

 of Eiionvmus and other food-plants of Hvponomettta in order to 

 encourage the latter's parasites, which are also parasites of the vine- 

 moths [R.A.E., A, viii, 356]. 



The successful establishment in North America of Tachinid parasites 

 of the brown-tail moth ^Nygmia phaeorrhoea] and gipsy moth \Porthetria 

 dispar] is instanced ; in new territory hyperparasites are absent 

 at first if care is taken not to import them with the beneficial species. 



When an outbreak is decUning in a given area and spreading else- 

 where, it is profitable to use the superfluous parasites in the first 

 district, thus counterbalancing the advantage of time usually possessed 

 b}' the host. After an outbreak of the nun-moth Liparis monacha] 

 or pine-moth [Panolis flammea], the cocoons of Parasetigena and 

 Ernestia should be collected for use elsewhere. Large numbers may 

 be obtained by confining parasitised mature caterpillars in a narrow 

 space. Lydella nigripes, Fall., and Carcelia riitilla, B.B., leave the 

 pupa [of Bupaliis piniariiis] a few weeks before the latter's flight 



