448 
The Progress of the Hessian Fly. 
Fig. II. — Platygasler HerricJcii. Packard. 
Insect m.i"nilied. Lines showing natural size. 
Concerning the parasites of the Plessian FJy, erroneous ideas 
have been formed and promulgated in the newspapers. The prin- 
cipal of these is that no measures should be employed to check 
its progress, because at the same time the parasites would also 
be checked or destroyed. This means that we are to allow the 
insect to multiply without taking any action, and to trust to 
the parasites to rid us of the pest. This is not advocated by 
American authorities, with their long experience of nearly one 
hundred years. On the contrary, it is shown by the reports of 
Fitch, Packard, and Riley, that in some seasons the parasites are 
few and far between, from certain causes, climatal in all proba- 
bility, and are perfectly unable to cope with and stay the increase 
of the insects. Packard, speaking of the " periodicity " in the 
abundance and scarcity of the Hessian Fly in the United States, 
observes that " this is due to the influence of the weather, of 
favourable and unfavourable seasons, and partly, in most cases, 
to the absence or abundance of the insect parasites, although the 
latter cause is largely influenced by climatic agencies." Ft might 
happen that if the farmers in Great Britain were unwise enough 
to put implicit faith in the parasites and allow the Hessian Fly 
to breed without let or hindrance, a season would come in 
which they would find that they had trusted to a mere broken 
reed. Hop planters have been advised to let alone their Aphis- 
infested hop-plants, and not wash them with soft soap or 
quassia, as this would kill or drive away the ladybirds, CocciiicUa;, 
from their hunting-grounds. Those who have taken this advice 
have frequently found too late that the ladybirds have been of 
no avail against the quickly generated hosts of Aphides which 
have ultimately ruined their crops. The Chalcid parasites, in 
short, are most useful allies against the Hessian Fly, but must 
