470 QUEENSLAND AGRICULTURAL JOURNAL. {1 Dec., 1897. 
Beyond this outcome of his knowledge that will reach him, when to stay his hand 
in destroying insects, the fruitgrower will seldom be able to put his information 
regarding the predaceous or parasitic insects to any profitable account. There are a 
few occasions, however, when this information may serve him, as will appear from the 
following illustrations :—Queensland, amongst other insect pests, is endowed with a 
special tree defoliator, a species of case-moth caterpillar, that often makes sad havoc 
with some of the most esteemed ornamental plants—e.g., the Shea Oaks or Grevilleas, 
and the members of the genus Pinus, especially P. insignis. In many cases it is 
. practicable to deal with these insects by hand-picking Now, inasmuch as the female 
insect is a wingless insect, and cannot therefore travel far, and is moreover very much 
addicted to having parasites, it is expedient to place all the case-moth caterpillars in 
the centre ofan open field, and so let their parasites effect their escape in order that 
they may renew their useful work, and not at once destroy them, as should be done 
generally when other destructive insects are collected. 
Again, in spraying for the purpose of destroying Scale Insects, a knowledge of 
the fact that some examples harbour parasites may be taken advantage of, and sv the 
friendly insect suffered to co-operate. “Let us suppose,” to quote F. M. Webster, 
“that we have 1,000 individuals of some species of Scale Insect, each capable of 
producing 100 young, so that, if all survived, we should have for a single generation 
109,000 individuals ; and, further, that we also have 100 parasites (or parasitised scale 
insects) capable of producing in one generation 10,000 individuals—a ratio as between 
scale insect and parasite of 10 to1. Now, if we apply an insecticide and destroy one- 
half of the Scale Insects, and none of the parasites, we have reduced the ratio of 5 
to 1, thus doubling their effective strength.” Now it has been found possible to 
accomplish this in the case of the plum Lecanium, by applying the insecticide when 
the parasites were still beneath the old scales and protected by them, whilst the young 
unparasitised scales were so thin that the kerosene emulsion, the agent used, penetrated 
them easily in many cases, “so that while 50 per cent. of the young scale were killed, 
the parasites were uninjured and appeared in spring in. such numbers as to overcome 
the balance of the young scale that survived. ‘These, in fact, did no injury, then: but 
for the effect of the parasites, they would in all probability have worked serious 
damage.’ * 
With regard to the practicability of introducing predaceous or parasitic insects 
from countries or regions in which they exist, to others in which they are absent, one 
is again confronted with difficulties, and discriminatory knowledge is again demanded. 
Tntroduced insects, as a rule, “show a greater power of increase than do indigenous 
species, and in a large number of instances have taken the place of the native forms, 
which have not been able to compete with them in the struggle for existence.” Now, 
unless the predaceous insects or parasite is kept in check in its native home by another 
insect parasitic upon it, and which will not serve to check its increase in its adopted 
country, we should expect as an outcome of this increased prolificness that the insects 
which are useful in subduing insect pests in their native home would, in proportion to 
to this increased vitality, be less so in the country in which they have been introduced. 
It is, however, often difficult to introduce beneficial insects without parasites to prey 
upon them, or .so place them, when introduced that they shall be surrounded by 
congenial circumstances. Moreover, introduced friendly insects may be brought into 
competition with native ones, not always to the advantage of the latter. The following 
will illustrate the absence of results that may follow the introduction of imported friendly 
insects:—A few years ago the able Entomologist of the Hawaiian Government, Albert 
Koebele, introduced into California no less than sixty species of predaceous and 
parasitic insects, for the express purpose of coping with the injurious insects of that 
State, but with the exception of the signal service rendered by Vedalia cardinalis and 
Novis Kebelei in exterminating the Cottony Cushion Scale, “in many thousands of 
specimens there is not one that can be considered a success at the present timet”; 
that is according to the testimony of John B. Smith, the well known Entomologist of 
the New Brunswick Experiment Station, who specially visited California to investigate 
the subject. 
The experience of Australia in the matter of the importations of predaceous and 
parasitic insects that have already been made, is not altogether encouraging, with the 
peeable exception of that afforded in the introduction of the English Humble Bee to 
ew Zealand. With the object of effecting by this means, the fertilisation of the Red 
* ¥, M. Webster: Indiana Horticultural Report, 1896. 
+ John B. Smith, Proc. of the 8th Annual Meeting of the Association of Economic 
Entomologists, 1896, p. 48. 
