PART IV. 29
on the reason for its initial spread a few years ago, but it will be shown
presently how, having once escaped even only to a small extent from
the controlling agencies which must have held it in subjection so long,
its further rapid multiplication was probably inevitable.
The life-history of this mealy-bug, when growing on coffee, may
be shortly summarised as follows: Generations are continuous
throughout the the year: it is almost impossible to estimate accurately
the time required for development from egg to adult under natural
field conditions, but in the laboratory this can be accomplished in
about 36 days for the female, and 33 for the male. The males
number some 109, of the total, or less; it appears that shade and a
high humidity increase the percentage of males produced, but the
significance of this is not yet understood. Neither is it yet known to
what extent the male is necessary for the continued reproduction of
the species—several generations can certainly be produced parthenogenetically,
 but experiments which are still in progress seem to be
pointing to the fact that this method of reproduction cannot go on
indefinitely.
There appear to be two main periods of abundance, each
commencing with or soon after the main flowering periods of the coffee
tree, in October and March. The causes influencing these seasons
are not yet known with certainty. To some extent the damp but not
unduly cold weather at these times of the year is directly favourable
to the mealy-bug; it also seems probable that the mealy-bug is able
to breed faster and more prolifically when the buds, flowers, and
young cherries are available as food, then when it has to feed on the
more mature cherries and the leaves. Again the ants attending the
mealy-bug are more abundant and active during rainy weather, while
the rains have an adverse effect on some, at least, of the predatory
enemies.
No reliable figures have been obtained of the actual number of
eggs laid by a coffee mealy-bug in this country, but under laboratory
conditions in the absence of all enemies, the number of young mealybugs
 which survive to about half way through the first instar, averages
about 150. Borden (2, p .5)} and other workers give 300—600 as the
number of eggs laid by P. citri when feeding on citrus.
For the benefit of those who delight in statistics, it may be stated
that at a conservative estimate, at the end of one year, the progeny
of a single mealy-bug, supposing all to survive, would weigh forty
thousand million tons.
Fortunately, however, for the coffee planter, some few tons of
these mealy-bugs fail to survive; we will therefore now consider the
factors which normally destroy the greater proportion of each
generation before it reaches maturity, and which, with a little human
aid, may well destroy all but an infinitesimal part.
Firstly, there are the climatic factors. Little as we know at
present about certain aspects of climate in relation to the Mealy-bug,
it can be definitely stated that a free circulation of air, resulting in a
lowered relative humidity, has a controlling effect, and that the worst
attacks of Mealy-bug are to be found in positions sheltered from the
prevailing N.E. wind, whether owing to the lie of the land or to the
prefence of wind-breaks. For this reason, an attack is much less
likely to be severe in a young plantation, and when it does occur in a
t See list of references on page 194.

185: