16 THE RELATIONSHIP OF OX WARBLE-FLY AND SOIL MOISTURE [Vol. XIX, Part I,
working in the Punjab, observed that the percentage of goats harbouring the larvae
of this fly varied from 48.3 in some flocks to 91.8 per cent in others. A very high
incidence of warble-fly infestation in goats has also been recorded from the North-
West Frontier Province, Sind and Baluchistan.
In as much as the adult and the egg-stage periods of H. lineatum are very short
and its entire larval life is spent inside the animal's body, atmospheric humidity and
temperature can have little influence on these stages of its life-cycle. On the other
hand, the pupal period normally lasts for from 4 to 6 weeks and is spent underneath
the soil surface. It would, therefore, seem more than probable that one, if not the
only, condition that ensures the viability of the fly in its pupal stage is the freedom
of the soil from excessive moisture. This is illustrated in the case of the district of
Gujranwala in the Punjab, where, inspite of heavy warble infestation in the ad-
joining areas, the water-logged condition of the soil in certain localities of this dis-
trict has apparently operated against the development of H. lineatum in its pupal
stage. A similar example is provided by the marshy tract of tehsil Laki in the
district of Bannu, North-West Frontier Province. Support to the foregoing hypo-
thesis is also lent by the fact that, whilst varying degrees of infestations have been
seen in the hilly districts of Darjeeling, Kalimpong, Nilgiris and Shillong, the pest
has been found to be wholly or almost absent from the adjoining plains, this being
attributable, inspite of frequently heavy rains in these districts, to the quick loss
of the moisture content of the soil due to declivity and gravelly nature of the locali-
ties. It is of interest that, at Darjeeling and Kalimpong, the incidence of warble-fly
infestation was sometimes as high as 20 to 30 per cent. Bruce [1938], working in
the United States of America, has also referred to the influence of soil moisture on
pupal development in H. lineatum. He observes, Cattle grubs, Hypoderma bovis
De G. and Hypoderma lineatum (De Vill.), are found throughout the United States,
but there are certain areas where they are scarce or periodically absent. Most of
these areas are small and scattered, but there is one large area, the Red River Valley
of the North, where, under average climatic conditions, cattle grubs are entirely
absent. To most stockmen of this area cattle grubs are unknown. Quite naturally
the question arises, what factor or set of factors is responsible for the scarcity of
cattle grubs in this area ? This question was studied by the Bureau of Entomology
from 1929 through 1932. Apparently the dominant controlling factor is excessive
soil moisture.'
Observations made on H. lineatum larvae and pupae kept under known moisture
conditions at the Mukteswar laboratory have shown that grubs placed on soil saturat-
ed with moisture died without pupating. Out of a dozen pupae left over for 50
days on soil containing 20 per cent moisture and over, not a single emergence took
place, and the pupae on dissection showed no trace of adult development. Also
no fly emerged from a batch of twelve pupae kept at 15 per cent soil moisture and
on dissection only two of the puparia showed partially developed adults. In a
batch of six pupae kept at 10 per cent soil moisture, only one emergence took place,
the pupal period in this case being 42 days. Pupae kept in containers with soil
moisture varying from 1 to 5 per cent yielded the maximum number of emergences,