MALAYSIAN PARASITES X 
FEEDING TIMES OF TROMBICULID LARVAE 
By 
J. L. HARRISON 
Mites of the family Trombiculidae are, for most of their lives, free-living predators in the 
surface layer of the soil. Once in their lifetime, however, they are parasitic, for the newly 
hatched larva, or “ chigger ”, climbs onto a larger animal, attaches, and spends some day 
sucking the lysed tissues of the host. When it is engorged it drops off again and spends the 
rest of its life as a non-parasitic soil mite. While there is evidence that a chigger which has 
not been able to engorge may attach to a second host (e.g., Mackie et al. 1946, p. 213), the fact 
that engorged larvae will readily transform into nymphs (which are never known 
to be parasitic) argues that normally the mite feeds parasitically only once in its lifetime. 
The host varies with the species of mite, and with the opportunity for infestation, but may be 
a mammal, a bird, a reptile, or more rarely an amphibian or an arthropod. For any one species 
of mite there is usually one host, or one group of hosts, which is clearly the principal support of 
the species, but the range of possible hosts is often very wide. 
Mite populations 
A convenient measure of the population of mites may thus be obtained from the infestation 
rate of a principal host. For any one species of mite in any one kind of habitat a direct 
comparison of infestation rates is probably valid as a comparison of the populations, and thus 
infestation rates alone may be used to compare the effects of such variables as weather, soil, 
acaricides, and burning. 
W T hen different species of mites are involved, however, such comparisons are no longer 
valid, for different species spend different times attached to the host, and this must be taken 
into account. Thus if chigger A has a feeding time twice as long as that of B, then a certain 
population of chigger A will produce twice the average infestation of its host as will the same 
population of chigger B. The average number of fresh attachments daily will be the same for 
each species, but since A feeds for twice as long, the average number of A present at any one 
time (which infestation rate measures) will be twice that of B. 
A convenient measure, which takes this into account, is the “ turnover ” of chiggers, that 
is the number of chiggers attaching (or detaching) in unit time. It may be estimated by dividing 
the infestation rate by the mean feeding time. For example a mean infestation of 100 chiggers 
per rat, mean feeding time 3 days, represents a turnover of : 
^^=33.3 chiggers per rat per day 
OR 
— =1,000 chiggers per rat per month (since 3 days = i/ioth of a 30 day month). 
If the rat population is known (and it is now possible to give reasonably good estimates) 
the chigger population can be expressed as a turnover of so many chiggers per hectare per 
month. If the mean length of life of the mite is known this may be further expressed as an 
actual density of mites at any one time. 
It will be seen that the statistic needed is the mean duration of the time spent by the chigger 
on the host, and it is in this sense that the expression “ feeding time ” is used here. Since the 
chigger might take some time to settle down to its meal, this “ feeding time ” will be slightly 
MALAYA, No. 26. 1953 
