A simple method for improving estimates of percentage
parasitism by insect parasitoids from field sampling of hosts
DEREKA. RUSSELL*
Department of Zoology, Victoria University of Wellington,
Private Bag, Wellington, New Zealand
ABSTRACT
A method is presented for improving the estimation of percentage parasitism in insect
field sampling data, by multiplying the apparent percent parasitism as determined
in the field by the ratio of the development periods during which the parasitoid and
the corresponding unparasitised host stage are present in the field.
Keywords: Parasitism, percentage, parasitoid, calculation.
The calculation of percentage parasitism from field data consisting of regular host samples
is fraught with difficulties which are generally ignored in the presentation of results. This
note attempts an improvement in the standard calculation of percent parasitism for insect
parasitoids.
PROBLEMS IN CALCULATING PERCENTAGE PARASITISM
Ideally the measure of parasite effect on the host should include a component for hosts
destroyed during adult parasite feeding, those which die of paralysis or trauma due to
parasite activity but in which no eggs are in fact laid, and hosts disrupted in their normal
behaviour patterns to the point of suffering higher mortality. I n most field studies none
of these factors can be measured and percent parasitism is defined as the percentage of
susceptible hosts parasitised in each generation. This limited definition is employed here.
Where the parasitoid and host leave some evidence of their presence it may be possible
to accurately calculate percentage parasitism at the end of each generation as
No. parasitised host remains
No. non-parasitised host remains No. parasitised host remains
(equation 1)
However such remains are not always unequivocal evidence of the presence of a
particular parasitoid species and, more seriously, host and parasite remains, if any, are
not always readily attributable to a particular generation in multi-voltine species when
+
* Present address: Station de Zoologie et d'Apidologie, Institut National de la Recherche
Agronomique, Montfavet 84140, France.
New Zealand Entomologist, 1987, Vol. 10
39
development is rapid. In such cases a subjective assessment of the 'freshness' of the remains
is necessary while sampling.
There are a number of further considerations which should be taken into account when
calculating percentage parasitism (Van Driesche 1983). The rates at which hosts enter
and leave the susceptible stage and the rates of parasite oviposition and emergence can
greatly affect the perceived levels of parasitism. Ideally one would calculate the ratio of
the standing crop of immature parasitoids to the standing crop of the susceptible host
stage. In the case of species in which the following 2 requirements are met for a particular
point in time:
1. All hosts have entered the susceptible stage but none have left
2. All parasitoid ovipositions are complete but no parasitoid emergences have yet taken
place
then an accurate measure of parasitism can be made with 1 or more samples at that point
in time. However, in many cases host entry into, and exist from, the susceptible stage
is either continuous or confusingly overlapped and parasitoid development may be effectively
continuous or unknown, especially if alternative hosts are utilised.
For this, normal, situation there are problems in the assessment of percentage parasitism
due to the differing phenologies of parasitised and unparasitised hosts. The parasitised host
often remains susceptible to sampling in the field for the full length of the parasitoid
developmental period, from egg to adult. Frequently this is longer than that of the
unparasitised host stage. There is thus a tendency to over estimate the number of parasitised
hosts in comparison with the number unparasitised.
AN ARITHMETIC METHOD OF IMPROVING ESTIMATES OF
PERCENTAGE PARASITISM
To help to overcome this latter problem, the number of parasitised hosts found per sample
period can be adjusted in line with the relative length of time parasitised and unparasitised
individuals are present in the field and susceptible to sampling. A correction factor can
be calculated from the ratio of known developmental periods of parasitoid and host, derived
from field or laboratory studies or from the literature.
As an example I consider the oriental fruit moth (O.F.M.) (Grapholita molesta (Busck)),
a tortricid pest of stone and pip fruit and its ichneumonid pupal parasite Glabridorsum stokesii
(Cameron). The interaction of these 2 species is discussed elsewhere (Russell, 1987). Where
the developmental response with respect to temperature on the part of either the parasitoid
or the host is known to be non-linear, and the shape of the curve(s) is known accurately,
then suitable figures for the mean developmental periods at the prevailing temperature
should be used. This information is not available for G. stokesii and so the ratio of host
to parasitoid developmental period at a known constant temperature was used here.
Development periods
Species
Grapholita molesta
Glabridorsum stokesii
Stage
Cocoon period
Egg to Adult
Developmental
Period at 26.7OC
Days (mean) Reference
10.9
Croft et al. (1980)
15.0
Haeussler (1945)
The pupal period given in Croft et al. (1980) is really the period for which the insect
is in the cocoon stage. As O.F.M. spend an average of 2.2 days as prepupae within the
cocoon at 26.7OC (Russell unpublished) the correction factor applied to G. stokesii numbers
was therefore 8.7115 =0.58.
Then % Parasitism =
O.F.M. parasitised x 0.58
x 100
O.F.M. not parasitised +(O.F.M. Parasitise x 0.58)
1
(equation 2)
40
New Zealand Entomologist, 1987, Vol. 4
It will readily be appreciated that this decreases the estimate of percentage parasitism
by a maximum of 42 % at the lowest observed levels, reducing to 0 % at 100 % apparent
parasitism. Where intervals between samples are very short in relation to the field
development periods of host and parasitoid, this correction greatly improves the accuracy
of the estimated percentage parasitism. Where the sample intervals are an appreciable
part of the development period the computed result is less reliable but is still a considerable
improvement over the uncorrected figures.
This analysis also assumes the hosts are parasitised on the first day on which they are
present in the field in the susceptible stage. In general, ichneumonid pupal parasites
oviposit onfin the pupa at an early stage (Valentine, Entomology Division DSIR pers.
comm.) but this period no doubt extends over at least several days in the field and could
be corrected for in particular cases.
In studies of parasitism, even when parsitism has been considered as a mortality factor
in life table work, e.g. Danthanarayana (1980), this type of adjustment has not been made.
REFERENCES
Croft, B. A.; Michels, M. F . ; Rice, R . E., 1980: Validation of a P . E . T . E . timing model for
the oriental fruit moth (Lepidoptra: Olethreutidae) in Michigan and central California. Great
Lakes entomologist 1 3 : 2 1 1-217.
Danthanarayana, W., 1980: Parasitism of the light brown apple moth, Epihyaspostuittana (Walker),
by its larval ectoparasite, Goniozus jacintae Farrugia (Hymenoptera: Bethylidae) in natural
populations in Victoria. Australian journal of zoo lo^ 28: 605-692.
Haeussler, G. J., 1945: Gambrus stokesii Cam. -an Australian parasite of codling moth and oriental
fruit moth. Journal of economic entomology 3 8 : 103-106.
Russell, D. A , , 1987: Glabridorsum stokesii (Cameron) (Hymenoptera: Ichneumonidae) - a parasitoid
of the oriental fruit moth (Grapholita molesta) in New Zealand. N e w Zealand entomolqist 10: 26-38.
Van Driesche, R . G . , 1983: Meaning of 'percent parasitism' in studies of insect parasitoids.
Enuironmental entomology 1 2 : 1 161- 1622.