234
€ I .
RLOMP
percentage parasitism was established and the proportion affected by
disease determined. The latter value, it is true, cannot be transferred
directly to the field population, but by comparing the rearing results of
different years a relative measure of the health of the insects can be
gained.
A. R E A R I N G O F EGGS
As reported (p. 212) egg density was determined by way of twig
sampling. The eggs were classified as “hatched”, “parasitized”, “unfertilized”, “empty”, or “dead”. The green, unhatched eggs present in
the samples were reared, and then classified. The data over three years
with high, intermediate, and low density are presented in Table XI1 and
the summarized results for all years are presented in Table XIII.
The eggs may be infected by one species of parasite, Trichogramma
embryophagum Htg., such eggs turn black during development, and are
readily recognized. The sampling dates guarantee that all, or nearly all,
of the unhatched host eggs found in the samples are older t h m 2 days.
Based on the results of some preliminary experiments it was originally
assumed that Trichogrumma does not infect host eggs older than two
days (under summer temperature conditions). Therefore, at the beginning of this study it was thought that all samples provided an unbiased
estimation of the percentage of eggs infected in the field. Later experiments showed that the parasite is able to oviposit successfully in host
eggs in which development has progressed. This implies that, the values
obtained at the beginning of the sampling period may represent too low
estimations. Therefore, such values were eliminated where this was the
case, as for instance in 1951 (see Table XII).
As demonstrated by Laing (1937) Trichogrammu is adapted for infecting egg batches and nearly always parasitizes the whole batch. This
can lead to an abnormally high percentage of parasitism when egg density is so low that the number of batches per sample is small. Thus, in
1958, sample 138 contained seven batches, one of which consisted of
eight infected eggs, resulting in an overestimation of the fraction parasitized (see Table XII).
Unfertilized eggs colour deep green, and dent gradually after about
10 days. It appears that such eggs are occasionally deposited by fertilized females in a batch’ of normal eggs. Empty eggs include all those
eggs whose content is partly or wholly absent without a visible perforation of the chorion. The shell of empty eggs usually becomes indented.
The category “embryo dead” comprises all eggs in which the embryo
dies during development due to unknown causes, and those in which the
fully developed first instar larva fails to leave the chorion through the
hole actually made by the larva.
€ I .
RLOMP
percentage parasitism was established and the proportion affected by
disease determined. The latter value, it is true, cannot be transferred
directly to the field population, but by comparing the rearing results of
different years a relative measure of the health of the insects can be
gained.
A. R E A R I N G O F EGGS
As reported (p. 212) egg density was determined by way of twig
sampling. The eggs were classified as “hatched”, “parasitized”, “unfertilized”, “empty”, or “dead”. The green, unhatched eggs present in
the samples were reared, and then classified. The data over three years
with high, intermediate, and low density are presented in Table XI1 and
the summarized results for all years are presented in Table XIII.
The eggs may be infected by one species of parasite, Trichogramma
embryophagum Htg., such eggs turn black during development, and are
readily recognized. The sampling dates guarantee that all, or nearly all,
of the unhatched host eggs found in the samples are older t h m 2 days.
Based on the results of some preliminary experiments it was originally
assumed that Trichogrumma does not infect host eggs older than two
days (under summer temperature conditions). Therefore, at the beginning of this study it was thought that all samples provided an unbiased
estimation of the percentage of eggs infected in the field. Later experiments showed that the parasite is able to oviposit successfully in host
eggs in which development has progressed. This implies that, the values
obtained at the beginning of the sampling period may represent too low
estimations. Therefore, such values were eliminated where this was the
case, as for instance in 1951 (see Table XII).
As demonstrated by Laing (1937) Trichogrammu is adapted for infecting egg batches and nearly always parasitizes the whole batch. This
can lead to an abnormally high percentage of parasitism when egg density is so low that the number of batches per sample is small. Thus, in
1958, sample 138 contained seven batches, one of which consisted of
eight infected eggs, resulting in an overestimation of the fraction parasitized (see Table XII).
Unfertilized eggs colour deep green, and dent gradually after about
10 days. It appears that such eggs are occasionally deposited by fertilized females in a batch’ of normal eggs. Empty eggs include all those
eggs whose content is partly or wholly absent without a visible perforation of the chorion. The shell of empty eggs usually becomes indented.
The category “embryo dead” comprises all eggs in which the embryo
dies during development due to unknown causes, and those in which the
fully developed first instar larva fails to leave the chorion through the
hole actually made by the larva.