BTUDIES ON THE INS&<;T FAUNA ON SCOTCH BROOM
191
flight on time. The data for the latter were obtained from the catches
in suction traps and on trap plante.
It ~ a a
found that very large proportions of populations emigrate, for
instance in the peak year of 0. vireecena (1962) it was calculated that
out of the total number of 660 OOO adults that emerged, 249 000276000 flew out of the plantation. The two estimates are bawd on
Waloff and Bakker’s and on Dempster’s (1961) methods.
I n 1960 the adult population of 0. &nocarpi was relatively small,
with a maximum 182 000 and it was calculatd that 64 000 emigrated.
The following was the peak year of 0. adenoearpi, the maximum number of adults in the field was 524 000, and 443 000 flew out of the
breeding site.
As far as is known, the density of adults had no detectable effect on
the emigrant population of H e t e r d y E u s , Asciodema and possibly of
0. a d e n m r p i , but in 0. virescena the numbers emigrating may have
been related to density. In 1962 when the adult population of 0. virescena doubled, 4.6 aa many individuals flew out of the plantation as in
the preceding year. Although the numbers of individuals collected on
the trap plants are a less reliable index than the catches in suction traps,
they were interesting as the total of 39 0. Uireacens in 1961 rose to 481
in 1962 and then again fell to 33 in 1963 when the population of adults
was halved.
Bince a large proportion of mirid populations emigrate a question of
equal interest was why do not all the adults disperse from the original
breeding site, and it was with this in mind that we (Lewis and Waloff,
1964) labeiled 0. virescens with two radioactive isotopes which could be
readily discriminated from each other autoradiographically. The two
seta of 0. virescen.9 were released in two section8 of the plantation, one
lot marked with P-32 near ita centre and the other marked with 5-35
near its edge. Samples collected inside the plantation and also outside it
on the trap plants provided evidence that there ia a considerable edge
effect o€ the habitat on the movement of the mirids and all the individuals marked with S-35 emigmted out of the plantation within the
first 24 h. There was very little movement in the individuals released
in the centre of Area 4. No doubt this “trapping” is temporary, but the
combination of its effect, chemo-attraction to the host plant and the
fall in flight-activity with maturation may account for a large proportion of the population staying within the breeding site. It is also probable that the attraction to the host plant may cause some return
migration of the populations milling in the air.
A mmparable set of observations on the fight of the moth LeuCoptera
eprtifoliella waa carried out by Agwu (1967). T h e mothe ale0 showed
activity which could be subdivided into “flitting” and ‘‘true flight”
191
flight on time. The data for the latter were obtained from the catches
in suction traps and on trap plante.
It ~ a a
found that very large proportions of populations emigrate, for
instance in the peak year of 0. vireecena (1962) it was calculated that
out of the total number of 660 OOO adults that emerged, 249 000276000 flew out of the plantation. The two estimates are bawd on
Waloff and Bakker’s and on Dempster’s (1961) methods.
I n 1960 the adult population of 0. &nocarpi was relatively small,
with a maximum 182 000 and it was calculatd that 64 000 emigrated.
The following was the peak year of 0. adenoearpi, the maximum number of adults in the field was 524 000, and 443 000 flew out of the
breeding site.
As far as is known, the density of adults had no detectable effect on
the emigrant population of H e t e r d y E u s , Asciodema and possibly of
0. a d e n m r p i , but in 0. virescena the numbers emigrating may have
been related to density. In 1962 when the adult population of 0. virescena doubled, 4.6 aa many individuals flew out of the plantation as in
the preceding year. Although the numbers of individuals collected on
the trap plants are a less reliable index than the catches in suction traps,
they were interesting as the total of 39 0. Uireacens in 1961 rose to 481
in 1962 and then again fell to 33 in 1963 when the population of adults
was halved.
Bince a large proportion of mirid populations emigrate a question of
equal interest was why do not all the adults disperse from the original
breeding site, and it was with this in mind that we (Lewis and Waloff,
1964) labeiled 0. virescens with two radioactive isotopes which could be
readily discriminated from each other autoradiographically. The two
seta of 0. virescen.9 were released in two section8 of the plantation, one
lot marked with P-32 near ita centre and the other marked with 5-35
near its edge. Samples collected inside the plantation and also outside it
on the trap plants provided evidence that there ia a considerable edge
effect o€ the habitat on the movement of the mirids and all the individuals marked with S-35 emigmted out of the plantation within the
first 24 h. There was very little movement in the individuals released
in the centre of Area 4. No doubt this “trapping” is temporary, but the
combination of its effect, chemo-attraction to the host plant and the
fall in flight-activity with maturation may account for a large proportion of the population staying within the breeding site. It is also probable that the attraction to the host plant may cause some return
migration of the populations milling in the air.
A mmparable set of observations on the fight of the moth LeuCoptera
eprtifoliella waa carried out by Agwu (1967). T h e mothe ale0 showed
activity which could be subdivided into “flitting” and ‘‘true flight”