DYNAMICS OF FIELD POPULATION OF PINE LOOPER
217
From relation (2) :
B m = 0.1821 x 2 515 = 458 shoots
(c) The mean number of eggs or larvae/m2. Finally, we have to compute the mean number of larvae/m2. Having estimated the mean number of larvae per shoot (gs) and the mean number of shoots/m2 (8,)
larval density can simply be derived according to relation (1).
As already stated on p. 212 this relation can be applied only when
certain conditions are fulfilled. These conditions will now be discussed.
First, the eggs of Bupalus are deposited on the older needles, probably
because new shoots are very small and the needles are too short during
the period of oviposition. After hatching, the larvae disperse and can be
found later on needles of all ages, i.e. both 3n yearling shoots and on
older twigs. Nevertheless, we expressed density as a number per yearling shoot (cf. Table I), meaning the eggs or larvae occurring in a mass
of needles (of all year classes) corresponding i;o one yearling shoot. This
is permissible only if the twigs are cut off i L t the older leafless parts
Secondly, a t the top of a tree the shoots are very long and stout and
bear quite a number of needles. I n the lower part of the crown there are
small shoots formed. Consequently, per unit of weight, the number of
shoots a t the top is much fewer than a t the base of the crown, with
intermediate values in the middle. This means that the twigs sampled
must be taken from all levels of the crown itnd in proportional quantities (cf. p. 212). If the bottom part of the crown is overcut, then the
number of shoots in the sample will be too high and its size overestimated, because the value s, is based on all levels of the crown. It
proved to be extremely difficult to verify exactly the correctness of this
aspect of sampling, because of the irregular growth of the branches.
Therefore, control has been by sight only, trjing to cut off all the twigs
occurring in a definite sector of the crown (Fig. 2 ) .
Finally, the method may be biased because yearling shoots fall off
during the period between sampling and measuring the value of B m .
Sampling has been carried out from June up to the beginning of October,
the felling of trees to determine b m has normally taken place in November, in a few years during September or 0ctol)er. On the lower branches
some shoots may lose all their needles and drop off. The quantity of
such shoots in three successive years was:
(cf. p. 212).
1954 during the period 29 June-27 October
5.3 shoots/m2.
1955 during the period 29 June-19 November 7.0 shoots/m2.
1956 during the period 8 July-25 October 13.4 shoots/m2.
which comprised 1, 1.5, and 2.5% of the shoots present. These relatively
217
From relation (2) :
B m = 0.1821 x 2 515 = 458 shoots
(c) The mean number of eggs or larvae/m2. Finally, we have to compute the mean number of larvae/m2. Having estimated the mean number of larvae per shoot (gs) and the mean number of shoots/m2 (8,)
larval density can simply be derived according to relation (1).
As already stated on p. 212 this relation can be applied only when
certain conditions are fulfilled. These conditions will now be discussed.
First, the eggs of Bupalus are deposited on the older needles, probably
because new shoots are very small and the needles are too short during
the period of oviposition. After hatching, the larvae disperse and can be
found later on needles of all ages, i.e. both 3n yearling shoots and on
older twigs. Nevertheless, we expressed density as a number per yearling shoot (cf. Table I), meaning the eggs or larvae occurring in a mass
of needles (of all year classes) corresponding i;o one yearling shoot. This
is permissible only if the twigs are cut off i L t the older leafless parts
Secondly, a t the top of a tree the shoots are very long and stout and
bear quite a number of needles. I n the lower part of the crown there are
small shoots formed. Consequently, per unit of weight, the number of
shoots a t the top is much fewer than a t the base of the crown, with
intermediate values in the middle. This means that the twigs sampled
must be taken from all levels of the crown itnd in proportional quantities (cf. p. 212). If the bottom part of the crown is overcut, then the
number of shoots in the sample will be too high and its size overestimated, because the value s, is based on all levels of the crown. It
proved to be extremely difficult to verify exactly the correctness of this
aspect of sampling, because of the irregular growth of the branches.
Therefore, control has been by sight only, trjing to cut off all the twigs
occurring in a definite sector of the crown (Fig. 2 ) .
Finally, the method may be biased because yearling shoots fall off
during the period between sampling and measuring the value of B m .
Sampling has been carried out from June up to the beginning of October,
the felling of trees to determine b m has normally taken place in November, in a few years during September or 0ctol)er. On the lower branches
some shoots may lose all their needles and drop off. The quantity of
such shoots in three successive years was:
(cf. p. 212).
1954 during the period 29 June-27 October
5.3 shoots/m2.
1955 during the period 29 June-19 November 7.0 shoots/m2.
1956 during the period 8 July-25 October 13.4 shoots/m2.
which comprised 1, 1.5, and 2.5% of the shoots present. These relatively