travel to find it. First instar larvae are physically incapable of biting through the cuticle of
even moderately-toughened leaves, so they must find newly-flushed leaves. Scarcity of
foliage is caused by defoliation or the abscission of damaged foliage which often follows
severe attacks (Jacobs 1955); by a lower rate of foliage production; or by a faster rate of leaf
BIOLOGY OF P.CHA.RYBDIS
toughening. The distance a larva must travel is also affected by several factors. If egg
numbers are high, clusters may be laid further back from young foliage because of the
consequent competition for oviposition sites. Furthermore, if the ratio of incubation time to the rate of foliage toughening increases, then there will be more toughened leaves and a greater distance to travel between the egg cluster and new foliage.
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Larval food location seems to depend largely on gustation or mechanoreception. Simple tests suggested that olfaction may occur over distances of a few centimetres. This is unlikely to provide any directional guidance under field conditions. My observations suggest that larvae may distinguish petioles of old and young leaves by any or all of their diameters, taste or resistance to biting. Larsson and Ohmart (1988) noted that
P.atomaria
larvae were also able rapidly to distinguish young and old leaves by palpating the base of the leaf. Bifurcations in the stem create more difficult problems for the larvae, particularly if both branches are similar in diameter. The female's oviposition behaviour minimises the chance of larvae encountering a bifurcation and even if shoot growth presents larvae with such a choice, both branches will probably carry newly-flushing foliage.It is apparent, therefore, that mortality in the first larval stadium could be a result of both density-dependent and density-independent factors. Scarcity of foliage caused by defoliation
clearly results in density-dependent mortality, but larval density probably does not
significantly affect the rate of leaf toughening. The results of the experimental study of 1st instar mortality suggest that when new leaves are abundant, mortality is independent of density.
The estimate of high mortality of
P.charybdis
on the ground at Kahu Farms is subject topossible error if fourth instar larvae dropped from the tree before they were capable of successfully pupating. Factors like wind gusts or death of larvae on the tree could have caused this effect. These would increase the apparent number of larvae surviving the fourth
BIOLOGY OF P.CHARYBDIS
instar on the tree. There are two reasons why this error is likely to be small. Firstly, the larvae have well-developed abdominal adhesive organs which make them very difficult to
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dislodge. The most probable way in which larvae could be dislodged from the tree is if the leaf or branch itself is blown down and because larvae are found predominantly on young foliage rather than the older leaves which are far more prone to abscission, then loss of larvae this way is probably small. However, larvae feeding on new foliage are at risk because other larvae feeding on the same leaf may excise leaf sections that support larvae. Secondly, diseases that may have cause moribund larvae to fall were not observed. The only pathogen I found was a fungus which attacked occasional late second-generation larvae, but in this case the cadaver was stuck firmly to the leaf by fungal hyphae. The number of larvae caught, then, may slightly overestimate the actual number leaving the tree to pupate.
The very high apparent mortality at Kahu Farms during this stage of the life history contrasts with Styles' (1969,1970) results. He found a maximum mortality of only 36.6% for mature
larvae placed in 203 mm diameter x 127mm deep metal cylinders sited in grass turf near his laboratory. Each contained 76mm of sieved soil covered with a layer of sphagnum moss and was closed at both ends with fme nylon mesh. Maximum mortality was 34.6% in similar
containers open at one end to admit soil-inhabiting predators. However, Styles' experiments were predisposed in several ways to underestimate mortality. Larvae in his containers had an excellent medium in which to pupate and this would protect them from physical hazards such as desiccation or from predation while searching for a site. Predators had only restricted access to the containers which were left open at the base, and this may have caused similarity between the two treatments (open vs closed containers), rather than Styles' interpretation that predation was unimportant in field populations. The loose pumice soils at Kahu Farms are generally free-draining and pupae may often have encountered very dry soil conditions. Unlike Styles' larvae, fourth instars at Kahu Farms were exposed to predation by such litter- searching birds as thrushes, blackbirds, Californian quail and pheasants and in some places suitable pupation sites may have been scarce, particularly where the ground was very dry and
BIOWGY OF P.CHA.RYBDIS