Skógræktarritið - 15.05.2001, Blaðsíða 177

SKÓGRÆKT HANDAN SKÓGARMARKA / NSSE
CONCHITA ALONSO
Is plant chemistry determining
mortality and dispersal of young
Epirrita autumnata larvae?
SAMANTEKT
Það er nauðsynlegt að ákvarða þá þætti sem valda mismunandi lauf-
skaða ef skilja á þróun varna gegn jurtaætum. Úti í náttúrunni er tvennt
sem orsakar það að einstök tré verða fyrir meiri laufskaða en önnur tré
sömu tegundar, nefnilega: það getur staðið undir fleiri jurtaætum, eða
hvereinstök jurtaæta geturétið meira magn laufa mælt í lífmassa. Mis-
munur í atferli við fæðunám sem ekki orsakar dauða lífverunnar mun
aðallega hafa áhrif það hvað lirfustigið varir lengi og endanlega stærð
jurtaætunnar, á meðan breytileiki í fjölda jurtaætna mun að líkindum
ákvarða mun í laufskaða plantna innan plöntustofnsins.
Val skordýra á varpstað, og dreifing og afföll á ungum Iirfum ákvarða
sennilega fjölda skordýra sem nærast á tiltekinni plöntu. Það er vitað
fyrir haustfiðrildið, Epirriia autumnata, að mæður eru ekki vandfýsnar
þegar þær eru að verpa. Hinsvegar er lítið vitað um dreifingu og afföll
á ungum E. autumnata lirfum. Hér sýni ég niðurstöður úr tveimur til-
raunum þar sem svifdreifing á spunaþræði, og afföll á ungum lirfum
voru athuguð á einstökum birkitrjám sem vitað var að höfðu mismun-
andi lífefnasamsetningu. Niðurstöður úr þessum tilraunum benda til
að gæði laufanna geti haft áhrif á afföll hjá ungum E. aulumnata lirfum,
en séu ekki líkleg til að hafa áhrif á dreifingu þeirra.
Traits affecting oviposition selec-
tion, dispersal and mortality of
young larvae could be consid-
ered as the most efficient plant
defenses against herbivorous
insects since they will determine
the final number of consumers
sustained and, thus, the defolia-
tion experienced by different
plants. Instead, plant characteris-
tics modifying feeding behavior
without affecting mortality of
individuals (i.e., percapita con-
sumption), will mostly determine
affect the length of larval period
and the final size of individuals
that will not drastically change
the defoliation of the plant in the
current season. Furthermore,
both aspects may be determined
caused by the same factors with
additive or non-additive effects,
and they can also modulate the
responses of the natural enemies
of the herbivores (Leather and
Walsh 1993, Thompson 1988a,
Hunterand Elkinton 2000).
Furthermore, distinguishing
these different steps in the inter-
action between plants and her-
bivorous insects can be also is
relevant to understand the evolu-
tion of their relationships since
mother selection and young lar-
vae selection both imply active
selection by the herbivore who
would in turn play the role of
selection pressure on plant char-
acteristics, whereas, survival of
young larvae implies differential
mortality and thus plants would
be in this case the selection
pressure on herbivores (Thomp-
son 1988b). Only detailed field
studies of herbivore densities
and defoliation can distinguish
these different sources of varia-
tion under natural conditions
(e.g., Hunteretal. 1997, Hunter
and Elkinton 2000). In addition,
indoor controlled experiments
may be useful, however, to evalu-
ate the potential relevance of
these different stages and spe-
cially to discard those with low
possibilities to affect the inter-
action between particular
species.
The interaction between the
autumnal moth (Epirrita autumnata
Bkh.) and one of its main host
plants the white birch (Betula
pubescens), has been studied from
many different perspectives
(Ruohomaki et al. 2000 and refer-
ences therein). Epirrita autumnata is
a univoltine geometrid species.
individuals overwinter as eggs,
and the new generation hatches in
spring, when synchrony with leaf
flush is important for larval devel-
opment (Ayres and MacLean
1987). Duration of the larval stage
depends on temperature and
foliage quality. The pupal mass
reached at the end of larval devel-
opment is a good estimate of real-
ized adult fecundity (Tammaru et
al. 1996). The short-lived adults
eclose in autumn. Females do not
usually fly before oviposition and
they are not selective while ovi-
SKÓGRÆKTARRITIÐ 2001 l.tbl.
175