Skógræktarritið - 15.05.2001, Blaðsíða 178

positing (Tammaru etal. 1995,
1996). Although E. autumnata lar-
vae are polyphagous leaf chewers,
mountain birch Betula pubescens
subsp. czerepanovii (Orlova) Hamet-
Ahti, due to its abundance, is their
main host plant in Northern
Fennoscandia (Kallio and
Lehtonen 1973), where the
species periodically cause severe
defoliations. Mountain birch
leaves contain relatively high lev-
els of different phenolic com-
pounds (Ossipovet al. 1997)
whose quantities vary among indi-
vidual trees and with leaf develop-
ment (Suomela et al. 1995, Nurmi
et al. 1996), and can affect E.
autumnata performance (e.g.,
Kauseetal. 1999). Among these
phenolic compounds high gal-
lotannin concentrations are char-
acteristic of young developing
leaves (Ossipov et al. 1997, Kause
etal. 1999) and thus, theyare
potentially suitable defensive
compounds against the earliest
season leaf feeders the neonate
larvae of E. autumnata. However,
little is known about the effects of
birch chemistry on dispersal and
mortality of young E. aulumnata
larvae.
The dispersal of neonate larvae
by ballooning has been describ-
ed in some other Lepidopteran
species such as Lymantria dispar
(Lymantriidae) (Hunterand
Elkinton 2000), Operophtera bruma-
ta (Geometridae) (Tikkanen
2000), Orgyia vetusta (Lymantri-
idae) (Harrison 1995), and Thyrid-
opterix ephemeraeformis (Psychidae)
(Ghent 1999), in relation to host
plant species, budburst phenolo-
gy, natural enemies and abiotic
conditions. None of these stud-
ies has tried, however, to test if
larvae can use the same mecha-
nism to discriminate conspecific
plants differing in leaf character-
istics other than phenology (but
see Harrison 1995). Here 1 pre-
sent results from two experi-
ments where ballooning disper-
sal, and mortality of young larvae
were studied in individual moun-
tain birches known to differ in
their foliage chemistry.
Materials and methods
Ballooning experiment
Ballooning studies were carried
out on early ]une 1998 using the
same 30 mountain birch trees
whose leaves had been previous-
ly analyzed and tested for quality
as food for E. autumnata larvae
(Lempa et al. 2000). Two other
substrates than mountain birch,
a glass bar and a branch of pine,
were also used to test the capa-
bility of E. aulumnata larvae to
balloon under laboratory condi-
tions. As a standard procedure a
table home ventilator (Finca©)
was used to produce a continue
air flow that allow larvae to es-
cape from the host using silk fila-
ments ('ballooning'). Both larvae
found on the table and those
observed while ballooning were
recorded as 'ballooning' individ-
uals. All the experiments were
done at room temperature
(22-23 °C). Larvae used in both
experiments belonged to labora-
tory reared strains maintained at
Kevo Subarctic Research insti-
tute Field Station.
Fourteen neonate larvae of E.
autumnata were placed with a fine
brush at different portions of a
thin glass bar in which three rub-
ber elastic bands were placed to
provided larvae with irregulari-
ties that helped them larvae to
stay in the artificial branchbar.
The number of larvae remaining
in the bar and ballooning were
recorded every five minutes for
150 min. In addition, a small por-
tion of a pine branch was cut and
placed in water. Epirrita autumnata
larvae were transferred there and
subsequently monitored record-
ing the number of individuals
ballooning. This procedure was
conducted on three different
dates 11,17 and 22 of June.
On 12 |une, a branch containing
at least 18 short shoots was cut
from every study tree
(N = 30). Branches were kept in
cold while collecting and immedi-
ately carried to the lab where they
were placed in water to avoid des-
iccation of Ieaves. Branches were
all starting to open their buds but
leaves could not be observed yet.
Eggs from six different broods
were mixed and the hatched lar-
vae were randomly distributed
amongtrees. Fifteen larvae were
transferred to each stembranch
trying to place them around the
same point, selecting some stem
bifurcation ifavailable.
Greenhouse experiment
The experiment was carried out
in March 1999 at Satakunta
Environmental Research Centre.
i used 3 years potted mountain
birch saplings obtained from
seeds that were three years old
at the time of the experiment.
These saplings were obtained
from seeds that belonged to
seven identified trees whose
foliage chemistry was well-known
(Lempa et al. 2000). For this
experiment we selected seven
mother the selected trees that
comprised a broad variation in
concentrations of proteins and
total gallotannins (Table 1). Four
saplings per mother tree of simi-
lar size, phenology and appear-
ance were used as replicates.
Pots were placed on Petri dishes
(12 cm diameter) containing
water to avoid larval movements
among plants. The system was
proved to be effective for this
purpose, since some of the dead
larvae were found within the
Petri dishes, and it was also used
as watering system and its level
was checked every day adding
more water when necessary con-
trolling the level of water daily.
176
SKÓGRÆKTARRITIÐ 2001 l.tbl.