Ecologia mediterranea 1999-25(2)

Linhart et al. Selective herbivory ofthyme chemotypes by a mollusk and a grasshopper
pers, no artificial diet was available, so that this type
of experiment could not be carried out.
Containers for individual animals consisted of:
i) circular, soil-filled glass dishes 15 cm in diameter
for the mollusks, covered with damp muslin, and
ii) rectangular planting trays 20x30 cm in size covered
with metal mesh cages about 5 cm in height for the
grasshoppers.
All experiments that followed the protocols described
here were carried out once. In the case of
slugs, different animals were used in the two experi­
ments, to prevent the possibility that learning in the
first trial with the living plants, might influence slug
behaviour in the second experiment. Some further tri­
als were run, usually with fewer animals, and lasting
longer periods of time. Because of these differences,
they were not directly comparable to the experiments
reported here, and were not analysed statistically.
However, all observations germane to the results re­
ported here are also noted.
RESULTS
Deroceras
The slugs ate primarily plants of the G and A che­
motypes, and ate little of the U, C, and T chemotypes
(Table I) of the nine individuals tested, five ate mostly
G, three ate more A than G and one ate mostly A and
L. In subsequent tests with fewer animals, G was also
the chemotype most eaten.
Tests with gel cakes showed that animals ate more
of G than the other chemotypes with L also selected,
and C the least eaten. Once again, there were differ­
ences among individuals, with 4/10 individuals prefer­
ring G, two a combination of G and L, one feeding
mostly on L, one on U, and two showing no clear
preference.
Leptophyes
The grasshoppers ate all chemotypes, but ate most
of chemotype T (Table 1). There was marked hetero­
geneity among individuals. In additional runs (data not
shown) of 8 individuals exposed to 6 chemotypes,
three showed a strong preference for T while the oth­
ers were less selective in their feeding.
DISCUSSION
Both herbivores ate a diversity of thyme chemo­
types but concentrated their feeding on some chemo­
types and were deterred by others. There was a
general consensus about the most and least eaten che­
motypes for each species, although there was also
some heterogeneity among individuals. This heteroge­
neity reflects phenotypic variation among individuals,
observed in most studies of this type, and underscores
the need to carry out such tests with adequate numbers
of animals. The two species differed dramatically in
their preferences, with Leptophyes feeding more on
thymol (T), while Deroceras fed very little on that
chemotype, eating primarily plants of geraniol (G) and
terpineol (A) chemotypes.
Species Chemotype P (3)
Plant material G L A U C T
SLUG
Plants (I) 4.1±0.6 2.7±0.5 3.9±0.6 1.0±0.2 0.9±0.2 1.0±0.3