Blanka KALINOVÁ, Kauel STRÁNSKÝ, Richard CTVRTECKA,
Juraj HARMATHA, Jan ZDAREK
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech
Republic, Flemingovo nám. 2, 166 10 Praha 6, Czech Republic
The apple blossom weevil (ABW), Anthonomus pomorum (L.) is a pest of apple trees throughout the Holarctic region. It is a univoltine species hibernating as an imago. Early in spring (March, April) the over-wintering beetles look for the host trees where they feed and mate, and where fertilised females lay their eggs into the flower beds. The infested buds become sterile and the blossoms never open. Larval and pupal develoopment of the ABW proceeds inside the dried buds and the beetles of the next generation emerge in May or June.
Some apple cultivars are more vulnerable to the ABW's attack than the others. The present study verified the existence of the host-cultivar preference by extensive field collections of post-hibernating weevils in commercial orchards containing mixed apple tree cultivars and analysed its causes. One of the reasons for the unequal infestation appeared to be variations in density of blossom buds in different cultivars. However, the numbers of buds per a length unit of branch were not directly correlated with the numbers of weevils collected and thus cannot fully account for differences in relative attractiveness of different plants. It can be hyppothesised that still other factors, most likely chemical ones, underlie cultivar-specific preference of oviposting ABW females. To further investigate this possibility a detailed chromatographic analysis of volatiles emanating from apple buds of two cultivars that greatly differ in ABW infestation rates (Boskop Red, James Grieve) was carried out. The respective chromatograms revealed apparent differences. Moreover, relative contents of volatiles also showed significant fluctuations at different phenological stages of buds in the two cultivars. These results suggest that chemical cues (host plant kairomones) may play a role in cultivar discrimination by feeding and oviposting ABW adults.
In a parallel experiment 43 compounds were separated by GC from the apple bud emanations and majority of them identified by means of GC-MS. The most abundant compounds were (Z)-3-hexen-1-ol (27%), (Z)-3-hexenylacetate (14%), (3E,6E)-a-farnesene (6.5%), (E)-ß-ocimine (3.7%) and methylbutanol (2.4%). Many of the identified compounds elicited EAG responses in male and female antennae with no appreciable differences between the sexes. (Z)-3-Hexen-1-ol, perylene, ß-caryophyllene, (3E,6E)-a-farnesene and 3-carene were most active in GC-EAG tests. The electrophysical tests thus demonstrate that the antennal receptors of both males and females of the ABW are capable to detect and possibly discriminate apple bud volatiles.
In behavioural tests (Y-olfactometer, pitfall trap olfactometer), the ABW's were able to discriminate emanations of the host tree buds from those of a non- host tree (cherry tree).