Ecole Nationale Supérieure Agronomique de Montpellier ... - CIAM

fed on the plants, they did not transmit more than the adults set later: they appeared to be
unable to infect the plants at this stage. This is also in agreement with the high quantity (10 8 )
phytoplasma observed in the unique infectious C. pruni tested.
Old emigrants were able to inoculate a few test plants in the transmission experiments,
but the proportion of infectious insects remained low (0.6%). This is in agreement with the
results of Carraro et al. (2003; 2004). The quantity of phytoplasma measured in these insects
did not explain the low proportion of infection, as it reached its highest value in the old
emigrants. The feeding behavior of the emigrants may be the cause of the low transmission
rate. In natural conditions, emigrants did not remain a long time on Prunus and in
transmission experiments most insects died when they were set on the healthy test plants. We
hypothesize that the newly emerged adults are repelled by the Prunus and generally do not
feed on it.
The quantity of phytoplasma in reimmigrants that have had an access to an infected plant
is quite low (around 10 4 ) and almost the same for acquisition periods between 1 and 21 days.
But at the end of the transmission period, 20 days later, the quantity of phytoplasma measured
in some insects was much higher. It can be assumed that during the first step the phytoplasma
just accumulated in the insects but that a multiplication took place after at least 3 weeks post
acquisition. Nine C. pruni out of the 28 tested showed such an increase of the quantity of
phytoplasma. Only one insect exhibited a value higher than 10 7 , similar to what was measured
in the old emigrants reared on infected plants. Even in this case, it cannot be certain that the
quantity of phytoplasma resulted from the acquisition on the infected plant as the 28 insects
came from a natural population in which a proportion of about 3% of insects are infected. The
experiments made by comparing the proportions of transmission from natural populations of
reimmigrants with populations that have had an access to an infected plant demonstrated no
difference, even with the insects which had 40 days post acquisition. Thus, these results
question the ability of the reimmigrants to acquire and transmit ‘Ca. P. prunorum’ if they are
not infected before the overwintering period: even if they acquire the phytoplasma by landing
and feeding on an infected plant, only very few of them will multiply the phytoplasma fast
enough to be infectious before their death.
Thus, from the transmission experiments and the measures of the quantity of
phytoplasma, the nymphs, the emigrants, and the healthy reimmigrants appeared to be much
less efficient vectors of ‘Ca. P. prunorum’ than the reimmigrants which arrive infected from
the overwintering sites.
The dynamics of C. pruni on the observed P. spinosa hedge indicated that the emerging
adults leave their host plants quickly: the peak of emergence lasted only 2 weeks. As there
seemed to be a good correlation between the dynamics of C. pruni at several sites in the same
area (Labonne & Lichou, 2004), a large number of C. pruni will leave their host plant at the
same time to go to another plant or area. Most of the reports of C. pruni out of Prunus plants
indicated conifers as shelter plants, and we were also unable to detect the insect on other
plants. The search for C. pruni on conifers in the plain, near the sites where P. spinosa were
abundant and where the insect reproduce gave an almost negative result. The closest site
where a significant density of C. pruni was detected is the first border of the Larzac plateau
(site 1), about 15 km west from the reproduction area that we observed. The highest density of
C. pruni was found in a mountainous site (site 2) were P. spinosa and other wild Prunus are
absent or very rare at a distance of several kilometers. All of these facts suggest the
hypothesis that C. pruni migrates on distances of several kilometers or more after its
emergence on Prunus, probably by using the dominant winds which blow from the sea to the
land at this season. The results of forced overwintering on conifers suggested also that the
survival, while possible in plain (Table 3), might be more efficient when the insects are in
altitude.
The synthesis of the results on the life cycle of C. pruni and of its transmission efficiency
at different stages of its life suggests the following scenario for the dissemination of ‘Ca. P.
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