Page 19 - Volume 2,Issue 2, July 2009

Honey bees and bumble bees as dispersers of a biofungicide

63

that observed in other studies focused on

the dispersion of several biocontrol agents

in open field conditions (Table 1) (11, 30, 31).

Shafir

et al.

(27) have detected 2.2 x 10

4

CFU

of

T. harzianum

per flower on strawberry.

As mentioned earlier, these comparisons

should be made with caution since most of

these studies differ in some methodological

details, namely in the features - and hence in

their performance - of the dispensers used,

in the visiting rates and in the respective

length of visits to which flowers have been

exposed. For example, while in most exper-

imental studies the flowers are exposed to

foragers for two (11) or three days (27), in our

honey bee assay, flowers were exposed for

only one day. Although attention was paid

to collecting flower samples during favour-

able sunny days, we observed that foraging

activity in the experimental field was rela-

tively low. The low foraging activity was at-

tributed to the existence of competitor flo-

ra in the vicinities of the experimental field

or the relatively low attractivity of the cho-

sen cultivar. Nevertheless, this result shows

that this biological control method may be

compromised by factors that have an effect

on pollinator activity, such as climatic con-

ditions, attractiveness of the chosen cultivar

and presence of competitor flora in the vi-

cinity of the target crop.

Factors mentioned earlier to explain the

high variability of

T. harzianum

densities

on flowers exposed to pollinators, togeth-

er with low foraging activity, may explain

the inexistence of significant differences be-

tween the densities of

T. harzianum

deposit-

ed on freely exposed flowers and on those

that had a single honey bee visit. As the for-

aging activity was so low, the

T. harzianum

that is brushed off from freely exposed flow-

ers by different factors, such as air currents

and redistribution of inocula by the honey

bees and other organisms, is probably not

resettled by successive visits of honey bees.

As a consequence, the resulting density of

inoculum would then be statistically com-

parable to that obtained with a single visit.

In the experiment using bumble bees,

undertaken in a closed greenhouse and

with a limiting number of available flowers,

inoculum losses during flight and groom-

ing could only occur in a restricted space

and through the repeated transfer of bio-

fungicide within the same set of flowers. As

a result, the bumble bees in foraging activi-

ty retained a high amount of inoculum and

consequently all the flowers that were free-

ly exposed to their visits showed inoculum

densities comparable to those mentioned in

other works (Table 2). More specifically, Yu

and Sutton (31) detected densities of about

0.45 x 10

3

– 2.5 x 10

3

of

G. roseum

on raspberry

flowers and Maccagnani

et al

. (21) observed

densities of about 0.14 x 10

3

of

T. harzianum

on tomato flowers, using the OP-dispenser.

Under greenhouse conditions, not only the

probability of

T. harzianum

being brushed

off from flowers was lower, since the fac-

tors early mentioned were controlled (for

example air currents, rain and the presence

of other organisms that could act as inocu-

lum dispersers), but also bumble bee activi-

ty was greater allowing the accumulation of

biofungicide throughout several visits.

This study also suggests the importance

of some parameters like the number of CFU

per bumble bee and the length of visit in

determining the amount of inoculum that

is deposited on flowers. We also determined

the density of inoculum that was deposited

after one single visit of honey bee or bum-

ble bee. Even though the present work does

not allow a direct comparison between hon-

ey bees and bumble bees in their effective-

ness as dispersers, since the assays were

undertaken at different experimental condi-

tions, the results demonstrated that bumble

bees ensured, in greenhouse conditions, a

higher percentage of flowers dusted with

T.

harzianum

– even after one single visit - than

honey bees, in open-field conditions. Previ-

ous works (31) showed that bumble bee col-

onies, although possessing fewer individ-

uals, were at least as efficient vectors of

G.

roseum

as honey bee colonies. The same au-

thors demonstrated that

B. impatiens

would

be the preferred vector when the weather

is cool and wet, and honey bees would be

preferable under warm conditions.