More data on Re-infestation by varroa

Greatti, M., Milani, N. and Nazzi, F., 1992. Reinfestation of an acaricide-treated apiary by Varroa
jacobsoni Oud. Exp. Appl. Acarol., 16: 279-286.

The sources of reinfestation of a treated apiary by Varroa jacobsoni Oud. were studied in Friuli
(North-Eastern Italy), in an area with a high density of colonies. Ten colonies, initially Varroa-free,
were treated with Apistan or Bayvarol strips and mites killed by these treatments were counted twice
a week for 1 year. Five hives were provided with "drone excluders", to avoid the entrance of drones
from other apiaries. Nectar and honeydew flow were monitored.

The reinfestation rate was low during spring, varied between 1.6-13.7 mites/day/colony during
June, July and first week of August, and rose impressively during September and October (up to a
mean of 75.6 mites/day/colony); it was relatively high when nectar flow was scarce. The presence of
drone excluders did not help to limit the number of mites imported: drones did not seem to be the
main cause of reinfestation. The coincidence between the increase in the reinfestation rate and the
scarcity of nectar flow and the massive importation of mites, observed especially in September and
October, suggest that reinfestation was

The data show the importance of the phenomenon of reinfestation of colonies
by Varroa jacobsoni. The number of mites imported from other apiaries
and feral swarms greatly exceeds that of mites which are likely to have survived
acaricide treatments and their offspring.

Under the conditions of this experiment, reinfestation may to be due, to an
important extent, to robbing of infested colonies and swarms, especially in
September and in October. The importation of a large number of mites in
autumn after acaricide treatments is particularly harmful, since they can reproduce
during one year before further treatments will be carried out.

These results indicate some practical means for keeping reinfestation rates
low:

(i)treatments should be carried out at the same time in all the colonies by all
the beekeepers in a given area;

(ii)treatments should not be carried out too early, since the risk of reinfestation
is maximal in the end of the season; products with a long-lasting action
could be introduced into the hive twice, i.e. for about 30 days at the
end of the summer (so unparasitized winter bees will develop) and for
15 days before the bees cease their activity and form the winter cluster.

(iii)since feral colonies are a source - perhaps the main source - of mites,
effective swarm control would lower the number of mites that can reinfest
treated apiaries.

PLB

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