It's said that if you have bees you have varroa, and the effects of varroa can be devastating.
Varroa, viruses and other pathogens.
There are many bee viruses which are naturally present in the bee population at a low level without normally causing significant harm. When Varroa feeds, as well as taking essential nourishment away from the developing bee, the mite, acting as the vector, can aid virus spread so that in heavily infested colonies they become much more widespread and potentially harmful by reducing the bees lifespan.
Effects on colonies.
Small numbers of Varroa mites infesting a colony will usually cause no obvious harm. However, as the level of infestation rises, the risk of harmful effects also rises. In poorly managed colonies where infestation is allowed to increase, signs of damage to the entire colony start to become evident. Severe infestation slows the replacement of old adult bees with healthy young bees, and may lead to the rapid spread of harmful bee viruses in the colony. At this stage, the normal processes of foraging, brood rearing and colony defence diminish and the colony’s entire social organisation begins to deteriorate – a process known as colony collapse. Colony collapse is usually very rapid (taking only a few weeks). Colony collapse can occur at any time of the year but in the UK seems to occur most often in August and September. However, spring colony collapse, can be quite common in March, April, and possibly into May.
We can't get rid of varroa completely, we can only manage the population, using oxalic acid at this time of year is just one part of the Integrated Pest Management system. But it is a very important part.