Hi Brian, like Darren, I've never found Mozzies to be much of a problem in Cuba and especially not out of the rainy season. If you are somebody who is susceptible to insect bites then by all means take a decent insect repellent if you are likely to be spending time of an evening outdoors where there is a lot of foliage (eg if your tour includes an overnight at the Hotel Moka in Las Terrazzas which is up in the mouintains and always more humid than the coastal areas at any time of year).
And whilst they have never bothered me, some people do react to bites from the sandflies that are found on some beaches. I don't recall any complaints about them on Cayo Levisa but as it's essentially one big sandbank and the layout means that you'll be virtually living on the beach, taking a sunscreen which contains an insect repellent might be a good precaution if you know that you are one of the people who does react to sandfly bites.
If you do get bitten and the bites become troublesome (eg infected) then rest assured that in my experience the Cuban medical services are excellent
Do have a great holiday and sit back and enjoy a mojito for me. Haven't been able to get anything sorted yet but hoping to be back there myself a bit later in the year including spending a few days on Cayo Levisa myself.