Asian ladybirds spoiling our plums?

A plague of Asian Ladybirds not only ate all the aphids on our espaliered plum tree, but suckered on the fruit and spoiled it with marks and a horrible black deposit (poo?) Even the larvae sort of hooked themselves on and left their husks attached to the fruit. What is the right time of year to prevent this happening again - and how?

I suspect the black might have been sotty mould from the ahid exudatins, rather than ladybird poo 

Plums can be very badly affected, so you might like to think about spraying for aphids early in the season next year, if your tree is not too large to do this.