I think you have the wrong outlook on viruses. Since in combat there is no guarantee that the virus will work on the enemy bot, viruses used for killing are not favorable. So, instead, you could have a virus that improves eating, similar to my veggie cancer virus.
I mean, you could create a virus that causes a veggie to consume all of its body except 10 and fire all of its energy in the direction it is shot from. While this does not act as a weapon of mass destruction and it wont be something the bot can fire at every veggie, it will increase the speed and efficiency with which the bot eats.
Also, you could have a spreading virus that makes veggies inedible to other bots. Any number of methods come to mind. The veggie could attack things that are not your species. Or it could blindly fire at everything with -1 shots -- but your species has an anti-firing poison. It could react by moving away from -6 shots, while your bots eat vegs with -1s.