The omnivores' argument is based on the faulty (and never admitted) premise that no pests are killed for meat production. So they think that they are comparing the death of one cow against the death of thousands of insects and rodents. It is a faulty comparison, because it is just false.
If you drive the 20 km road between the town and where I live, you will see two fields of grazing cattle and about twenty fields of corn. So tourists driving that road think they see free-range cattle, for whom they think no pests are killed, and acres of crops, which are heavily sprayed with pesticides. What they are not told is that all of that corn is grown for animal feed, both for cattle and for poultry. Local residents can tell which corn is grown for feed because of the way it is harvested. It turns out that nearly 100% in this valley is for animal feed, but tourists never see that. Nor do they see the feedlots where the non-free-range cattle are warehoused and fed.
They never see the poultry farms. either. Huge sheds that house 30,000 birds each in conditions you do not want to think about. There are dozens of them in the area, but they just look like some kind of storage facility to the unthinking tourist. They don't register the existence of the big transport trucks that haul the chicken feed from the processing plants to the farms. Just another big truck. But all the tourist sees is acres of corn that they assume is intended for human consumption.
All those pests that the pesticides kill in the corn fields are killed for the production of meat. And given that it takes ten pounds of feed to produce one pound of meat, the impact of meat production is vastly greater than the impact of growing veggies for human consumption.
Yes, there are crops here that are grown for human consumption, and yes, many of them are sprayed with pesticides. And yes, it is regrettable that we vegans cannot be totally free from causing harm. My wife and I try to buy organic or un-sprayed local veggies when we can: there is a local market garden where we buy most of our veggies.
But the truth is that the majority of pests killed in agriculture are killed for meat production.