Insurer Hartford Financial Services Group warned Monday that its third-quarter catastrophe losses would be substantially higher than a year earlier, mostly as a result of last month’s Hurricane Irene.

The Hartford, in a slide presentation for a Barclays Capital conference, said it expects pretax catastrophe losses for July and August of $150 million to $250 million, of which $75 million to $175 million comes from Irene.

In the third quarter of 2010, in which no hurricanes made landfall in the United States, the Hartford’s catastrophe losses were $13 million.

For most insurers, though, disaster losses this year have far exceeded expectations and historical comparison. Between Irene, devastating tornadoes in April and May, drought and a severe winter, many U.S. insurers have already lost well more in 2011 than they lost in 2010.

Worldwide, insurers lost more than $70 billion just in the first six months of the year due to natural disasters, reinsurer Swiss Re has said.