Summer's Bumper Crop

Don't call it rerun season anymore: This summer 18 new series will be dotting the prime-time landscape. ABC has already launched Jack's Place and Julie, and shows from NBC (Home Fires) and Fox (the beach-house comedy Down the Shore) are on the way. But CBS is trying even harder to keep viewers from straying. This summer the network will offer nine new series.

''For summer, we've developed shows that are a little offbeat,'' says Steve Warner, CBS vice president of program planning. Offbeat can mean anything from Howie, a Howie Mandel skit show, to the sexy, anecdotal Grapevine, to The Hollywood Game, an entertainment trivia test. If the shows catch on, the rewards are vast. In 1990, two of CBS' summer tryouts were Northern Exposure, which ranked 23rd this season, and Top Cops, now a Thursday mainstay.

But if every network tries to beat the September rush, summer itself becomes a traffic jam. So CBS will wait until August to unveil the 90210-like Freshman Dorm, the glossy soap 2000 Malibu Road, with Drew Barrymore and Jennifer Beals, and Middle Ages, a rueful drama about Chicago salesmen that may be the prize of CBS' summer stock. ''If we get two hits,'' Warner says, ''we'll be ecstatic. But if we get zero, we'll still do it again. Exposure and Top Cops have more than paid for this summer's whole schedule.''