from the Univ. of Florida site:
The body fluids are slightly acid; however, if the egg mass (average about 350 per female) and body parts are allowed to remain on the car for several days, bacterial action increases the acidity and etches the paint. A soaking with water for about five minutes followed by a scrubbing within 15 to 20 minutes should remove most of the lovebugs without harm to automobile paint.
There are several things that can be done to lessen the problem facing motorists. By traveling at night motorists can avoid the insects; lovebugs reach peak activity at 10:00 am and stop flying at dusk.
Lovebugs are more easily removed, and the chance of damaging the car's finish is lessened if the car has been waxed recently. When the remains are left on an unwaxed car for several days, the finish will often be permanently damaged. Soaking for several minutes with water aids in their removal. When lovebugs are numerous, some motorists spread a light film of baby oil over the front of the hood, above the windshield and on the grill and bumper. This practice will make their removal a simpler task.