Fleas are in tall grass: Flea larvae live in the sod. Whether the grass is 1" tall or 5" tall, the flea larvae live in the sod. Get rid of the flea larvae? Let the grass grow so there's cover for predators like Lightening Bug larvae and Ladybug larvae and others which hunt down and eat bugs and other larvae, including Fleas, Grubs and others! Tall grass actually gives you less fleas! I mow at the highest setting on my mower and never more than once a week, even when it's hot and wet and the grass is growing fast. We've got two dogs and two cats. Fleas? Occasionally. Who doesn't? We have rabbits around, where do the fleas come from?? We also have more Lightening Bugs now than when we moved in.
Taller grass equals deeper roots. Deeper roots equal better reach for moisture and food which equals healthier grass and more drought resistant grasses and plants. We don't water, no matter how dry it gets, and we still have summer-long green.
But weeds will grow! Yes! Yay! I've got White (Dutch) Clover, Heal-All, Plantain, Dandelions, Strawberries, Periwinkles, Creeping Charlie, Creeping Jenny, Violets, Straw Flowers, Day Lilies, Peppermint (yes, 'invasive' Peppermint in my lawn! Smells wonderful when I mow!), several types of Sorrel, Daisies, Lambsquarters, Canadian Lettuce, several types of Chives, dozens of types of grass, not just one or two so if disease hits my lawn, it'll stay green even then, but with healthier grass, its more disease resistant to begin with!
My wife and I could easily eat our lawn all summer long if we had to, and in the fall we could dry, can or whatever and eat for quite a bit of the winter - out of our in-town lawn. That's even with the town rabbits eating their share! They love our lawn ... and our garden too, but hey, plant enough for everyone! Our neighbors would be eating their lawn treatments, which of course is where they let their children and pets romp ... I'm not even going there.
Yes, after a few years I started gardening again. When we first moved in, I tilled a 10' x 10' patch for the start of a garden and turned up three worms - three worms! I let it lay fallow with no 'lawn' treatment other than a couple light doses of lime. Today, anywhere I dig, I usually turn up no less than three worms per shovel full!! I grow Sunchokes, three varieties, and we can them and eat them all year around. We don't worry about poison in the lawn any more. For the first couple of years we had brown patches from grubs as the poisons leached out, but as the longer grass roots and so-called 'weeds' took over, the brown patches disappeared. Oh the grubs are still there, but with healthier greens, they don't seem to kill a single plant! Plus, there aren't many grubs, as I said, our Lightening Bug population has exploded!