Storm Over Warlock — Andre Norton

The Throg task force struck the Terran Survey camp a few minutes after dawn, without warning, and with a deadly precision which argued that the aliens had fully reconnoitered and prepared that attack. Eye-searing lances of energy lashed back and forth across the base with methodical accuracy. And a single cowering witness, flattened on a ledge in the heights above, knew that when the last of those yellow-red bolts fell, nothing human would be left alive down there. His teeth closed hard upon the thick stuff of the sleeve covering his thin forearm, and in his throat a scream of terror and rage was stillborn.
More than caution kept him pinned on that narrow shelf of rock. Watching that holocaust below, Shann Lantee could not force himself to move. The sheer ruthlessness of the Throg move-in left him momentarily weak. To listen to a tale of Throgs in action, and to be an eye-witness to such action, were two vastly different things. He shivered in spite of the warmth of the Survey Corps uniform.
As yet he had sighted none of the aliens, only their plate-shaped flyers. They would stay aloft until their long-range weapon cleared out all opposition. But how had they been able to make such a complete annihilation of the Terran force? The last report had placed the nearest Throg nest at least two systems away from Warlock. And a patrol lane had been drawn about the Circe system the minute that Survey had marked its second planet ready for colonization. Somehow the beetles had slipped through that supposedly tight cordon and would now consolidate their gains with their usual speed at rooting. First an energy attack to finish the small Terran force; then they would simply take over.
A month later, or maybe two months, and they could not have done it. The grids would have been up, and any Throg ship venturing into Warlock's amber-tinted sky would abruptly cease to be. In the race for survival as a galactic power, Terra had that one small edge over the swarms of the enemy. They need only stake out their new-found world and get the grids assembled on its surface; then that planet would be locked to the beetles. The critical period was between the first discovery of a suitable colony world and the erection of grid control. Planets in the past had been lost during that time lag, just as Warlock was lost now...