Planet of Giants — Terrance Dicks

The TARDIS has landed on Earth, in England, in 1963. But when he and his companions Susan, Ian, and Barbara venture outside, they are lost in a maze and menaced by gigantic insects. However, the insects are dying -- every living thing is dying. Meanwhile, in a cottage garden on a perfect summer's day, the man from the Ministry arrives to put a stop to the production of DN6, a pesticide with the power to destroy all life-forms. But the men who invented DN6 will stop at nothing to see DN6 succeed. Unabridged.