a method has been found for protecting a crop by killing eggs of insects , or insects at the egg stage wherein the insects are detrimental to the crop . this method comprises applying to a locus where the eggs of insects are located or are expected to be located an effective amount of a compound of formula , or a salt thereof , r1 - x - n - c ═ z - r4 wherein r1 represents an optionally substituted aromatic hetero ring containing nitrogen atom ; x represents an optionally substituted alkylene or alkylidene ; r2 and r3 represent hydrogen or optionally substituted alkyl ; r4 represents a cyano or a nitro , and z represents ch or n . the insect eggs which can be killed include tobacco budworms , pyrethroid resistant tobacco budworm , mexican bean beetle or colorado potato beetle .