From time to time we get bugs in the lucerne. At the moment we have the odd Bluegreen Aphid & Heliothis grub. The aphids are very tiny at only 3mm long and are hard to spot with the naked eye; the grubs are a little bigger growing to about 30mm long.
Rather than spraying pesticides we much prefer to rely on beneficials & natural predators. Mowing the paddock every 28 days also helps; the birds have a field day when we mow a paddock as the insects are easy pickings.