Sorghum shoot fly

in the first few weeks after your sorghum comes up, the central youngest leaf — the spike in the middle of the whorl — wilts, dries and turns straw-brown while the outer leaves are still green. Give it a gentle tug: a dead-heart slides out easily, like pulling a straw from a bundle, and the soft base smells rotten. That dead central shoot is the surest sign.

the culprit is a small ash-grey fly, a bit smaller than a housefly (about half a centimetre), that you will see flitting low over the young seedlings in the cool of the early morning. The real damage is done by its maggot — the fly's young, legless grub — pale and no bigger than a grain of rice — which you find if you split open the base of a dead-heart shoot.

drought or heat wilt droops the whole plant, not just the central leaf, and the shoot won't pull out or smell rotten. The stalk borer attacks bigger, older plants and leaves neat rows of shot-holes across the leaves — shoot fly hits only young seedlings and leaves the outer leaves whole. A tip nipped by a bird or goat looks torn and ragged; a dead-heart is whole on the outside but dead and rotten inside. Later, struck plants turn bushy and stunted, throwing many weak side-tillers instead of one strong stem.

the danger window is short and early. Start the day your seedlings emerge and keep checking every 2–3 days through the first 3–4 weeks (up to about 30 days after emergence). After that the plants are big enough to be safe and you can stop. Watch late-sown fields hardest — they stay in danger the longest.

cross the field in a big "W" shape so you cover all four corners and the middle, not just the edge by the path. Make about 5 stops along the W and check 20 seedlings at each stop — 100 seedlings in all.

at each plant look into the centre of the whorl — is the youngest central leaf green and standing, or wilted and brown? Tug any suspect central shoot; if it slides out and smells rotten, that is a dead-heart. Also look for the small grey flies resting on the seedlings in the early morning, and for tiny white, cigar-shaped eggs laid singly on the underside of the young leaves — both mean dead-hearts are on the way.

count the dead-hearts. If more than 1 in every 10 seedlings (more than 10 out of 100) shows a dead-heart in these first three weeks, you have crossed the line — act now. Every day you wait, more shoots are cut and the chance to save the stand slips away.

the most reliable protection, especially if you must sow late, is a systemic-insecticide seed treatment — seed coated with the chemical before you sow. Systemic means the young plant draws the chemical up inside its own sap, so the maggot boring into the shoot is poisoned from within, where an outside spray can never reach. Treated seed is poison, and is dyed a warning colour — never eat it or feed it to animals, keep it well away from food and children, and wash your hands after handling it.

only if you still cross the threshold (more than 1 in 10 seedlings with dead-hearts), spray the seedlings with a systemic insecticide. Handle it safely: cover your skin, eyes, nose and mouth, spray in the cool of morning or evening with the wind at your back, and keep it away from food, drinking water, children and bees. Wash yourself and your clothes afterwards, respect the pre-harvest interval (the wait-days printed on the label between your last spray and harvest), and rotate to a different chemical group each season so the fly never gets used to one poison — that build-up of immunity is called resistance.

once seedlings pass about 30 days / the 4–5 leaf stage they have grown out of danger — the shoot fly cannot hurt an established plant, so stop spraying then. If a patch is thin but the rains still allow, gap-fill or re-sow the bare spots quickly and let survivors tiller (send up side-shoots) to close the gaps. If a whole late-sown field is wiped out at the seedling stage, cut your losses and put your effort into a fresh, early sowing — with this pest, sowing time is the real medicine.