Written for ranch hands, hunters, 4-H families, veterinarians and anyone who checks animals. Ten minutes. Nothing here is a diagnosis; it tells you what to look for and how to get a usable photo, video or sound clip to the people who can confirm it.
Screwworm does not spread animal to animal; the fly lays eggs at the edge of a wound or a body opening. The risk to people handling an infested animal is the same wound-care risk as any maggot wound: gloves, wash hands, keep your own cuts covered.
Screwworm larvae feed on living tissue (USDA APHIS). That is the whole difference from the common maggots you have seen on dead tissue or a neglected sore.
| Points toward screwworm | Points away |
|---|---|
| The wound gets deeper and larger over days instead of drying up; often a small opening with a large pocket underneath | Maggots on the surface only, on dead or necrotic tissue, wound not deepening |
| Larvae packed head-down into the tissue, only the tail ends showing; they do not scatter quickly when exposed | Larvae crawling loose on the surface, scattering when disturbed |
| Bloody or straw-coloured discharge, a strong foul smell, the animal restless, off feed, separated from the herd | Dry crust, mild smell, animal behaving normally |
| Typical sites: navel of a newborn, dehorning, castration, branding or ear-tag wounds, tick bites, shearing cuts, the vulva after calving, nostrils, even a fresh scratch | No wound at all (screwworm needs an opening) |
The eggs are laid in a creamy white shingle-like mass at the wound edge; they hatch in about a day. Larvae feed for 5 to 7 days, grow to about half an inch, then drop to the ground to pupate. New flies emerge in about a week in warm weather. That timing is why one missed wound becomes many flies within a month.
Mature screwworm larvae are 12 to 17 mm, creamy white to pinkish, tapered toward the head like a wood screw, with rings of small dark spines around each segment. The one feature that most helps separate New World screwworm from the common secondary screwworm, Cochliomyia macellaria, is visible through the skin at the back end:
Other things a trained eye checks: the shape of the posterior spiracles (the breathing openings at the tail), the rows of spines, and the mouth hooks. You do not need to judge these; you need to photograph them.
About 8 to 10 mm, a little larger than a house fly, metallic blue-green with an orange face and reddish-orange eyes. Look at the top of the thorax (the section behind the head):
| New World screwworm (C. hominivorax) | Secondary screwworm (C. macellaria) |
|---|---|
| Three dark lengthwise stripes on the thorax, the middle stripe short or incomplete; a dark basicosta (the small plate where the wing joins the thorax) | Three dark stripes of about equal length running from the head down the thorax; basicosta lighter brown |
| Rarely seen away from wounds; females seek living animals | Common everywhere around carcasses, garbage, manure |
Source: UF/IFAS EDIS IN1146, New World screwworm; Texas Animal Health Commission NWS page.
Honest statement: an adult fly is very hard to identify from a phone photo, and most blue-green flies you see are not screwworm. Photograph it anyway if it is on or near a wound, and note that. The Picket Line trap nodes and the laboratory do the fly identification.
| What | How it differs |
|---|---|
| Secondary screwworm, green and blue bottle flies (Lucilia, Calliphora, Phormia) | Larvae feed on dead tissue and carcasses; do not burrow into healthy flesh; trunks pale |
| Flesh flies (Sarcophaga) | Grey with checkerboard abdomen, larvae deposited live, mostly on carrion |
| Bot and warble fly larvae (Cuterebra, Hypoderma) | Single large larva in a swelling with a breathing hole, not a packed open wound |
| Wool maggots (sheep strike) | Lucilia on soiled fleece; surface damage first |
| Old World screwworm (Chrysomya bezziana) | Same behaviour, different continent; not expected in the Americas but would be reported the same way |
A 10-second steady clip of the wound shows what a photo cannot: larvae that stay buried and wriggle in place versus larvae that crawl off. Use normal speed. Slow-motion at 240 frames per second is useful for a fly's behaviour at the wound, but it is not fast enough to measure wingbeat; leave that to the nodes.
Flies have a flight tone. Phone microphones capture it well enough that researchers identify mosquito species from phone recordings. Hold the phone within a hand's width of a fly in flight or buzzing in a container and record 10 to 20 seconds in a quiet moment. The app's audio clip is sent for analysis of the wingbeat fundamental; it is a supporting cue, never the call on its own.
| Capability | Reality |
|---|---|
| Ultraviolet light to check for the sterile-fly dye | No phone has a UV emitter. A $5 to $10 365 nm UV flashlight makes the dye fluoresce and the phone camera sees it. Picket Line will sell or recommend one as the kit accessory, and the trap nodes carry their own UV LED. |
| Macro photography | Many recent phones switch to a macro lens automatically; a $10 clip-on macro lens helps older phones with the trunks and spines. |
| Slow-motion video | 240 fps on most phones; good for behaviour, not for wingbeat frequency (needs more than 500 fps). |
| Audio | Fine for flight tone at close range in quiet conditions. |
| LiDAR depth (iPhone Pro, some Android) | Too coarse for a wound; use a coin or ruler for scale instead. |
| Thermal camera accessories | Can show inflammation around a wound; a nice-to-have, not needed. |
| Infrared / night vision | Face-recognition IR is not available to apps; not useful here. |
Use the Report page or the app. The triage you get is a prompt, not a verdict. If it says Likely or Possible, the handoff buttons dial or open the official channel for your state and for USDA APHIS. Confirmed cases are published only by officials.
Last reviewed 2026-10-06 against the sources above. This guide is maintained by 4DAT LLC and reviewed against official sources; it will be reviewed by a veterinary entomologist before the app's public launch. Corrections: hello@ai4dat.com.