Imagine a field of vibrant carrots, their orange hues promising a bountiful harvest. But beneath the surface, a hidden menace lurks. The carrot rust fly, a Dipteran (true fly) insect, lays its eggs near the base of these plants. When the eggs hatch, the resulting larvae, or maggots, burrow into the roots, causing them to rot and become unmarketable.

Carrot rust fly (CRF; Chamaepsila rosae, formerly Psila rosae), also known as carrot root fly, can devastate entire crops, leaving growers with significant financial losses. It targets vegetables such as carrots, parsnips, celery, celeriac, caraway, dill, fennel, and parsley crops (CABI 2009).

Fortunately, Australia is currently free from this destructive insect, thanks to our commitment to the best biosecurity practices.

Carrot rust fly: a root rot menace

This pest rears its ugly self from tunnels that it burrows through carrots. The tunnels can be seen on the surface of the root vegetables and are rusty in colour due to the maggot’s excrement – hence the name ‘carrot rust fly’. The tunnels created by the maggots can lead to secondary infection with soft rot bacteria or soil-borne fungal diseases. The yellowish maggot is the larva of the adult fly.

In spring, the fly lays its eggs on the ground at the base of the carrot. The young larvae burrow into the soil and feed on the plant’s young root system. On older plants, the older larvae will feed on the taproot, mining their way to the nutritional cortex of the root. When the larvae mature, they leave the taproot and pupate in the soil before emerging as the adult fly and starting the cycle again.

As winter approaches, the larvae can over-winter as pupae and emerge in spring when conditions are suitable. These infestations cause economic crop damage; while the plant will survive, the taproot becomes unmarketable due to larval damage and secondary infections that may occur (Poole 2009:21).

Carrot rust fly damage on carrots

What to look out for

This pest is best recognised not by its appearance, but by the type of damage it causes to the crop. The first signs of CRF infestation are a red or yellow discolouration of the foliage, potentially followed by wilting and plant stunting or death. Infestation might also lead to distortion or forking of roots.

In New Zealand, where CRF was first detected in 1931, there may be up to four generations of CRF annually. Both larvae and pupae can survive the winter in the soil and roots. In the Netherlands, two to three generations per year are often observed, sometimes four if unseasonally warm. Frost kills larvae/pupae.

In Australia, due to the absence of frost in many places where our carrots are produced, we could potentially see year-round infections.

Carrot rust flies have four life stages:

• Adult flies have a shining black body, yellow legs and a reddish-brown head. They are small; about 6-8mm in size and a wingspan of roughly 12mm (CABI 2019). Adults may be found on the foliage of both host and non-host plants, and around field margins (Poole 2009:22).
• Eggs are ovoid, white and <1mm in length. Adult female flies will be attracted to Apiaceae crops to deposit their eggs at the base of the plant (Poole 2009:22).
• Larvae (maggots) are initially colourless, with dark mouth-hooks. The maggot gradually grows through three instars, with the mature larvae reaching about 8-10mm in size with a creamy-white to white colour. Young larvae feed on smaller roots, like lateral and side roots. This may cause crop damage, like leaf discolouration, wilting and plant death. The older larvae, however, will feed on the taproot, creating mines to get to the root cortex. While the plant often survives this damage, it is this larval stage that causes economic damage as it renders the carrots unmarketable. It also leaves the crop prone to secondary infections.
• Pupae are 4.5-6mm long, with a yellowish-brown colour. Pupation occurs in the soil (Poole 2009:22).

Prepare and protect your crops

A strong monitoring program that includes record keeping of observations made during crop monitoring activities, coupled with routine yellow sticky trapping at high-risk periods is key to early detection.

At the first sign of unusual symptoms, yellow sticky traps can be inspected for signs of the adult flies followed by targeting plants with symptoms to inspect the root for signs of maggot damage. At this stage, take photos of the plant and any maggots or flies present, record the coordinates of the observations and call Plant Health Australia’s Exotic Plant Pest Hotline (1800 084 881) for further advice.

The pest may travel as pupae on infected host plants, therefore the importance of adopting a ‘come clean, go clean’ approach is key to reducing the risk of introduction to your property and/or spread beyond.

Like Liriomyza leafminers, CRF is a notoriously difficult pest to manage. Chemical management options are increasingly limited. Most management relies on cultural practices and prevention of (high load) population establishment. Adult flies are weak fliers but are attracted to Apiaceae crops through smell, and this knowledge is used to prevent populations from establishing through site selection decisions (Andrews 2009; van Rozen & van den Broek 2011).

Key messages

• CRF is a native European pest that has spread globally.
• It causes significant physiological and economic damage to root crops.
• Australia is currently free from CRF.
• The temperate climate of Australia’s carrot-growing regions may favour CRF’s establishment.
• Early detection is key to managing CRF infestations.
• Inspecting plants for symptoms and root damage is essential.
• Biosecurity measures are vital to prevent the introduction and spread of CRF.

FOR MORE INFORMATION
AUSVEG biosecurity activities, please get in touch with the AUSVEG Biosecurity & Extension Team on 03 9882 0277 or email science@ausveg.com.au