Photo 1. Upper leaf surface of cowpea with snake-like mines and blotches damaged by cowpea leafminer, Phodoryctis caerulea.
Photo 4. Adult cowpea leafminer, Phodoryctis caerulea, showing upright angle when at rest, and the fringed upturned trailing edges of the wings
Phodoryctis caerulea. It was previously known as Acrocercops caerulea, and it has been recorded as Phodoryctis centrometra from Fiji. It is a moth of the Gracillariidae.
Widespread within Asia. There are unconfirmed reports that it is in India, and Malaysia (CABI), and also reports (Wikipedia) that it is present in Indonesia, Sri Lanka, Japan, Taiwan, and Oceania. It is recorded from Australia, Fiji, Guam, Solomon Islands, and Tonga.
Cowpea (Vigna unguiculata spp. unguiculata). It is also known from several other edible legumes, e.g., black gram (Vigna mungro), broad bean (Vicia faba), French bean (Phaseolus vulgaris), yardlong bean (Vigna unguiculata spp. sesquipedalis), mungbean (Vigna radiata), soybean (Glycine max), and ground-cover legumes, e.g., Centrosema, Pueraria, as well as fodder legumes, e.g., Crotalaria, and weeds, e.g., beach morning glory.
The larva of the moth does the damage burrowing in the upper surface of the leaf. At first, the larva makes a narrow winding snake-like mine, at the end of which is a large conspicuous blotch, with a thin translucent covering (Photos 1&2).
The immature larva is flattened, carrot-shaped, yellow, and has a row of golden dots on each side of its back. The head is triangular. When mature, the larva is 4.5 mm long, bright red, with a golden-brown head. The pupa develops inside a flat silky cocoon. Pupae are found on crop plants as well as weeds.
The adult moth is 3.5 mm long, with a dirty-cream body, and smoky forewings (Photos 3-5). The antennae are longer than the wings (Photos 4&5). The end of the wing are fringed and upturned. The first two pairs of legs are well-developed, the upper parts are black, and they hold the insect at a steep angle when at rest (Photo 4).
The leafminer survives as pupae in plant remains, in soil after harvest or on weeds.
Details of the impact of this leafminer are unknown. In Fiji, it is said that cowpeas are badly affected, and damage increases with age of the crop. In Australia, where it has been present for 20 years or more, it has not been reported by growers as a significant pest.
Look for the mines which begin as thin trails and expand into large blotches. Look for the smoky grey forewings with pale markings on the front edge of the forewing of the adult, which seem to be distinctive of the cowpea leafminer.
Chemical control is difficult because the larvae are protected inside their mines. Also, insecticides are likely to destroy natural enemies, and this can lead to outbreaks of other insects, and also mites.
AUTHOR Grahame Jackson
Information from Swaine G (1971) Agricultural Zoology in Fiji. Her Majesty's Stationery Office. London; Photos 1-4 Mani Mua, SPC. Sigatoka Research Station, Fiji. Photo 5 Wikipedia. (https://en.wikipedia.org/wiki/Phodoryctis_caerulea).
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/185: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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