Bean flower thrips, Oriental bean thrips, Asian bean thrips
Megalurothrips usitatus; previously known as Taeniothrips nigricornis
Widespread. Asia, Oceania. It is recorded from Australia, Fiji1, French Polynesia, Kiribati, Papua New Guinea, Tonga, and Tuvalu.
Mostly members of the bean family (Fabaceae); common hosts are cowpea, French bean, pea, peanut, pigeon pea, mung bean, and soybean.
Thrips damage the pods of beans. Infestations begin in the flowers (Photo 1), and pods become twisted,deformedwith reddish-brown russet marks as the thrips feed. Large infestations cause poor pod set, stunted plants, and leaves and flowers to wilt. This species of thrips is not known to spread viruses.
Eggs are laid in slots flowers and leaves cut with the ovipositor; nymphs are yellow at first, but later deep yellow or orange-red. Pupae are formed in the soil. Adults and nymphs are readily seen when opening the flowers. The adults are greyish-brown, with deepest colours on the head and striped abdominal segments (Photo 2).
Spread is by active flight, but also winged adults and nymphs can be picked up by winds and carried long distances.
It is not a major pest of beans in Fiji and it is only occasionally so in Asia. The related species Megalurothrips sjostedtiis considered much more important.In Java, for instance, Megalurothrips usitatus was common on soybean, but crop yields were not affected. In contrast, a 30% yield loss in peanut was recorded in Taiwan, but Megalurothrips usitatus was not the only species present.
Look for the wilting of flowers and leaves, and adult thrips, and nymphs, in the flowers. Because of similarity of Megalurothrips usitatus with other species, specimens should be sent for expert identification. Sticky traps are used to monitor and control some species of thrips, and could be tried for this species.
Several natural enemies feed on thrips. There are predatory thrips, minute pirate bugs, predatory mites, lacewing larvae and ladybird beetles (adults and larvae). Parasitoids have been report (eulophids), but their impact is not well known.
Note, it is best to avoid pesticides to manage thrips, especially those pesticides that are long lasting, as they will destroy populations of beneficial insects. Also, thrips develop resistance to pesticides easily.
It is unlikely that thrips populations will be high enough to justify use of pesticide. In any case, the use of pesticides is likely to do more harm than good, as they will kill natural enemies. Also, thrips tend to hide in sheltered places on plants, and within flower buds they are difficult to reach with pesticides. If pesticides are applied, try plant-derived products (botanical sprays) first, and always try to treat the undersides of the leaves. Do the following:
AUTHOR Grahame Jackson
1Information from Swaine G (1971) Agricultural Zoology in Fiji. Her Majesty's Stationery Office. London; and CABI (2016) Megalurothrips usitatus (bean flower thrips) Crop Protection Compendium. (www.cabi.org/cpc); and from Bean flower thrips (Megalurothrips usitatus) Plantwise Knowledge Bank. (http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsID=33140). Photo 1 Desley Tree (2010) Oriental bean flower thrips, Asian bean thrips (Megalurothrips usitatus) PaDIL - http://www.padil.gov.au. Photo 2. Ko Ko Maung, Bugwood.org.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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