November 2003. North Malaita, Solomon Islands, has for quite some time experienced heavy damage to slippery cabbage, Abelmoschus manihot (which is known in the eastern Pacific as aibeka, pele or
bele). This crop is the most important vegetable to subsistence farmers and, in addition to the identity of the beetle, we would like to know any control measures that we could test. The beetle is approx 0.5 cm in length. Any information on the life cycle and ecology of the beetle is very welcome.
The beetle appears to be a problem year-round, and attacks both young and old leaves. Most varieties suffer heavy damage, affecting the food security of the people. Applications of commercial pesticides is no option: people have no means of affording pesticides, nor would they have easy access to them in this remote area of Malaita. And since the kabis is harvested and consumed daily, pesticides have the potential to harm human health.
Species of this genus are normally polyphagus. Females lay eggs in the soil at the base of the host plant and hatch after 7-11 days. Larvae feed for 11- 28 days on the rootlets. Pupation takes place in the soil
with the adult beetles emerging after 10-17 days. If conditions are adverse they may remain in the soil for up to 2 months. Adults feed on the young growth, skeletizing the leaves, and will remain on the host
plants as long as they can. During the dry season or in adverse conditions they will diapause in cracks in the soil. Once the food plant is used up, adults will disperse by flight. Sexual maturity takes place
several days after emergence, mating and egg laying takes place soon afterward. There may be up to three generations per season. Many Podagrica species are considered serious crop pests, principally because they can occur in large numbers causing damage to both the roots and leaves of the host plant.
Members said that the beetle entered Solomon Islands in the 1980s from Papua New Guinea. It was also said that the beetle was similar to N gemella in China. The accidental introduction into Solomon Islands from Papua New Guinea coincided with the introduction of the weevil Elaedibius kameronunicus to pollinate oil palm. It was initially mistaken for this pollinator, but later correctly identified. At that time the British Museum idenfitied the beetle as Podagrica basselae. Subsequently, the genus was changed to Nisotra. However, according to SPC, it was identified as Podagrica basselae Bryant, 1937 (Coleoptera: family: Chrysomelidae, subfamily Alticinae) det. Shute by the Natural History Museum, UK in 2004.
In PNG, it is most commonly known as the aibika (local name for slippery cabbage) shot-hole beetle. It is indeed one of the most important pests of the crop. It causes serious damage to leaves and once the leaves are completely defoliated it feeds on the petioles. It is an important pest all year around, but the damage is more severe during the dry season (especially the Central Province). It was first described from New guinea from Basella sp., and it has been recorded from Bougainville from Okra.
As far as natural enemies are concerned only a spider predator Onyopes sp. has been documented. The only effective control measure developed in PNG is the use of Derris eliptica as a home grown insecticide. The rotenone content of this plant was analysed in 2007 and found to be 3.2% w/w (Editor).
In Solomon Islands, the narrow leaf variety are said to be less affected by the beetle, but it is the large leafed varieties that are the most popular.
Specimens were sent to the New Zealand Arthrpod Collection for identification and this is what was said:
“Comparisons were made with five specimens of Nisotra basselae collected on slippery cabbage at Ringi,
New Georgia, on 13 May 1999. The image hasn’t got the detail of the pronotum, which would make it more certain, but it definitely conforms to the specimens collected”.
“In Macfarlane R. 1986. Important insect pests of smallholder crops in Solomon Islands and their control. Dodo Creek Research Station Technical Bulletin 4A, p. 10.), there is mention of Podagrica (Nisotra) basselae as just having arrived in Guadacanal. At that stage, there was no cultural control recommendation, and chemical control was Cypermethrin at 0.02%, spray every 10-14 days. Waiting period: 3 days”.
“In Kimoto S, Ismay JW & Samuelson GA (1984) Distribution of chrysomelid pests associated with certain agricultural plants in Papua New Guinea (Coleoptera). Esakia 21: 49-57, there is a black and white photograph of Nisotra obliterata on page 53, which shows the same general features as the Ringi specimens. The pronotum tips do not appear to be quite as “pointed” as the Ringi specimens; however, this could simply be because the quality of the photograph is not sufficient to show this feature”.
As for chemical control, it was suggested that Bt should be tried.