Impact and management of Oribius weevils in Papua New Guinea

• National Agricultural Research Institute, Papua New Guinea
• Fresh Produce Development Company Ltd., Papua New Guinea

Oribius weevils occur in Papua New Guinea, Irian Jaya and far north Queensland. They are a major agricultural pest in PNG, attacking the leaves, young shoots and developing fruits of a wide range of crop plants. Loss of plant vigour and surface scarring of fresh produce can reduce crop yields by 50-100 per cent.

At present there is almost no information on Oribius weevils. It is known that adults rather than larvae cause the damage to plants, but little is known about their status, systematics or ecology.

The project is estimating the impact of Oribius weevils on fruit and vegetable crops in Papua New Guinea (PNG), by obtaining information on Oribius systematics and ecology, to help develop methods to control infestations of these weevils.

This project will involve laboratory, glasshouse and field experimentation. The project team will first assess the damage caused by Oribius weevils by introducing weevils to citrus and coffee plants or branches under controlled conditions, and by an economic analysis of the size and worth of horticultural crops in the PNG highlands. Researchers will then determine the effectiveness of different types and concentrations of insecticide against the weevils, initially using laboratory trials and later using field trials of the most promising candidates.
In the next stage of the project, researchers will study weevil ecology using regular field sampling, movement studies and laboratory studies on lifecycles and larval feeding. These studies will provide information about seasonal patterns of abundance and sexual phenology, the relationship between the damaging adult stage and the enigmatic larval stage, and general movement patterns. Such information will be crucial in developing an integrated pest management strategy.
Finally, researchers will try to gain a better understanding of weevil systematics by studying existing collections and collecting new material for analysis. This approach will allow individual specimens to be accurately identified and named. The PNG National Agricultural Research Institute will be involved in all aspects of the project.

Nearly all Highlands' crops were found to suffer at least some oribius damage and the project team considered that the impact of the weevil was underestimated

Biological impact surveys were carried out for avocado, cabbage, capsicum, citrus coffee and strawberry. In all trials oribius management reduced damage significantly, with yield from protected plots up to 10 times greater than from unprotected plots. Coffee bean yield was not significantly affected by oribius, but oribius feeding did affect the growth of coffee seedlings and coffee regrowth after pruning. An extensive farmer-based crop impact survey was also undertaken. Twenty-four commonly grown crops were surveyed, based on a relative damage scale (high, medium and low). Only three crops (carrot, spring onion & asparagus) always scored as low damage, while 15 crops were reported as having some level of high damage. Based on their extensive and intensive research, the project scientists deemed Oribius to be the most serious pest of agriculture in the PNG Highlands.

Five insecticides were lab tested on both O. destructor and O. inimicus. Karate, Target and Chlophyrifos had the highest efficacy, whilst Orthene and Malathion were significantly less effective. Direct spraying of insecticide onto weevils was the most efficient exposure technique. There was no apparent difference in the efficacy of insecticides based on Oribius species. The three most effective insecticides were subsequently further tested at one-half and one-quarter manufacturers' recommended concentration. Karate and Target were still highly efficacious at the lower dosages.

A simple cost:benefit analysis was undertaken on insecticide control techniques, and results indicated that treatments to control oribius were highly profitable, with strength Karate control yielding a K66.40 increase in return compared with no control of oribius.

Soil-dwelling larval stages were confirmed to feed on the roots of selected plant species. The flightless adult beetles walk onto host plants where they feed and mate: it appears that beetle movement between host plants is common. At Aiyura and Goroka adults can be found in the field all year round, but are most abundant between November and April.

Oribius weevils feed on a very large number of different crop and non-crop plants, but they do not utilise all plants equally. In a survey of 32 of the most common crop and weed plants around the Aiyura Valley, O. inimicus was collected from all but one plant species (spring onion), with 34% of beetles collected from only three plant species (pepino, guava and thickhead). For O. destructor, the second most common Oribius species at Aiyura, seven of the surveyed plant species yielded no beetles, while the three most commonly utilised hosts, yielding 41% of individuals, were pepino (again), cassia and poinsettia.

Ob Nothing has been published on the taxonomy of the genus Oribius since 1959 and the exact number of species in the genus remains unclear. However, the project team estimated there were probably a minimum of 50 species in the genus. Following collation of existing collections, plus new collections, seven Oribius species were identified as pests of PNG horticulture. The diagnostic features and distribution of each of these species were recognised and may be available to users via a poster, paper and technical document.

Based on all components of the study the project team made the following recommendations for control.
Where oribius damage is quantitatively assessed as medium to high, then chemical control is warranted.
Crops should be watched for damage, most closely from December through to June, when beetle numbers are highest.
Karate, followed by Target, are the preferred chemicals for control. Application at - manufacturer's recommended rates is as efficacious as applying at full rate.
Spraying will be most efficient if carried out in the morning (before 10.00 am) or mid-afternoon as beetles are on plants at this time: during the middle of the day they rest in protected areas off the crop. Early morning spraying during peak flowering period of a crop should be restricted as pollinators are also commonly active at this time.
Thickhead should be removed from gardens wherever possible because it will attract beetles.

Intercropping or border planting with non-preferred plants, such as spring onion and carrot is likely to reduce oribius damage.