Pheromone: Rhabdoscelus obscurus Minimize

April 2000. A male-produced aggregation pheromone for Rhabdoscelus obsurus has been identified and tested successfully in Australia and Hawaii. This insect can be a problem in Australia in sugarcane and tests are in progress in Queensland to determine if pheromone mass trapping will reduce populations. 

Pheromone lures and testing protocol are available in Costa Rica (chemtica@sol.racsa.co.cr). In Costa Rica, Brazil and several other countries several groups have successfully mass trapped related weevils - Metamasius hemipterus (West Indian sugarcane weevil) and Rhynchophorus palmarum (American palm weevil). Mass trapping of Metamasius hemipterus is in palmito palm and sugarcane. Mass trapping of Rhynchophorus palmarum is in palmito, oil palm and coconut palm.

In PNG, researchers at the Coconut and Cocoa Research Institute have experience using pheromone traps against Rhynchophorus bilineatus in coconuts.

In a long email, Cam Oehlschlager provided information on  experiments from Central America
(R. palmarum) and the Middle East (R. ferrugineus).

1. Weevil  Behavior

R. palmarum

During sunny days most weevils flew in the early morning between 6 and 9 am and again in the late afternoon between 3 and 6 pm. During periods of maximum flight the temperatures were usually below 25°C. On cloudy days, in which the temperature did not exceed 25°C, weevil flights were more evenly distributed throughout the day.

During 2 days, 114 R. palmarum were observed as they approached a 19 l bucket trap strapped to a palm oil palm plantation (Oehlschlager et al. (
1993). Journal Economic Entomology 86: 1381). Weevils usually circled the palm with the trap a few times before landing. Of the ones that circled, 34% landed. Of these, 87% landed on the palm in the vicinity of the trap. The remainder landed on the trap, but were not observed to fly in. Once weevils landed, 82% crawled into the entry holes in the sides and dropped onto the sugarcane. Entry from the side requires weevils to crawl on the vertical surface of traps and a roughened surface is probably beneficial. Because weevils preferred to land on palms they could be using the latter as silhouettes. This led to a test of the relative attractiveness of 19 l bucket traps strapped to palms in which the pheromone lure was attached in the normal location under the lid, on the palm near the bucket and in the bottom of the bucket. There was no statistical difference in capture rates for bucket traps with the pheromone lure in these three locations (even when the lure was placed on the palm outside the trap).

2. Trap Location - Vertical Direction

R. palmarum

In red ring diseased palms the larvae of R. palmarum are found predominately in the upper portion of the stem, but attack may occur at any location where there is injured tissue. There are more weevils near the tops of palms since there is more damaged tissue here because of harvesting and pruning. In mature oil
palm stands, weevils were observed to fly in zig-zag patterns 2-3 m above the ground.  Surprisingly, standard pheromone-baited bucket traps located between palms are most effective when they were placed on the ground (Oehlschlager et al., 1993). The higher capture rates of traps on the ground could be due to the higher humidity associated with these traps or the availability of landing surfaces in the vicinity of these traps. When traps were strapped to palms, weevils preferred to land on the palms then crawl into traps. There was no statistical difference between trap capture rates at various heights above ground. Because of human activity in plantations, traps were routinely strapped to palms at 1.4 m.

R. ferrugineus

In Egypt, R. ferrugineus attack is 80% at the base (assumed to be due to entry at sites of offshoot removal) and 20% in the trunk and upper portion of the date palms. Studies there determined that traps placed on the ground are more efficient than traps hung on date palms. The important point is that the weevils can walk into the traps at ground level. Ground traps captured 7.4 times as many R. ferrugineus as traps hung on date palms at chest height. Since 1996, the Ministry of Agriculture and Fisheries of the UAE uses buried upright bucket traps containing fermenting date fruit which do not contain insecticide to capture R. ferrugineus. The food appears to be attractive for at least a month (FAO Workshop on Date Palm Weevil, Cairo, Egypt,  December 1998).

3. Trap location - Horizontal Direction

R. palmarum

During the period 1990-1992 several trap density experiments were conducted in Costa Rica using pheromone+sugarcane baited bucket traps to capture R palmarum in oil palm (Chinchilla et al., 1993. PORIM Proceedings, Kuala Lumpur, Malaysia). Traps were deployed at 4-5, 3.5, and 1/ha, and 1/3.5ha, on a grid basis. In all these experiments, lowering of R. palmarum populations were judged from a decrease in capture rates and lowering of red ring disease. Over a year, using even the lowest trap density of these experiments (1 trap/3.5ha) there was a reduction in the populations of R. palmarum to an economically acceptable red ring disease level (Chinchilla et al. 1993. PORIM Proceedings, Kuala Lumpur, Malaysia). In 1992, mass trapping was conducted using 1 trap/5 ha (average) in 5,500 ha and 8,000 ha oil palm in Costa Rica, and in one year the incidence of weevil associated red ring disease was lowered by 80% without any insecticide spraying program (except on RRD trees which were removed). Since 1992, trapping has been practiced in many oil palm plantations with weevil associated red ring disease using trap densities of 1 trap/3 ha to 1 trap/7 ha depending on the initial red ring disease level. In the successful applications of this strategy grid placement of traps has been used.

In 1998-1999 Brazilian workers mass trapped R. palmarum in 50 ha of coconut using 1 trap/ha (grid trapping) (Brazilian Chemical Ecology Meeting Proceedings Curitiba, Brazil, November 1999).  Using this trap density and grid trapping weevil associated damage (larvae and red ring disease) were lowered over 90% during the trial.

There is an alternative strategy to use pheromone traps to reduce palm weevil populations which is to place traps around the edge of the plantation. This strategy was not successful in the single trial in which it was conducted in 83 ha of oil palm by Chinchilla et al. (1993). (PORIM Proceedings, Kuala Lumpur, Malaysia).

4. Tests on barrier trapping

The results of experiments indicated that barrier trapping is not likely to be effective. While perimeter traps in a mass trap area consistently capture more weevils than internal traps, the internal traps are important to maintain a low population. For maximum effectiveness ihe pheromone traps should be placed on a grid basis inside palm groves.

5. Palms on which traps are placed

Early in the development of mass trapping for R. palmarum the question of attack of palms containing traps vs other palms was examined. Mass trapping began in 1991 in a  40 ha plot of oil palm that was heavily infested with red ring disease. During one year, trap position, capture rate and red ring infestation were followed (Oehlschlager et al., 1995. Environmental Entomology 24: 1004.)  While  some palms on which traps were hung did become infested with red ring disease or with weevil larvae the rate at which these palms bacame infested was no greater than the rate at which palms within the plot on which traps were not hung became infested.

Later (May 2011),the following paper: Reddy GVP, BalakrishnanS, Remolona JE, Kikuchi R, Bamba JP (2011) Influence of Trap Type, Size, Color, and Trapping Location on Capture of Rhabdoscelus obscurus (Coleoptera: Curculionidae). Annals of the Entomological Society of America 104(3): 594-603.

http://www.ingentaconnect.com/content/esa/aesa/2011/00000104/00000003/art00026

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