Abstract

Several trap designs are described which are effective to attract and capture adults of the America palm weevil (Rhynchophorus palmarum). Two elements are important in an efficient trap: the male produced aggregation pheromone (6-methyl-2(E)-hepten-4-ol) and a food source. Pheromone-baited traps captured from 6 to 30 times more weevils than un baited traps. The food source can be stem pieces of oil palm arranged in different ways, however, they lose attractiveness in about a week. A 40 cm stem section can be hollowed with a chain saw to form a container, when baited with the pheromone and sugar cane those traps may be effective for more than three months in the field. Plastic containers (5-20 l) with insecticide-treated sugar cane pieces and the pheromone are very effective traps.

Mass trapping of the weevil over several months in a commercial oil palm plantation, using some of these traps, have caused a drastic reduction in the weevil population, which was associated with a decline in the rate of increase of the red ring disease.

Introduction

The American palm weevil, Rhynchophorus palmarum (L) is an important pest of several palms in tropical America. Larvae may cause direct damage when feeding in the stem. It is also the only known vector of the red ring disease, caused by the nematode, Rhadinaphelenchus cocophilus (Cobb, 1922; Hagley, 1963; Griffith, 1987). This disease is of widespread distribution in coconuts and oil palm on all Tropical America, particularly in the continental platform. Disease control is achieved through the early eradication of affected palms, elimination of the vector's breeding sites and trapping of the adult insects (Griffith, 1987; Chinchilla, 1992).

Several trap designs have been described that would attract and capture adults of R. palmarum. Most of them use insecticide-treated palm stem pieces (Mariau, 1968; Griffith, 1969; Morin, 1986; Griffith, 1987; Morales and Chinchilla, 1990; Chinchilla et al , 1990; Moura,1991). Some of the models described are rather laborious to build and the cost per trap is high. Also, when the trap materials have some other potential uses for workers and occasional visitors to the plantation, the losses due to robbery can be quite high.

We recently reported (Oehlschlager et al , 1992a,1992b) the effectiveness of a trap have proved to be very efficient to attract and capture adults of R. palmarum when used in combination of the male-produced aggregation pheromone.

Material and Methods

Field experiments were carried out in a commercial oil palm plantation, located in the Coto-Colorado valley, South Pacific coast of Costa Rica. All traps design were compared against a control which was a 19 l plastic bucket ( Fig. 1F ). All experiments were arranged as completely randomized blocks with ten replications for each treatment (trap design).

Following the planting pattern of the plantation (9 m triangular), each treatment (trap) within a replication was separated three palms across from the next one. Separation between replicates was of approximately 90 m (10 rows). Buckets and other similar traps were hung from old petiolar leaf bases at chest high. Heavier traps, as those made from palm trunks, were placed directly on the ground. The pheromone was released from a plastic devise at the rate of 20 mg in 24 h at 30ºC. The field life of one of such devices was about three months.

Data analysis

Assumption of normality and homogeneity of variance were tested on all data by graphical assessment of log variance versus log mean and Bartlett's test, respectively (SAS Inst. 1985). Some data were transformed by (X + 0.5) 0.5 (Zar, 1984) and subjected to an analysis of variance using PROC GLM (SAS Inst. 1985). Means differences were separated using the Bonferroni's t test.

Traps made from oil palm stem pieces

Solid stem pieces. One of such traps is made by cutting stem pieces 20-25 cm long. These transversal sections are cracked with a heavy axe at right angles to obtain four sections, that remain together hold by the stem fibers ( Fig. 1B ). Two of such pieces form a trap (one on top of the other). Some dried palm leaves on top help to protect the trap from rapid desiccation.

A variant of this trap is made when longer pieces (40 cm) of palm stem are obtained, and then cut lengthwise in four pieces. Seven of such pieces form a trap; four pieces are placed on the ground, cut surfaces up, and other three pieces are place on top ( Fig. 1A ). Other trap designs are illustrated on Fig. 1 .

Hollow trunk.This trap is made from a stem section about 30 to 40 cm long, which is hallowed with a chain saw ( Fig. 1E ). The top and bottom of these stem containers are also made from stem sections about 10 cm in thickness. Some three or four "V" cuts were made on the rims of the hallowed section to facilitate the entrance of the insects into the trap (some pebbles may also provide enough space for the weevils to enter). This trap is used in combination of insecticide-treated sugar cane in a similar way as in the plastic bucket trap. The amount of sugar cane to add depends on the diameter of the stem, but must be enough to cover the bottom of the container. sugar cane pieces should lay horizontal on the bottom.

In order to kill the insects rapidly, reducing the number of poisoned insects that are able to scape, it is preferred to use an insecticide with a knock-down action, such as carbofuran (Furadan F: 1.5 cc\l). Large amounts of sugar cane pieces can be treated by immersing them in an insecticide solution.

Plastic containers

Plastic buckets. This trap consists of a white 19 l plastic bucket, with drainage holes at the bottom, and entry slots on the side and top ( Fig. 1F ). The device with the pheromone was placed directly hanging from the lid. Traps contained some 12-15 pieces of halved sugar cane stalk that were previously immersed in a insecticide solution. The traps were checked weekly, and 3-4 pieces of fresh sugar cane stalk were added. All the cane was replaced every two weeks. This has been our standard trap against which all other models have been compared.

Other food sources tried in the buckets were halved pieces of leaf petioles (from coconut and oil palm), a sponge impregnated with molasses, and palm heart pieces, both of coconut and oil palm. A sticky substance smeared on the bottom and detergent laced water (5 cm) in the bucket were also tested as substitutes of the sugar cane to retain the insects in the traps. The bucket was also modified by placing a metal funnel to fit the upper part in order to impede the weevils to escape.

Other containers. Disposable plastic containers, five and eight liters, have been used similar as the 19 l buckets ( Fig. 1G ). Halved sugarcane pieces are also added to these traps, but their length has to be reduced accordingly.

Other designs. Some of the following examples have not been extensively tested, but all of them attract and capture a fair amount of insects. Designs in Figs. 1C and 1D have been used commercially. A bag trap consisted of a wire frame (30 cm high x 20 cm diameter) covered with a black polyethylene bag (600 gauge) and having a polyethylene covered wire mesh (0.6 cm) top. Sugar cane was added in a similar way as to the buckets. Another trap consisted of a hole in the ground of the approximate size of a 19 l bucket, with 2-3 cm of gravel on the bottom, and 12-15 halved sugar cane pieces treated with carbofuran. The hole was covered with several pieces of dried oil palm leaves and the pheromone lure was hung on a stick 40 cm above the hole.

Results and Discussions

A pheromone-based trapping program can effectively reduce the population of the palm weevil in an oil palm plantation, and this has been associated with a reduction in the incidence of the red ring disease (Oehlschlager et al , 1992c). The male-produced aggregation pheromone is available as RHYNKO-LURE from ASD de Costa Rica (P.O.Box 30, 1000 San José, COSTA RICA). As the pheromone lures will be fixed costs in any trapping program, it is important to lower the cost of other elements such as the trap itself. Savings may come from the use of cheaper materials, easier to construct traps and from the need of less maintenance in the field. Additionally, traps should not be attractive to be stolen. However, efficiency should not be sacrificed.

For some plantations where old stands are available, the traps made from palm stems are very convenient. However, in young plantings, normally there is not enough material available to make the traps, so buckets or similar containers are the option to choose.

Traps made from palm stem pieces

Typically, captures in these traps increase after two days, reach a peak on day three or four, and then decrease until its necessary to replace them, after a week or so. When trunk pieces are large enough, the trap attractiveness can be extended for a few more days by removing the superficial tissue that has deteriorated (rotten). If pheromone is added to this traps, captures increase and the trap maintains its attractiveness for about 10 days. Fig. 2 compares weevil captures between the trap design in Fig. 1B , and the bucket trap with sugar cane and pheromone.

The insecticide used should not have a strong smell, since it may repel the insects. Some insecticides that have prove effective are: methomyl (Lannate), carbofuran (Furadan F) and carbaryl (Sevin 80),among others.

Hollow palm trunks

These traps will also attract insects when used without the pheromone and sugar cane, but captures are relatively low, and its effective period is similar to those traps described above. When sugar cane is used, captures increased significantly. The field life of these "containers" is estimated in at least three months, and the trap maintains its attractiveness as long as fresh sugar cane is added periodically. Furthermore, when the top section of the trap is replaced with a fresh one, captures increase accordingly. This effect is illustrated in Fig. 3, where the top of the traps was changed the on 23rd day.

Accumulated captures in the hallow trunk traps over a period of time can be even higher than in the bucket traps (Fig. 3). Here, accumulated captures in 10 hallow stem traps over a period of 84 days was 25% higher than in the bucket traps (1,840 insects versus 1,381 in the buckets). It is thought that stem volatile compounds released from fresh cuts, sugar cane and the pheromone act synergetically to increase captures. Additionally, the fact of these stem traps being on the ground would increases its efficiency. This last effect was demonstrated during a 10 replication experiment where bucket trap placed on the ground captured significantly more insects than traps at 1.7 and 3.1 m above ground level (Fig. 4). Bucket traps are routinely hanged at chest high to facilitate handling, and to reduce number of traps stolen or being vandalized.

Plastic containers

Buckets. This design ( Fig. 1F ) has been the tester in most of our studies on weevil population dynamics and the red ring disease (Oehlschlager, 1992b, 1992c). A good quality bucket will last in the field for more than a year, but the unitary cost can be relatively high. Under some circumstances, buckets may be stolen since even with holes it may have many uses.

All sugar cane pieces are changed every two weeks, but three or four fresh pieces are added weekly to "reactivate" the trap. This way, the bucket traps captured on a accumulated basis more weevils than all stem traps tested (except the hallow trunk plus sugar cane).

It is quite possible that trap interference may have occurred for a few days, when palm stem traps were placed near bucket traps. The pattern of daily captures indicate that insects attracted to a trapping area, may prefer the stem traps to the buckets. After a few days, however, attractive stem volatile compounds stop being released, and captures start increasing in the buckets (Figs. 2 and 3).

Lateral entry holes on the bucket were modified to provide a landing platform for the insects. The upper rectangular holes on the bucket side were not cut completely, leaving the base bent downward. This modification did not improve captures significantly.

Sugar cane molasses, detergent laced water or a sticky substance on the bottom of the bucket were poor substitutes for sugar cane. A metal funnel fit to the mouth of the bucket decreased captures significantly, even when used along with sugar cane.

Oil palm stem pieces, and particularly soft tissue from the palm heart were very efficient, but its attractiveness decreased quite fast after a few days. This materials may be used in the bucket traps but maintenance cost per trap increases, as they must be visited more frequently to replace the deteriorated tissue. However, halved leaf petioles (both from coconut and sugarcane) allowed the capture of a fair amount of insects in the buckets, but captures were significantly lower than with sugar cane (Fig. 5).

Other plastic containers. Captures in some of these containers (5 liter capacity) were similar and sometimes even higher than in bucket traps. Several low toxicity pesticides (i.e. Bacillus thuringiensis) are sold in appropriate containers for trap making. These small vessels are also easy to transport and not so attractive to be stolen. All these traps, as well the bucket traps should be placed in close contact with the palm stem in order to facilitate the weevil entrance. Most insects that are attracted to the trap land on the palm stem and from there moves to the trap.

Other traps. Of the several designs tested, holes in the ground with sugar cane and the pheromone attracted some insects, but the captures were significantly lower than the bucket traps. However, the efficiency of the bag trap described was similar to the standard trap. The bag trap is cheaper to build than the bucket one, but it deteriorates easily in the field.

It is possible that in the near future some synthetic palm volatile compounds can be used to substitute the sugar cane. The effect wanted is a low cost trap that will not require much maintenance in the field.

Conclusions

Several trap designs have been described that effectively attract and capture adults of the American palm weevil, R. palmarum . Two elements seem to be essential to get high captures in the field: a food source and the male-produced aggregation pheromone. Halved sugar cane stalk have been the more convenient food source so far tested. However, it is possible that some palm volatile could be used instead.

Traps can be constructed directly from oil palm (or coconut) stems or from plastic containers. The trap is attractive as long as the pheromone is released from an appropriate device and the food source is renewed often.

It has been shown that massive trapping of the weevil using the bucket trap, effectively lowers the insect population in an area, and this has been associated with a reduction of the red ring disease in oil palm.

Acknowledgments

The authors wish to thanks Palma Tica company in Costa Rica for permission to publish these results. Special thanks to D. L. Richardson for his support to this research and revision of the manuscript; G. Castrillo for help during the field work and A. Salas for computer work; R. S. Mc.Donald of the Department of Biological Sciences at Simon Fraser University for statistical analysis; the Natural Sciences and Engineering Research Council of Canada for Operating and International Collaboration Grants to A.C.O. An administrative leave granted to A.C.O. by the Simon Fraser University is also gratefully acknowledged.

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