Status of Biological Control of C. odorata (L.) R.M. King and H. Robinson on Pohnpei, Federated States
of Micronesia

by
Nelson M. Esguerra, William S. William
and Justino R. Smith
College of Micronesia Land Grant Programs,
P.O. Box 1179, Kolonia, Pohnpei, FSM 96941


 Introduction
 Materials/ Methods
 Results / Discussion
 References
Table 1 

ABSTRACT

Biological control of the Siam weed on Pohnpei, Federated States of Micronesia involved mass rearing the biological control agent, Pareuchaetes pseudoinsulata Rego Barros, in the laboratory and mass releasing either larvae or adults in field infestations of the weed.
Almost two years of field release did not result in establishment. It was observed that predators such as ground lizards, spiders, birds, red and black ants preyed heavily on the adults, larvae and eggs of P. pseudoinsulata. On October 24, 1990, feeding injury and the presence of a number of caterpillars were noticed in one of the release areas. It is planned that after further confirmation of the firm establishment, field adapted larvae will be collected for release in other areas. About 500 adults will be released on each site to facilitate the establishment in other areas.
Problems in mass rearing and mass releasing P. pseudoinsulata and techniques used to overcome the problems both in the field and in the laboratory are discussed.

INTRODUCTION

The Siam weed has become a serious weed pest in the States of Pohnpei, Kosrae and Yap in the Federated States of Micronesia (FSM). The weed is spreading rapidly and is now abundant in Madolenihmw and Kitti municipalities of the southern part of Pohnpei. It is spreading slowly toward Sokehs and isolated patches of the weed are appearing on Kolonia and Nett.
In the two municipalities where they are present, Chromolaena odorata is usually abundant on vacant lands, pastures, roadsides, and cultivated lands. The weed seems to grow well on Pohnpei, with an annual rainfall of about 500 cm and temperature range from 70° - 92° F. Once established, the weed forms a dense mass of green cover over land, impeding access to roads and pathways. It becomes a fire hazard in dry seasons.
Because of the seriousness of this weed in FSM, the College of Micronesia Land Grant Program decided to include C. odorata as a target for biological control under the Agricultural Development in the American Pacific (ADAP) Biological Control Agent Exchange Project. In February 1989, through the help of the University of Guam, larvae of Pareuchaetes pseudoinsulata Rego Barros (Lepidoptera: Arctiidae) were brought to Pohnpei.
This paper discusses the procedures and activities that were carried on and will be implemented on Pohnpei aimed at controlling the Siam weed with the arctiid moth, P. pseudoinsulata.

MATERIALS AND METHODS

Laboratory rearing of P. pseudoinsulata

Rearing the caterpillars of P. pseudoinsulata to produce large numbers in the laboratory followed the techniques used by Muniappan et al. (1989) and Seibert (1985) with some modifications. The technique used was as follows:
Eggs of P. pseudoinsulata were laid in groups on Chromolaena leaves. Leaves with eggs were detached from the stems and placed in ice cream cups with covers. Eggs laid on screens in rearing cages were allowed to hatch and newly hatched larvae were collected and placed in cups with covers and fresh detached leaves of Chromolaena as food. Ventilation was provided for the rearing cups by glueing muslin cloth on the cut portion of the lid. The first and second larval instars were maintained in these cups and fresh leaves were provided daily. The cups were cleaned every 2 days and larvae feeding on detached leaves were transferred to clean cups, thus avoiding accumulation of frass that could be toxic to the insect. Upon reaching the third instar, the larvae were transferred to 25 gallon-plastic containers. Chromolaena stems with leaves were kept in small bottles with water to prevent wilting of leaves and assuring larvae ample supply of fresh leaves for two days.
The 25 gallon containers were cleaned of frass and dried leaves every two days and caterpillars transferred to clean containers with fresh leaves. When the caterpillars were mature and about to pupate, fresh detached leaves were placed on the bottom of the containers for pupation. Three days after, the pupae were collected and placed inside the 30.4 x 30.4 cm organdie nylon cloth and 30.4 x 30.4 x 60.9 cm fine meshed wire rearing cages. Cut shoots of Chromolaena kept in small bottles with water were placed inside the rearing cages to provide as oviposition sites for female moths. Pieces of cotton soaked in sugar solution were kept hanging inside the cages and replaced every two days. Cotton soaked in salt solution was also hung inside the cages. These served as food for the moths.

Population Density of Chromolaena at the release site

A 5 x 5 meter wooden quadrat was thrown in the designated release site and the number of Chromolaena plants were counted. Ten weeds were randomly selected inside the quadrat. The size of stems and height of these ten weeds were measured using a meterstick.

Field Release and Monitoring of P. pseudoinsulata

Larvae

Laboratory reared third instar larvae of P. pseudoinsulata were placed in cups with covers and brought to the release sites. Each release site was heavily infested with luxuriantly growing Chromolaena. The larvae were individually picked from cups using a camel-hair brush and released on Chromolaena. Signs of feeding and presence of larvae were monitored two days and one week after their release.

Adults

Newly emerged adults of P. pseudoinsulata from laboratory cultures were released in the release site. One week after their release, observations were made for the presence of eggs and newly hatched larvae on Chromolaena.
Adult moths were also released in a 1.17 x 1.17 x1.22 m field cage with naturally growing Chromolaena inside for oviposition. Since earlier attempts at facilitating the establishment of adult moths inside the cage were a failure due to predators, the cage and Chromolaena including about 1.2 m around the cages were sprayed with diazinon at 1 kg ai/ha to kill predaceous insects and other organisms.

RESULTS AND DISCUSSION

Other Insects Feeding on Chromolaena in the field

In the course of surveying the extent of Chromolaena infestation on the southern part of the island, some insects were observed feeding on the weed although it is very unlikely that they could be utilized for a biological control program for this weed. These insects were observed to feed also on other plants and economic crops on Pohnpei.
Melon aphid, Aphis gossypii, infested the shoots and tender leaves causing curling and cupping of affected leaves. Negligible damage also occurred due to feeding by the Philippine katydid, Phaneroptera furcifera Stal. An unidentified mealybug also infested the tender shoots of Siam weed.

Population And Growth of Chromolaena

Prior to releasing the larvae of P. pseudoinsulata, the release site at Kitti was sampled for Chromolaena density in a 5 x 5 m quadrat. The total number of weeds was 116 and most of them were in the actively growing stage. On the release site, the weed formed tangled bushes 2-3 m high ( x = 2.35 ) with stem sizes ranging from 4 to 6 cm ( x = 5.45 ). The site was chosen primarily because the surrounding areas were also infested with the Siam weed and therefore assured the biocontrol agent ample food to support several generations when established.
Flowering of Chromolaena usually occurs in December through February each year on Pohnpei. Since the seeds of Chromolaena are wind-borne and can easily be carried by wind to new areas, it is expected that in 2 to 3 years, if left uncontrolled, it will be distributed throughout the island.

Rearing Problems in the Laboratory and Solutions
Rearing of larvae of P. pseudoinsulata started from 60 larvae received from Guam. Development of larvae, pupae, adults and eggs were normal and cultures were successfully completed up to 7 generations, and during that time periodic releases of the larvae were made in the field.
Unfortunately, in the 8th generation many mature larvae began to die from an unknown cause, and this almost eliminated the cultures despite cleaning of rearing cups and 25 gallon rearing containers every other day. Only a few larvae survived the 8th generation. To prevent further mass mortality of larvae in 25 gallon containers, the cloth covers were removed from 8:00 a.m. to 4:30 p.m. to assure oxygen in containers for the larvae. The containers were covered in the evening to prevent the escape of larvae.
It was also observed in the process of culturing that a few eggs from the majority of egg masses laid on the leaves of Chromolaena and on the screen of the rearing cages failed to hatch for unknown reasons, despite providing cotton balls soaked in sugar solution as food for the adults. Napompeth (1990) mentioned a similar problem in his laboratory cultures of P. pseudoinsulata and felt that trace amounts of sodium (Na) played a role in the hatchability of eggs. He recommended adding NaCl to the diets of both adults and larvae. Hence, in addition to sugar solution, cotton balls soaked in salt solution were hung in rearing cages to supplement the adult nutrition.
Ants were observed to interfere with laboratory cultures by preying on eggs and larvae of P. pseudoinsulata in rearing cups and 25 gallon containers. Pupae were collected, placed in rearing cups and brought to another room for 2 days. The rearing room was sprayed with diazinon at 1 kg ai/100 gallons of water to kill ants. Legs of chairs and tables where rearing was being done were placed in cans with water to prevent ants from interfering with the culture.

Field Releases of P. pseudoinsulata: Problems Encountered and Solutions

Larvae

Table I shows the number of larvae that were released in the field. However, field released larvae could not be recovered from the release site one week after. It was observed that spiders, red ants and black ants, birds and ground lizards were attracted readily to the release sites. Spiders were seen preying on the larvae.

Table 1. Number of laboratory-reared P. pseudoinsulata released on Siam weed infested area

 Date

Location

Stage

Number released

 2/28/89

Wenik Elem. Sch. (Kitti)

 larvae

 219

 4/27/89

 Wenik Elem. Sch. (Kitti)

 larvae

 1010

 5/3/89

 Wenik Elem. Sch. (Kitti)

 larvae

 210

 6/15/89

 Wenik Elem. Sch. (Kitti)

 larvae

 502

 7/14/89

 Wenik Elem. Sch. (Kitti)

 adult

 128

 8/23/89

 Nan Mal (Palikir)

 adult

 202

 9/25/89

 Nan Mal (Palikir)

 larvae

 350

 10/15/90

 Nan Mal (Palikir)

 larvae

 751

 11/21/89

 Nan Mal (Palikir)

 larvae

 462

 1/18/90

 Nan Mal (Palikir)

 larvae

 1503

 3/8/90

 Palikir (Sokehs)

 adults

 216

 4/12/90

 Palikir (Sokehs)

 adults (Field Cage)

 300

 5/2/90

 Palikir (Sokehs)

 adults (Field Cage)

 225

Since larvae could not get established despite many releases made, possibly due to predation, releases of larvae were made inside a 1.17 x 1.17 x 1.22 m field cage with Chromolaena growing naturally inside the cage. Again, the larvae released inside the cage could not be recovered after the release.

Adults

Field releases of adult moths of P. pseudoinsulata were made in July and August 1989. More than 300 adults were liberated at the release site. Again one week after, no eggs or young larvae could be recovered from Chromolaena. In March and April 1990 adults were again released, but after a week neither egg masses nor young larvae were evident at the release site. In May 1990 it was decided to release adults in field cages with Chromolaena inside. A week before releasing the adults, the cages were sprayed with diazinon at 1 kg ai/Ha, including the Chromolaena inside the cages and the surrounding areas about 1.2 m around the cages, to get rid of the existing predators and to prevent them being attracted to the cages with adult moths inside. Again, lizards and spiders were able to enter the cages and killed those adults inside.
The arctiid moth, P. pseudoinsulata, so far is the only biocontrol agent that has been reared and released on Pohnpei for the control of Siam weed. About 15 months of field release of larvae and adults since February 1989 did not result in establishment. However, in one release site feeding injury on leaves of the Siam weed and few caterpillars were noticed. This field population is now being monitored to observe its buildup in the release site as well as its natural spread.
In some parts of the world, like India and West Africa, the arctiid moths failed to establish because these areas have a very pronounced dry season. But in Sri Lanka where the dry season is less severe, as in Trinidad, the insect sucessfully established. CAB International Institute for Biological Control (1988) suggested that collections of P. pseudoinsulata should be made from climatic conditions similar to the proposed released zone: in our case, similar to the wetter conditions of Pohnpei should be the most promising approach.
Seibert (1985), based on his work in Guam, mentioned that in an effort to maximize the number of P. pseudoinsulata individuals in one location, adult moths were released. He found that approximately 500 adults if released in the same location would increase the chances of establishment. This is desirable because the high resulting density of larvae are believed necessary to overcome predatory pressure and to create an epidemic population.
Attempts are also being made to produce more adults in the laboratory. Five hundred adults will be released each time and releases will be made two or three times in the same release site to find out whether P. pseudoinsulata will become established in other areas of Pohnpei infested with Chromolaena. It is also planned that the larvae from the established site be transferred to new sites to assist in spreading and to cover the island within a short period.

REFERENCES

CAB International Institute for Biocontrol Control. 1988. Prospects for the biocontrol of Chromolaena odorata (L.) R.M. King and H. Robinson. In: R. Muniappan (ed.) First Intern.Workshop on Biol. Control Chromolaena odorata. pp.25-33. Agr. Expt. Sta., Univ. of Guam.

Muniappan, R., V.T. Sundaramurthy & C.A. Viraktamath. 1989. Distribution of Chromolaena odorata (Asteraceae) and bionomics, consumption and utilization of food by Pareuchaetes pseudoinsulata(Lepidoptera: Arctiidae) in India. Proc. VII Int. Sym. Biol. Contr. Weeds, 6-11 March1988, Rome,Italy. Delfosse, E.S. (ed.). Ist.Sper.Pathol.Veg. (MAF), pp. 401-409

Napompeth, B. 1990. Infertility of Pareuchaetes pseudoinsulata. In: Contributions from Thailand. Chromolaena odorata Newsletter. No. 3. p.6.

Seibert, T.F. 1985. Biological Control of Chromolaena odorata. Guam Agric. Expt. Station. Annual Report. 12-13.