In many tropical countries, lepidopterous stem borers are major pests of the rice crop. Study of the rice borers in Surinam, Rupela albinella
and Diatraea saccharalis,
was made in the Paramaribo area, at the experimental station 'CELOS' during 1971, 1972 and 1973, since data on the ecology and economic importance of these borers were incomplete and almost lacking for the small holders rice areas. Special attention was paid to the role of the parasites of these borers.
I. R. albinella
This 'white rice borer' deposits its scale-covered egg masses on the leaves. Newly hatched larvae disperse in a very active way both on plants and on the water surface and they may also use the flow of water. They bore into the stem cavity within 24 hours after hatching. Development to maturity takes place inside one internode. There are five larval instars and only the last two can not be separated by head capsule width. The full-grown larva cuts an exit opening in the stem wall for escape of the adult. The duration of the different developmental stages was determined.
Normally the percentage of larvae which entered diapause proved to be very low. It was not clear which factor was reponsible for diapause induction. A correlation with a slightly higher temperature (about 2°C) was detected but the data are still too limited to accept the temperature as a basic factor.
Moths are active during darkness and are attracted by ordinary incandescent light, but are seldom captured during the period of full moon.
requires an adequate stem cavity for its development, rice plants become vulnerable to attack only 60 days after sowing when a proper internodal space may be present. Rice varieties having a total growth period of only 105 days automatically possess an 'escape resistance', because the larval and pupal development takes about 50 days.
Generally R. albinella
is well adapted to the rice plant. Crop losses are very small since the stem tissue is only attacked superficially.
Four parasites of R. albinella
were found, viz.:
sp., an egg parasite with a short life cycle. This insect was not further studied.
2. Venturia ovivenans,
an egg-larval parasite with a high reproduction capacity, parasitizes the eggs. The growth of the parasite larva is moderate until the host is full-grown. Rapid growth follows and the larva leaves its host and pupates inside the stem.
Development of host and parasite are well sychronized, both in nondiapause and diapause situations. The average time of emergence of the wasp is two days earlier than of the moth.
The species is very common throughout Surinam.
3. Strabotes rupelae,
a larval and pupal parasite, deposits its eggs near the fullgrown host larva or pupa. The wasp even crawls into the water in search for hosts. The ectoparasitic larva grows very rapidly and sucks out its host. The duration of the life cycle is one third that of the host. The adult life span may be as long as two months.
The parasite was quite common at the 'CELOS' research centre with its continuous rice cultivation program.
sp., a gregarious and ectoparasitic wasp, lays its eggs in the stem cavity near a full-grown host larva. Larval growth is very rapid and the total life cycle is one third of that of its host.
The parasite was common at the 'CELOS' research centre.
The interactions between R. albinella
and its parasites are schematically given in Figure 15.
The greatest mortality occurs in the L 1
during dispersal and penetration of the host plant. Later on, the effect of parasites as well as pupal mortality are important.
The succession of the generations of the borer and its parasites under the 'CELOS' continuous rice cultivation system is given in Figure 18. Over a period of slightly more than two years, borer infestation slowly increased whereas parasitization of the borer slowly decreased. The percentage composition of the parasitic complex remained stable in these two years.
II. D. saccharalis
An existing aseptical diet for the 'brown borer' was improved and made it possible to rear large numbers of borers. The following good rearing results were obtained: total developmental time (35-38 days), pupal weights (males 75 mg, females 125 mg) and egg production (400 eggs/female). A clear correlation was found between pupal weight and egg production.
It was not possible to accurately separate the 6 larval instars by means of head capsule width. The behaviour of the larvae was followed. Moths emerged before midnight and sometimes copulated the same night. A male copulates once a night and at most, three successive nights. Copulation always occurs late in the night. The first eggs are laid the night after mating.
Larval and pupal mortality was very high under field circumstances.Agathis stigmaterus
was the only important parasite. It was reared in large quantities in host larvae feeding on the diet mentioned above. The wasp deposits its eggs in 6-14 day-old hosts. An average of 100 hosts were parasitized per wasp. The parasite first grows slowly; it then leaves the nearly full-grown host and sucks it out within a few hours. There is a good synchronization between the host and the parasite. Only 2 % of the wasps were males.
Ant nests were present in about 15 % of the rice hills. These ants, Paratrechina sp., feed on the young borer larvae.
incidentally may cause some local losses, the overall damage is negligible.