Idrus Abbas

Population dynamics of animals in unpredictably-changing tropical environments

We studied population dynamics of a solitary phytophagous beetle,Epilachna viqintioctopunctata and a social stingless bee,Trigona minangkabau, in Sumatra, Indonesia for 5 years from 1981.Population increase ofEpilachna vigintioctopunctata was suppressed in months of normal rainfall (≥300mm) but was released in the 1982–1983 El Nino-Southern. Oscillation when rainfall dropped to 50% of the long-term average. Mechanisms might be direct; rainfall lowered egg hatchability and the time of adult’s residence on host plants. When dry weather continued for more than three generations, theEpilachna vigintioctopunctata population reached a density at which food shortage due to defoliation occurred. Although parasitism of immature stages was high, it was not a population-regulating factor. Thus, there were two types of ecological crunch: competition for food resources at the end of favourable dry periods and high mortality during heavy rainfall periods that usually followed El Nino-Southern Oscillation dry conditions.By an experimental addition of artificial nest sites, colony density ofTrigona minangkabau increased 2.5 times the original density of natural colonies. One-half of artificial nest sites were occupied by arboreal ants and thus competition for nest sites with ants suppressed further increase ofTrigona minangkabau. Intermediate rainfall was favourable forTrigona minangkabau because the rate of colony foundation decreased both during dry El Niño-Southern Oscillation months and months with heavy rain. Colony death was independent from rainfall. Many colonies that survived for 6 months persisted for >2 years and colony density was quite stable.Trigona minangkabau colonies could survive even under unfavourable periods, by hoarding resources in the nest. There was no significant ecological crunch during the study period and colony density almost always tracked the carrying capacity of the habitat, which was basically determined by nest-site abundance.Climatic conditions, especially rainfall, changed with various periodicities, 4–5 years for El Nino-Southern Oscillation, and 2 years for the monsoon and other shorter periods. The contribution of periodicities of 1 and 0.5 years, that were linked to movement of the sun, were weak, indicating that animals could not use seasonal changes of environments,e.g. daylength, to predict environmental changes. We discuss traits adaptive to such unpredictably-changing tropical environments. Separation of predictability of temporal environmental change and synchronous changes among patches improves our understanding. Low oviposition rate and resulting prolonged life-span ofEpilachna vigintioctopunctata, usually associated withK-selected traits of life history, seem to be adaptations for unpredictable environmental changes.

Foraging behavior of individual workers and foraging dynamics of colonies of three sumatran stingless bees

The foraging behavior of three stingless bees,Trigona (Tetragonula) minangkabau, T. (Trigonella) moorei andT. (Heterotrigona) itama, was studied to describe patterns of resource harvest in disturbed forest areas in Sumatra, Indonesia. 1. Average daily total number of foraging flights per colony was 1200 inT. minangkabau, 2400 inT. moorei and 7000 inT. itama and it was proportional to colony population size. Foragers collecting nectar, pollen or plant resin were respectively 70–80%, 10–20% and <10% in the three species. Pollen was collected most in the morning. Nectar collection peaked in midday inT. itama but continued almost evenly until dusk inT. minangkabau andT. moorei. Resin was collected evenly throughout day. 2. In all the three species the volume of a nectar load carried by a returning forager did not decrease until 1600, followed by slight decline. In the morning the sugar concentration of nectar was almost constantly 30%, and in the afternoon its maximum value reached 60% although some remained 30%. As a result, the mean sugar weight in a nectar load gradually increased until dusk. 3. InT. minangkabau, resin was collected by specialized foragers. Nectar foragers switched to collect pollen and vice versa. There were two types of foraging: “exploitatory” flights, the repitition of short, rather constant flights bringing back full resource loads, and “exploratory” flights, prolonged flight and/or reduction in amount of resources carried. Exploitatory flights followed the exploratory flights which led to discovery of rich sources. Mean duration of exploitatory flights was 7 min during nectar collection, 12 min for pollen and 23 min for resin. Sites of exploited flowers were estimated to be 84–434 m distant from the nest site. Mean duration of flights for nectar collection was 13 min in the morning and 6 min in the late afternoon. Average daily total number of foraging flights per colony was 1200 inT. minangkabau, 2400 inT. moorei and 7000 inT. itama and it was proportional to colony population size. Foragers collecting nectar, pollen or plant resin were respectively 70–80%, 10–20% and <10% in the three species. Pollen was collected most in the morning. Nectar collection peaked in midday inT. itama but continued almost evenly until dusk inT. minangkabau andT. moorei. Resin was collected evenly throughout day. In all the three species the volume of a nectar load carried by a returning forager did not decrease until 1600, followed by slight decline. In the morning the sugar concentration of nectar was almost constantly 30%, and in the afternoon its maximum value reached 60% although some remained 30%. As a result, the mean sugar weight in a nectar load gradually increased until dusk. InT. minangkabau, resin was collected by specialized foragers. Nectar foragers switched to collect pollen and vice versa. There were two types of foraging: “exploitatory” flights, the repitition of short, rather constant flights bringing back full resource loads, and “exploratory” flights, prolonged flight and/or reduction in amount of resources carried. Exploitatory flights followed the exploratory flights which led to discovery of rich sources. Mean duration of exploitatory flights was 7 min during nectar collection, 12 min for pollen and 23 min for resin. Sites of exploited flowers were estimated to be 84–434 m distant from the nest site. Mean duration of flights for nectar collection was 13 min in the morning and 6 min in the late afternoon.

Population dynamics of the phytophagous lady beetle,Epilachna vigintioctopunctata, in an eggplant field in Sumatra

From December, 1981 to December 1982, the population dynamics of the phytophagous lady beetleEpilachna vigintioctopunctata (Fabricius) (Evp) was studied by mark-recapture of adults and the construction of a life table in an eggplant field in Padang, Sumatra, Indonesia. (1) After planting of the host plants, adults of Evp soon colonized and oviposited massively, resulting in a rapid population growth for 1–2 months; thereafter, the population increase slowed due to defoliation. Three–four months thereafter the host plants recovered their leaves, but leaf quality was less suitable for Evp and, as a result, the population remained at a low level during the rest of the study period. Adult population size fluctuated 7–8 fold during the study period. (2) Adult Evp showed a 1∶1 sex ratio. (3) The daily rate of residence, ψ i , derived by theJolly-Seber formula fluctuated from 0.9 to 1.0. (4) The estimated mean length of residence of adults was 16.5 days (male) and 15.2 (female), but this was probably much shorter than the actual longevity, because the adults were so active that they flew away or dropped off the plants when they were approached or were slightly disturbed. (5) Adults showed a rich variation in elytral spot patterns. The average number of non-persistent spots per elytron was 0.52 in males and 1.08 in females; 23.3% of males and 24.6% of females had confluence(s). (6) The life table shows that parasitism by wasps, which killed 9.7% of eggs and 35.0% of pupae, and starvation by overcrowding contributed most to the total mortality from egg to adult emergence (>98%). (7) The high dispersal power of adults, coupled with their prolongedl x −m x schedules was advantageous for exploiting food plants which were variable in space and time. After planting of the host plants, adults of Evp soon colonized and oviposited massively, resulting in a rapid population growth for 1–2 months; thereafter, the population increase slowed due to defoliation. Three–four months thereafter the host plants recovered their leaves, but leaf quality was less suitable for Evp and, as a result, the population remained at a low level during the rest of the study period. Adult population size fluctuated 7–8 fold during the study period. Adult Evp showed a 1∶1 sex ratio. The daily rate of residence, ψ i , derived by theJolly-Seber formula fluctuated from 0.9 to 1.0. The estimated mean length of residence of adults was 16.5 days (male) and 15.2 (female), but this was probably much shorter than the actual longevity, because the adults were so active that they flew away or dropped off the plants when they were approached or were slightly disturbed. Adults showed a rich variation in elytral spot patterns. The average number of non-persistent spots per elytron was 0.52 in males and 1.08 in females; 23.3% of males and 24.6% of females had confluence(s). The life table shows that parasitism by wasps, which killed 9.7% of eggs and 35.0% of pupae, and starvation by overcrowding contributed most to the total mortality from egg to adult emergence (>98%). The high dispersal power of adults, coupled with their prolongedl x −m x schedules was advantageous for exploiting food plants which were variable in space and time.

Adult population parameters and life tables of an epilachnine beetle (Coleoptera: Coccinellidae) feeding on bitter cucumber in Sumatra

The population dynamics of an epilachnine beetle, which is closely related toEpilachna sparsa Dieke (henceforth called “sp. C”) and feeds on bitter cucumberMomordica charantia, was studied by mark-recapture of adults and the construction of life tables. The study was repeated three times, i.e., March–May, July–September and October–December in 1982, in Padang, Sumatra, Indonesia. After the establishment of the host plants, adults of “sp. C” soon colonized, and each study period ended in the death of the plants due to defoliation by the larvae and adults. The estimated mean length of residence of adults ranged from 6–11 days, but this was probably much shorter than the actual longevity, because the adults were so active that they flew away, or dropped off the plants, when they were approached or slightly disturbed. Life tables indicated that egg mortality ranged from 17.8–53.9%, and a parasitic waspTetrastichus sp. B made up 41.1–64.2% of egg mortality. Two wasps,Tetrastichus sp. C andPediobius foveolatus killed 1.2–19.4% (7.6–100%)* of 4th instars and only the latter species attacked the pupae, killing 24.6–59.1% (45.1–72.4%). Parasitism and starvation by overcrowding contributed most to the total mortality from egg to adult emergence, which ranged from 89.4–99.5%. “Sp. C” had a higher diversity and level of parasitism than the Japanese species,E. vigintioctopunctata. The high dispersal power of “sp. C”, coupled with the prolongedl x−mx schedules shown under laboratory conditions, was advantageous for exploiting the food plant which was available throughout the year, but was rather patchily distributed in space.

Geographical variation of elytral spot patterns in the phytophagous ladybird,Epilachna vigintioctopunctata (Coleoptera: Coccinellidae) in the Province of Sumatera Barat, Indonesia

Geographical variation in the elytral spot patterns of a phytophagous ladybird,Epilachna vigintioctopunctata, was studied in the Province of Sumatera Barat, Indonesia. Populations ofE. vigintioctopunctata were divided into four major groups (I–IV) by the incidence of spot pattern variations. Group I, occurring in the coastal plains and inland lowlands, and Group IV, confined to the highlands, were the extremes of the spot pattern variations, the latter had many more non-persistent spots and confluences with larger body size and advanced melanism than the former. These two groups were connected with each other via the intermediate groups. A positive relationship was detected between the elevations of sample sites and the average number of non-persistent spots per elytron. Consequently, present results favor the view that the two previously recognized forms ofE. vigintioctopunctata (formae A and B inKatakura et al., 1988) represent a complicated intraspecific variation rather than two distinct sibling species. Elytral spot pattern variations were not different between the sexes or between the beetles collected from different kinds of host plants.

Generation cycles in Indonesian lady beetle populations may occur as a result of cannibalism.

Generation cycles, population cycles with a period of approximately one generation, have been observed in a variety of field and laboratory studies. Such dynamics are predicted to arise through the effects of resource competition and cannibalism or involve consumer–natural enemy interactions. We first show, using a new highly simplified model, that generation cycles are a very common outcome of strongly age–structured intraspecific interactions involving cannibalism. We then analyse a series of unique long–term time–series of ladybeetle (Coccinellidae) abundances from tropical Indonesia. Some of the time–series display clear generation cycles, and we argue that there is strong evidence that these are caused by intraspecific cannibalism.

Preliminary life table of the spotted tortoise beetle,Aspidomorpha miliaris (Coleoptera: Chrysomelidae) in Sumatra

From December, 1981 to February, 1982, a population study of the spotted tortoise beetle,Aspidomorpha miliaris, feeding on a shrub-like morning glory,Ipomea carnea, was conducted in Padang, Sumatra with the construction of a life table. (1) Dissection of oothecae collected from the field after hatching clarified that the average egg mass size was 43.4 and hatching rate was 25.0%. Causes for egg mortality included: parasitism by a wasp,Tetrastichus sp. A(Europhidae) (49.8% of eggs laid), predation (12.8%), disappearance of egg masses (5.3%) and hatching failure (7.1%). An ant,Dolichoderus bituberculatus, visiting the extrafloral nectaries of the host plnts was responsible for predation and disappearance of the egg masses. The ants again attacked the larvae and pupae. (2) Larvae showed a gregarious habit for almost the entire larval period. Survival rates between two successive instars were low and constant, ranging from 70 to 90%, but only 1.3% of final (5th) instars become pupae (six individuals). Since the growth of host plants was extremely rapid, shortage of food was rare in larval stages. The sudden drop in numbers after 5th instar may be due to predation and/or dispersal of matured larvae from the host plants for pupation. (3) Pupae were attacked by three species of parasitic wasps:Tetrastichus sp. C,Pediobius elasmi (Eulophidae) andCassidocida aspidomorphae (Tetracampidae). Among the six pupae, two were parasitized, one died of disease and two disappeared. Out of 4078 eggs laid, only one emerged to adult. Dissection of oothecae collected from the field after hatching clarified that the average egg mass size was 43.4 and hatching rate was 25.0%. Causes for egg mortality included: parasitism by a wasp,Tetrastichus sp. A(Europhidae) (49.8% of eggs laid), predation (12.8%), disappearance of egg masses (5.3%) and hatching failure (7.1%). An ant,Dolichoderus bituberculatus, visiting the extrafloral nectaries of the host plnts was responsible for predation and disappearance of the egg masses. The ants again attacked the larvae and pupae. Larvae showed a gregarious habit for almost the entire larval period. Survival rates between two successive instars were low and constant, ranging from 70 to 90%, but only 1.3% of final (5th) instars become pupae (six individuals). Since the growth of host plants was extremely rapid, shortage of food was rare in larval stages. The sudden drop in numbers after 5th instar may be due to predation and/or dispersal of matured larvae from the host plants for pupation. Pupae were attacked by three species of parasitic wasps:Tetrastichus sp. C,Pediobius elasmi (Eulophidae) andCassidocida aspidomorphae (Tetracampidae). Among the six pupae, two were parasitized, one died of disease and two disappeared. Out of 4078 eggs laid, only one emerged to adult.

Survivorship and fertility schedules of two Sumatran tortoise beetles,Aspidomorpha miliaris andA. sanctaecrucis (Coleoptera: Chrysomelidae) under laboratory conditions

Two species of tortoise beetles,Aspidomorpha miliaris (AM) andA. sanctaecrucis (AS) feeding on a shrub-like morning glory,Ipomoea carnea, were reared under laboratory conditions to study their survivorship and fertility schedules. AM and AS required 34–39 days and 30–37, respectively, for the development of the immature stages. The mean longevity of the males was 88.4 days in AM and 63.8 in AS, and that of females was 87.9 days in AM and 83.3 in AS. The mean length of the pre-reproductive period (27.2 days in AM and 33.8 in AS) was much longer than that of the post-reproductive period (10.9 days in AM and 14.3 in AS). Females laid eggs at a nearly constant rate throughout their reproductive period. The reproductive valueV x /V 0 of the two species remained high for most of their adult life, as a result of prolonged survivorship and fertility periods. The total number of eggs produced per female was 442.9 (AM) and 80.1 (AS). The intrinsic rate of natural increaser was 0.070 (AM) and 0.044 (AS) per capita per day. The prolonged reproductive schedules, coupled with strong dispersal power, of these species no doubt have an adaptive value for living in highly disturbed tropical environments, where rainfall is ample but unpredictable and food resources are available throughout the year in a wide area, but distributed in widely flung patches.

非害虫の食植性テントウムシ， Epilachna pytho (甲虫目，テントウムシ科)の生存ー繁殖スケジュール

A non-pest Sumatran phytophagous lady beetle, Epilachna pytho was reared under laboratory conditions to study survivorship and fertility schedules. The species is found above 600 m from sea level, mainly depending on wild cucurbits such as Trichosanthes tricuspidata, T. ovigera, Gynostemma pentaphyllum, Mukia javanica and rarely occurring in cultivated fields of Sechium edule. The development of the immature stages of E. pytho required 33.6 days. The mean longevity of males ( 94.4 days) was shorter than that of females (106.3 days). The mean length of the pre-reproductive period (25.4 days) was longer than that of the post-reproductive period (3.6 days). Females laid eggs continuously throughout their reproductive period (77.3 days). The average number of eggs produced per female was 609.9. The intrinsic rate of natural increase r was 0.067 per capita per day. These life history traits ofE. pytho was similar to E. enneasticta, a non-pest species feeding on Solanaceae and living in high lands. E. pytho exhibited 1) longer immature stages, 2) longer mean length of the pre-reproductive period, 3) lesser fertility and 4) smaller r and larger T values than four pest epilachnine species, living at a wide elevational range.