Population Dynamics of Thrips

Gall samples were collected from P. longifolia trees at Hasanuddin University campus and its vicinities in Makassar, South Sulawesi Province, Indonesia. Ten mature galls per tree were collected from 20 trees and placed in separate self-sealing plastic bags and then brought back to the laboratory for further processing. Because the 2 sides of leaf blades are closely folded together to form the gall, they usually needed to be broken apart in order to extract all life stages of the thrips. This was possible to do because the thrips are not very mobile, and neither are they active fliers. The thrips were washed off the galls by rinsing the leaves with 70% ethanol over a fine cloth filter placed on a Berlese funnel. The thrips retained on the cloth filter were rinsed with ethanol, collected onto a Petri dish, and then counted under a dissecting microscope. The numbers of adults and immatures were determined under a stereo microscope (50–100X). Sampling in this way was performed every 2 wk from 15 Aug 2010 to 1 Jul 2011, and samples were taken at the same location to examine the dynamics of the local population (number thrips per gall) through time. On each observation date, the number of young leaves (less than four weeks of fully expanded leaves) per tree was also recorded.

Life Cycle of Crotonothrips polyalthiae

The life history of the thrips was studied using ten 3-month old P. longifolia seedlings, individually planted in a 3-kg polybag. The plants were fertilized and watered as necessary. A couple of newly emerging adult thrips were transferred onto a young leaf (±21 d), and 2 leaves were used on each of the plants. The thrips-infested leaf was then caged using a cage made of a 400-ml plastic cup whose bottom had been removed and replaced with styrofoam. The styrofoam was sliced up to its midpoint to provide a gap for inserting the leaf petiole and the mouth of the cup was covered with a piece of fine cloth, which was held in place with a rubber band (Fig. 1). The main purpose of the caging was to protect thrips eggs from parasitoids and anthocorid predators. When about a half of the leaf blade had formed a gall (24–36 h after thrips infestation), the gall was checked for deposited eggs along the midrib inside the gall by using a magnifying lens. When eggs were present, the adults were removed from the gall, and each gall was observed every 24 h for emerging larvae.

As soon as first instar larvae were observed, the remaining eggs, if any, and all larvae except one were removed from each gall. The single remaining larva was then observed every 24 h until it became an adult. The number of instars and the length of each stadium were recorded. This experiment was conducted twice, during the rainy season from Dec 2010 to Jan 2011 and during the dry season from Jun 2011 to Jul 2011.

Adult Longevity, Egg Number, and Percent of Egg Hatch

A couple of newly emerging adults were transferred onto a young leaf (±21 days). The thrips-infested leaf was then caged using a cage as described above (Fig. 1). Ten 3-month old P. longifolia trees were used in this study and two cages were used on each tree. The cages were opened every 24 h to check the condition of the adults (alive or dead) and the presence of natural enemies and thrips larvae. Adult longevity was determined by counting the number of days from when the thrips were placed onto the leaves until they died. Dead adults were collected in a small vial with 70% ethanol for checking whether they were females or males under a microscope. Every newly formed larva was counted and recorded and then removed from the gall. Two weeks after both adults died, the number of unhatched eggs in each gall was counted under a dissecting microscope (50 to 100X). The percentage of hatching eggs was calculated using the following formula:

Fig. 1.

Thrips cage used in studying the life cycle of Crotonothrips polyalthiae on Polyalthia longifolia.

Thrips Effects on Plant Height, Stem Circumference, and Number of Uninfested Young Leaves

Thirty 3-mo old plants (average height of 62.5 cm and stem circumference of 4.8 cm) were individually planted in 5-liter polybags on 5 Jul 2010, and fertilized and watered as necessary. Fifteen of the plants were randomly selected for insec- ticide application. Thirty grams of carbofuran (Furadan 3GR, PT, Bina Guna Kimia, Jakarta, Indonesia) per plant were applied to the soil at planting, and followed by deltamethrin (Decis 25 EC, PT. Bayer Indonesia, Jakarta) applications as thrips re-infestation occurred. The other 15 plants were left unsprayed to let thrips infestations occur throughout the course of the experiment. The plants were placed between rows of 2 yr old P. longifolia that were heavily infested by C. polyalthiae.

Plant height, stem circumference, and the number of uninfested young leaves (not forming galls) were recorded every 4 mo for 1 yr: 5 Jul 2010; 6 Nov 2011; 4 Mar 2011; and 10 Jul 2011. Plant height was measured from the soil surface to the top of the plant main stem using a metering tape. Stem circumference was measured using a metering tape on the largest part of the stem, close to the soil surface. In addition, the number of uninfested young leaves was also counted on every observation date. Plant height, stem circumference, and number of unifested leaves were compared between insecticide-treated and untreated plants on each date with t-test.

Population Dynamics of Thrips

There was a general trend that the thrips population in mature leaf galls increased following increase in young leaves available, which in turn was associated with the rainfall rate (Fig. 2). The average number of thrips per gall peaked 3 times, on 18 Dec 2010, 9 Mar 2011, and 17 May 2011 with thrips numbers of 96, 85, and 125.6, respectively. The first thrips population peak occurred 4 wk after the peak in the number of young leaves per plant on 21 Nov 2010, which was 3 wk after the rainfall peak that occurred on 24 Oct 2011. Similarly, the second and third population peaks occurred 2 wk after the peak in the number of young leaves per plant that was observed on 12 Feb 2011 and 20 Apr 2011 respectively; about 4 wk after the rainfall had peaked.

Populations of C. polyalthiae fluctuated throughout the year and peaked 3 times during the study, following the peaks in the number of young leaves that occurred 2 to 4 wk earlier. This was similar to the population dynamics of Crotonothrips dantahasta on Memecylon edule (Melastomataceae), and of Teuchothrips longus on Pavetta hispidula (Rubiaceae), both of which were influenced by the presence of young leaves (Ananthakrishnan & Raman 1989). Both hosts produce new leaves for a period of 7 mo and the populations of C. dantahasta and T. longus were also high during the same 7 mo. In addition, the emergence of adult thrips coincided with the availability of new leaf growth.

Fig. 2.

Average number of Crotonothrips polyalthiae (mature and immature stages) found within each gall sample, rainfall rate, and the number of young leaves per Polyalthia longifolia tree (Makassar, 14 Aug 2010–25 Jul 2011). Thrips and young leaves expressed in numbers and rainfall accumulation in mm per 2 weeks.

The number of young leaves produced was affected by the rainfall. The number of young leaves peaked 3 to 4 wk after the peaks in the rainfall. The thrips populations were generally higher during the rainy season than the dry season. This is probably because of the abundance of young leaves available for gall-induction. Besides that the leaves produced during the rainy season probably have better quality suitable for higher fecundity of the thrips. The thrips complete their life cycle inside the gall, therefore, they are not directly exposed to rain. This is contrary to those thrips that do not live in galls, such as Frankliniella occidentalis and species of the genus Thrips. Population growth in these thrips is hampered by rain, because many of the eggs, larvae, and pupae are washed off the plants or killed by raindrops (Boissot et al. 1998; Kirk 1997; Room & Wardhaugh 1977). During the dry season, however, the gall thrips populations remain relatively low, due to the low number of young leaves available. Other than the rainfall, thrips population fluctuation is also affected by biotic factors such as predators and parasitoids (Ananthakrishnan & Raman, 1989; Ramachandran et al. 2001). We observed the presence of anthocorids feeding on the eggs, larvae, and adults of C. polyalthiae within galls. Dry, galled leaves remained on Polyalthia plants long after they were abandoned by the thrips, in contrast to the leaves galled by Gynaikothrips ficarum on Ficus that commonly drop off prematurely (Buss 2003).

Thrips Life Cycle

The life cycle experiment was conducted twice, one each during the dry season and rainy season. However, the data from the two experiments were not significantly different in adult longevity, egg number per female, and percent of egg hatch per female (p > 0.05), and the data were therefore pooled, doubling the number of replicates (n = 3040, depending on the number of missing or dead insects). The development time of C. polyalthiae from egg to adult took 17 to 25 d with an average of 20.8 d (Table 1). Soon after being placed onto a young leaf, the adults fed on the leaf midrib adjacent to the petiole. Twenty four hours after feeding commenced the leaf blade parts closest to the leaf petiole began moving upwards, and 24 h later they touched each other to initiate the formation of a leaf gall. At this point the female began laying eggs, scattered along the midrib. Eggs are elongate, somewhat cylindrical, rounded at both ends, shiny white turning gray as they mature. Egg incubation lasted five to 10 d. The first instar larva is a tiny, pale insect that soon turns dark pink with 2 solid white stripes across the abdomen, and lasts for 2 to 4 d. The antennae, head, nota, the tubular last segment of the abdomen, and legs are dark black. Second instar larvae lasted 2–4 d. They look similar to the first instar, apart from the larger size and slightly paler color with only the anterior part of the notum and head dark black. The prepupa lasted 2–3 d and is similar to the larvae but with the antennae short and pointed laterally; the antennae, head, and last abdominal segment are translucent, and the white stripes on the abdomen faded and less well defined than on the first and second instar larvae. The first pupa is similar to the prepupa but is larger and has wing buds, and the antennae are longer and directed backwards over the head. The abdominal white stripes are almost absent by this stage, and remain only on the lateral part of the terga. The second pupa is similar to the first pupa but with longer antennae, no white stripes, and the last abdominal segment darker.

TABLE 1.

DURATION IN DAYS OF GROWTH STAGES OF CROTONOTHRIPS POLYALTHIAE UNDER FIELD CONDITIONS, 27.3 ± 3.3 °C (RAINY SEASON) AND 29 ± 2.9 °C (DRY SEASON) AND A PHOTOPERIOD OF ABOUT 12:12 H L:D.

TABLE 2.

ADULT LONGEVITY (DAYS), EGG NUMBER, AND FEMALE FECUNDITY (%) OF CROTONOTHRIPS POLYALTHIAE UNDER FIELD CONDITIONS, 27.3 ± 3.3 °C AND A PHOTOPERIOD OF ABOUT 12:12 H L:D.

TABLE 3.

EFFECT OF CROTONOTHRIPS POLYALTHIAE DAMAGE ON PLANT HEIGHT, STEM CIRCUMFERENCE, AND THE NUMBER OF HEALTHY YOUNG LEAVES OF POLYALTHIA LONGIFOLIA.

Fig. 3.

(a) Polyalthia longifolia tree top damaged by Crotonothrips polyalthiae, (b) Normal tree top.

Adult Longevity, Egg Number, and Percent of Egg Hatch

Adult female and male longevities, the number of eggs laid per female, and the number of hatching eggs, are listed in Table 2. These observations were made under field conditions, 27.3 ± 3.3 °C and a photoperiod of about 12:12 h (L:D). Adult females could live from 16 to 29 days, with an average of 24.3 d; adult males could live from 4 to 18 d with an average of 12.2 d. An adult female laid an average of 41.5 eggs, ranging from 11–74 eggs, during her life time. Seventy one to 100% of those eggs hatched successfully, with an average of 94.9%.

Thrips Effects on Plant Height, Stem Circumference, and Uninfested Young Leaf Number

The presence of this leaf-galling thrips significantly affected plant height, stem circumference, and the number of healthy young leaves produced by the experimental plants (Table 3). Pre-treatment observations showed that there were no significant differences in plant height, stem circumference, and the number of uninfested young leaves between treated and untreated plants used in this experiment. However, in post-treatment observations, 4, 8, and 12 mo after the initiation of the study, there were significant differences in plant height, stem circumference, and the number of uninfested leaves between the sprayed and unsprayed plants.

Fig. 4.

A 6-month old Polyalthia longifolia plant severely damaged by Crotonothrips polyalthiae.

The thrips reduced the plant height and stem circumference by 49 and 37%, respectively, 1 yr after the initiation of the experiment. To our knowledge this is the first record of any gall thrips hampering the growth rate of a host plant. Although we did not find any dead plants due to attack by this thrips, the damaged plants lost their aesthetic and economic values (Fig. 3). Young plants, up to six months old, could suffer such severe injury that damages young leaves and inhibits plant growth (Fig. 4), that the local nursery owners currently rely on insecticides to control C. polyalthiae on young plants.