Brazilian peppertree, Schinus terebinthifolia Raddi (Sapindales: Anacardiaceae) is a perennial woody shrub/tree native to Brazil, Argentina, Uruguay and Paraguay (Barkley 1944, 1957). It was originally introduced to Florida in the 1840s as an ornamental (Mack 1991), but has since naturalized becoming an invasive weed occupying more than 283,000 ha in central and south Florida (Cuda et al. 2006; Rogers et al. 2012) and is expanding its range northward (Anon. 2007a,b; Mukherjee et al. 2012). Brazilian peppertree having numerous invasive properties such as fast growth, prolific seed production, tolerance to shade, fire, drought, salinity, and vigorous resprouting (Ewel 1979; Nilsen & Muller 1980; Doren et al. 1991; Ewe & Sternberg 2005), displaces native vegetation and reduces the biodiversity of native plants and animals (Bennett et al. 1990). Furthermore, Brazilian peppertree populations in Florida are the result of 2 separate introductions from different parts of the native range resulting in the establishment of 2 cpDNA haplotypes (A and B) and their hybrids (Williams et al. 2005).

In 2011, the South Florida Water Management District spent almost $1.8 million controlling Brazilian peppertree (Rodgers et al. 2012). Because of its invasive characteristics and resistance to conventional control methods, Brazilian peppertree was targeted for classical biological control in Florida in the late 1970s (Delfosse 1979; Campbell et al. 1980). The lack of native congeners minimizes the risk of damage to nontarget plants from introduced natural enemies, making it a good candidate for biological control (Pemberton 2000). Biological control of Brazilian peppertree could provide a more cost effective and sustainable approach for managing this invasive weed, especially when it is integrated with conventional control methods (Cuda et al. 2006). Several insect biological control agents have been studied for release in Florida (e.g., Medal et al. 1999; Martin et al. 2004; Cuda et al. 2005, 2008, 2009; Manrique et al. 2008; Moeri et al. 2009; Mound et al. 2010; Wheeler et al. 2010; Oleiro et al. 2011; McKay et al. 2012; Rendon et al. 2012). Calophya terebinthifolii Burckhardt & Basset (Hemiptera: Calophyidae), a leaflet galling psyllid of Brazilian peppertree (Vitorino et al. 2011), is the focus of this study.

Because local climatic conditions can play an important role in the distribution and effectiveness of a weed biocontrol agent (McClay & Hughes 1995; Julien 2000; Robertson et al. 2008), matching ecoclimatic conditions for optimal development of C. terebinthifolii in its native range is likely to increase the likelihood of its establishment post-release (Geiger & Gutierrez 2000). Ecological niche modelling is a tool for predicting the fundamental niche of a species (where a species could occur) by establishing a statistical relationship between species known occurrences and relevant predictor variables (for example, climatic variables) (Phillips et al. 2006). Several methods have been developed to model climate based species distributions (reviewed in Franklin 2009). The Maximum Entropy Species Distribution Model (MaxEnt) is a presence only modelling approach that predicts the ecological niche of a species using known occurrence data (latitude/ longitude point locations of the species) and environmental layers (Phillips et al. 2006). Using the input data for C. terebinthifolii, MaxEnt can generate a prediction of geographical locations climatically suitable for the psyllid to establish persistent populations in Florida.

The objectives of this research were to accelerate testing of C. terebinthifolii as a biological control agent for Brazilian peppertree in Florida by 1) investigating the biology of C. terebinthifolii in Brazil (fecundity, number/size of instars, and survivorship); 2) comparing the performance of C. terebinthifolii on the genotypes of Brazilian peppertree that occur in Florida; and 3) using ecological niche modelling to identify potential release areas in Florida that overlap climatically with the insect's native distribution in South America and locating additional sites in Brazil for collecting C. terebinthifolii adapted to Florida Brazilian peppertree genotypes.

MATERIALS AND METHODS

The study was conducted from 11 May–8 Aug 2009 in Gaspar, Santa Catarina, Brazil at the Laboratório de Monitoramento e Proteção Florestal (LAMPF) (S -26.9110° W -48.9362°). LAMPF is affiliated with the Forest Protection and Surveillance Laboratory that is part of the Universidade Regional de Blumenau.

RESULTS AND DISCUSSION

Fecundity, Instar Verification, Size, and Survivorship In total, 18 of 28 females (64.3%) in cages containing mating pairs of adult C. terebinthifolii deposited eggs. Mean number of eggs laid per female was 55.3 ± 8.9 (n = 18) with a range of 16–139 eggs (excluding the 10 females that did not lay eggs). These values are similar to those reported for C. schini; the average number of eggs laid for this species was 50 (no SEM was given) with a range of 25–110 eggs based on 10 ovipositing females (Alvarez-Zagoya & Cibrian-Tovar 1999). The oblong shaped eggs of C. terebinthifolii are 0.212 ± 0.002 mm in length (Table 1) and are a milky, translucent cream color when first laid then turn a black, iridescent color within 24 h. Similar to C. schini (Downer et al. 1988), the females laid their eggs on the new leaf flush along the leaflet margins, midribs, leaflet petiolules, and leaf buds. The number of eggs laid by a female increased with wing length (F = 5.15; df = 9; P = 0.0494; Fig. 1). The low R² value (0.364) reflected the high variability in the data. In total, 11 of the 18 females that laid eggs (61.1%) were recovered for measurement.

To determine the number of instars, a histogram of body length was constructed and examined for distinct peaks. The histogram (Fig. 2) confirmed 5 instars for C. terebinthifolii. Automontage images of the different instars and adults are shown in Fig. 3 in black and white and in color in Suppl. Fig. 3. The measurements of all life stages of C. terebinthifolii (body length, body width, and head capsule width) are provided in Table 1 and were compared with its congener C. schini in Fig. 4. Female body lengths for C. terebinthifolii were significantly larger than males (t = 3.65; df = 38; P = 0.008), which is typical for most psyllids (Hodkinson 2009). By comparison, C. schini is significantly larger than C. terebinthifolii in the following stages: egg (t = 10.85; df = 52; P < 0.0001), 3rd instar (t = 3.62; df = 42; P = 0.0008), 5th instar (t = 7.63; df = 157; P < 0.0001), and adult (t = 3.54; df = 68; P = 0.0007).

A survivorship curve (Fig. 5) was developed using daily observations on survivorship from 4 of the cages (n = 157 eggs) held in the environmental growth chambers. Eclosion of 1st instars commenced on day 9, and there was a sharp decrease in survival during the first wk after hatching when the nymphs are in the vulnerable crawler stage. Survivorship leveled off after the crawlers settled into pit galls and then survival gradually decreased during the 3rd, 4th, and 5th instars until adults began to emerge around day 43. The average duration from egg to adult stage was 43.7 ± 1.2 days, which was similar to the 43 days reported for C. schini (Alvarez-Zagoya & Cibrian-Tovar 1999).

Performance on Brazilian Peppertree Haplotypes Established in Florida

The results from the DNA analysis are shown in Table 2. From this test sample, psyllids only developed to adults on haplotype A and haplotype O plants, with the best performance on haplotype A plants. A G-test comparing all other haplotypes (O, K, D, and M), with haplotype A plants showed the psyllids performed significantly better on A than other haplotypes (G = 7.63; P < 0.01). Recent studies on the Brazilian peppertree invasion in Florida (Geiger et al. 2011; Mukherjee et al. 2012) and ‘fine scale’ adaptation by several of its natural enemies (Cuda et al. 2012) suggest that it may be necessary to introduce multiple biotypespecific control agents into Florida to achieve the desired impact.

TABLE 1.

DIMENSIONS OF ALL LIFE STAGES OF THE LEAFLET GALLING PSYLLID CALOPHYA TEREBINTHIFOLII.

Fig. 1.

The relationship between wing length and laboratory fecundity of female Calophya terebinthifolii (n = 11).

Fig. 2.

Frequency distribution of nymphal length measurements of Calophya terebinthifolii. Peaks within groupings indicate 5 instars.

Fig. 3.

Automontage images of Calophya terebinthifolii. A) 1st instar, B) 2nd instar, C) 3rd instar, D) 4th instar, E) 5th instar, F) 5th instar prior to darkening, G) adult male, and H) adult female. Photo credit L. R. Christ.

Ecological Niche Modelling

Tropical psyllids are especially vulnerable to cold temperatures and drought (Hodkinson 2009). Waring & Cobb (1992) reported that in 73% of published studies, gall-forming species reacted negatively to drought. Young nymphs of several species of gall-inducing psyllids often suffer high mortality before gall formation due to low humidity (Ferreira et al. 1990; personal observation). Because C. terebinthifolii reproduces continuously in Brazil, precipitation during the driest month (bio 14) is a critical factor to include in the model as reflected by this variable's overall contribution to the model prediction. It is noteworthy that Weins et al. (2006) found that temperature seasonality (bio4) was an important climate variable for predicting the distribution of tropical tree frogs in temperate North America.

The minimum training threshold dependent binomial test results for Florida (validated using Florida type A Brazilian peppertree points) were highly significant for all the data partitions (Table 3; z = 3.1623; P = 0.0008). The average FPA (Fractional Predictive Area) was 0.448. The extrinsic omission rates were zero for all partitions. Likewise, the results for the South American tests were similar (Table 4; z = 2.5-3.2; P < 0.0001), with all data partitions being highly significant. The average FPA for the South American tests was 0.173. The extrinsic omission rates were slightly higher with an average of 4%. Overall, the models generated by MaxEnt were accurate with predictions being significantly better than random.

Fig. 4.

Comparison of the body lengths of the life stages of Calophya terebinthifolii and C. schini. Measurements for C. schini obtained from Alvarez-Zagoya & Cibrian-Tovar (1999).

The predicted distribution of C. terebinthifolii in Florida is shown in Fig. 6 in black and white and in color in Suppl. Fig. 6. Establishment of C. terebinthifolii is expected to occur in a few coastal regions along the panhandle, west coast, and along the east coast in Florida. Areas of overlap where Brazilian peppertree haplotype A plants were known to occur and predicted to be suitable for establishment of C. terebinthifolii included the following Florida counties: Volusia, coastal Pasco and Hernando, and a small section of southwestern Polk. These counties should be targeted for planned releases of C. terebinthifolii if it is approved for release as a biological control agent of Brazilian peppertree. It is noteworthy that the psyllid is predicted to establish in Franklin Co. in the panhandle where Brazilian peppertree was recently discovered (EDDMaps 2012).

Fig. 5.

Survivorship curve of Calophya terebinthifolii in laboratory environmental growth chambers in Brazil at 21.9 ± 2.6 °C, RH 67.9 ± 7.6% and 16:8 h L:D.

Using the known locations of C. terebinthifolii, another map was generated in MaxEnt to predict other areas in South America likely to support psyllid populations. The map for South America (available upon request) predicted C. terebinthifolii to occur in the following countries: 1) southern Chile, 2) central Bolivia, 3) southeastern Paraguay, 4) Uruguay, and 5) southern Brazil in the states of Rio Grande do Sul, Santa Catarina, Paraná, and coastal São Paulo. These areas (except for Chile and Bolivia where Brazilian peppertree has not been reported) would be good locations to survey for C. terebinthifolii because the full native range for the psyllid is still unknown.

CONCLUSIONS

Results of this research provide baseline biological data for evaluating C. terebinthifolii as a candidate for biological control of Brazilian peppertree. The 5 instars observed for C. terebinthifolii in this study differed from the number reported by Downer et al. (1988) for C. schini in California. They observed this congener had only 4 distinct peaks after measuring 1,000 field-collected nymphs in all stages of development over a 10 wk period. More recently, Alvarez-Zagoya & Cibrian-Tovar (1999), who studied C. schini in Mexico, found evidence of 5 instars when nymphs were studied in a laboratory setting. It is conceivable that Downer et al. (1988) focused their field sampling efforts on nymphs that had already initiated gall formation, and simply overlooked the first instar crawler stage because of its small size and mobility (Table 1).

Three different studies on both species of Calophya; C. schini in Chapingo, Mexico (Alvarez-Zagoya & Cibrian-Tovar 1999), C. schini in Ventura County, California (Downer et al. 1988), and C. terebinthifolii in Santa Catarina, Brazil (Vitorino et al. 2011; personal observation) found the insects to have continuous generations. The polyvoltinism exhibited by Calophya psyllids makes them good candidates for biological control because they can provide year round control. Although not gall-forming, other psyllids such as Boreioglycaspis melaleucae Moore and Heteropsylla spinulosa Muddiman, Hodkinson and Hollis have been successfully used as biological control agents (Kuniata & Korowi 2004; Rayamajhi et al. 2007; Center 2007; Rayamajhi et al. 2008). Additionally, C. terebinthifolii may reach higher densities in Florida than Brazil because of escape from specialist parasitoids (Christ 2010).

Brazilian peppertree haplotypes suitable for psyllid development were unknown prior to the laboratory rearing experiments in Brazil. Few replicates were used for some of the haplotypes (K, D, and M) and no type B plants were included, making it difficult to draw clear conclusions regarding the possible suitability of Florida genotype plants. However, it is noteworthy that type O is most closely related to haplotype B (Mukherjee et al. 2012). If C. terebinthifolii is indeed locally adapted to haplotype A, it may have difficulty developing on the novel genotypes in Florida created by the hybridization of haplotypes A and B. In Brazil, these 2 haplotypes do not occur sympatrically (Williams et al. 2005; Mukherjee et al. 2012)). However, if C. terebinthifolii from coastal Santa Catarina, Brazil can develop on both haplotypes A and O, it may accept haplotype B as well. Because our preliminary data suggest that C. terebinthifolii is locally adapted to specific Brazilian peppertree genotypes, evidence of ‘fine scale’ adaptation to its host plant is consistent with other small natural enemies of Brazilian peppertree having limited dispersal capability (see Cuda et al. 2012).

TABLE 2.

REARING OF CALOPHYA TEREBINTHIFOLII ON 5 DIFFERENT BRAZILIAN PEPPERTREE HAPLOTYPES AT LAMPF, SANTA CATARINA, BRAZIL, MAY–AUG 2009. REARING SUCCESS EXPRESSED AS A PERCENTAGE OF THE TOTAL NUMBER OF PLANTS WITHIN EACH HAPLOTYPE PRODUCING NORMAL ADULTS.

TABLE 3.

MINIMUM TRAINING THRESHOLD DEPENDENT BINOMIAL TEST RESULTS OF OMISSION FOR PREDICTING THE DISTRIBUTION OF THE LEAFLET GALLING PSYLLID CALOPHYA TEREBINTHIFOLII IN FLORIDA WITH MAXENT.

TABLE 4.

MINIMUM TRAINING THRESHOLD DEPENDENT BINOMIAL TEST RESULTS OF OMISSION FOR PREDICTING THE DISTRIBUTION OF THE LEAFLET GALLING PSYLLID CALOPHYA TEREBINTHIFOLII IN SOUTH AMERICA WITH MAXENT.

Fig. 6.

Map of predicted climatic suitability for the leaflet galling psyllid Calophya terebinthifolii, its host plant Brazilian peppertree haplotype A, and their predicted overlap in Florida. Open squares (n = 50) indicate locations of Brazilian peppertrees identified as haplotype A through DNA analysis (D. A. Williams, unpubl. data).

The 2 predicted distribution maps produced in this study were constrained by the paucity of data points for C. terebinthifolii in its native range. Consequently, the maps generated should be considered preliminary. One of the assumptions made when generating niche models is that the entire native range of the species is known and is accounted for in the point locations (Phillips et al. 2006). However, the complete native range of C. terebinthifolii is still unknown. As more information about the native range distribution of the psyllid becomes available, a more accurate model can be developed. Nevertheless, these preliminary models are reasonable for selecting locations for future surveys in South America and potential habitats in Florida where the psyllid could be released as a biocontrol agent for Brazilian peppertree.

Finally, the most compelling argument for using C. terebinthifolii as a biological control agent for Brazilian peppertree in Florida is based on the discovery of the adventive C. schini in California in the mid 1980s (Downer et al. 1988). Calophya schini, which was first reported from Los Angeles County, California in Jul 1984, dispersed rapidly from San Diego County to the San Francisco Bay (a distance of about 800 km) in less than 4 yr (Downer et al. 1988). This congener of C. terebinthifolii caused extensive damage on Peruvian peppertree, Schinus molle L. In southern California where both Schinus species co-occur, Downer et al. (1988) reported that C. schini attacked only Peruvian peppertree but not its close relative Brazilian peppertree. Assuming C. terebinthifolii would respond similarly if it were introduced into Florida without its natural enemies, this leaflet galling psyllid may be a promising candidate for the biological control of Brazilian peppertree.