U BioControl 47: 327-347, 2002.

qv

O 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Natural mortality factors acting on citrus leafminer, Phyllocnistis citrella, in lime orchards in South Florida D.M. AMALIN~*, J.E. PENA~,R.E. DUNCAN', H.W. BROWNING~and R. MCSORLEY~ 0

I

1

4

'

Unrversity of Florida, Tropical Research and Education Centel; 18905 SW 280th Street, Homestead, FL 33031, USA 2~niversity of Florida, Citrus Research and Education Centeq 700 Experiment Road Station, Luke Alfred, FL, USA University of Florida, Department of Entomology and Nematology, Bldg 70, Gainesville, FL 32603, USA *author for correspondence; e-mail: [email protected] Received 19 January 2001; accepted in revised form 29 May 2001

Abstract. Seasonal mortality of the citrus leafminer, Phyllocnistis citrella Stainton, was studied from 1994 through 1998 in 'Tahiti' lime, Citrus aurantifolia (Christm.) Swingle, in Homestead, Florida. Survival of each developmental host stage and the proportion attacked by indigenous and introduced natural enemies were determined. Before the recovery of the introduced parasitoid, Ageniuspis citricola Logvinovskaya, in 1995, the third-instar host had the highest average proportion of parasitized individuals (0.14) followed by the prepupa (0.11) while the first instar had the lowest proportion parasitized (0.02). After the first recovery and establishment of A. citricola, the proportion of pupae parasitized increased to 0.56 followed by the prepupa (0.14) and the third instar (0.11). Before the introduction of A. citricola, the highest proportion of hosts lulled by predation was observed in second instar (0.17) and third instar (0.15). After the establishment of the introduced species, the proportion of dead individuals due to predation was greater for second instar (0.31) and third instar (0.21) larvae. Mortality caused by indigenous natural enemies was significantly correlated with increases of P citrella density. Parasitism of P citrella by the exotic parasitoid, A. citricola, correlated less well to host density over the season (r2 = 0.12) than did mortality caused by indigenous natural enemies (r2 = 0.76). Moreover. a higher percent mortality in population of P citrella was obtained from predation by the indigenous natural enemies than the introduced parasitoid as shown in the mortality estimates from 1995 to 1998.

Key words: Ageniaspis citricola, biological control, citrus leafminer, mortality estimates, mortality factors, parasitoids, Phyllocnistis citrella, Pnigalio minio, predators

Introduction The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae) invaded Florida in 1993, spreading rapidly throughout the commer-

cia1 citrus growing areas of the state (Knapp et al., 1995). Female citrus leafminers deposit eggs individually on the adaxial or abaxial sides of young leaves. The eggs hatch in 2 to 10 d depending on the temperature conditions (Knapp et al., 1995). Upon hatching, the three larval instars feed in leaf parenchyma and pupation occurs in the leaf. Phyllocnistis citrella is multivoltine in southern Florida, and total generation time can fluctuate between 13 to 52 d depending on temperature (Pefia et al., 1996). Native parasitic wasps and various predacious arthropods attacking t? citrella have been found throughout Florida since its arrival (Browning and Pefia, 1995). In a previous study, we determined that citrus leafminers are hosts for 8 species of resident or indigenous parasitoids in Florida (Pefia et al., 1996). Most of these parasitoids were Eulophidae (87.4%) (Pefia et al., 1996). Among these eulophids, Pnigalio minio (Walker) comprised 80% of the native parasitoids of P. citrella in south Florida. In 1994, the exotic encyrtid, Ageniaspis citricola Logvinoskaya, a highly specific endoparasitoid, was .introduced and released in Florida against t? citrella and subsequently established in south Florida in 1995 (Hoy et al., 1997). Several predators, among them lacewing larvae, ants, thrips, spiders, and the flower bug, also have been found feeding on t? citrella. Nevertheless, little is known about the impact of predators on citrus leafminer (Amalin et al., 1996). Measurement of the effect of natural enemies (i.e. parasitoids and predators) on host populations is complex, especially when the host is multivoltine. According to Morris (1959), frequent sampling of host populations, and collection of data on natural enemies, knowledge of climate effects on natural enemies and hosts are needed to accurately assess the effects of mortality factors. Van Driesche et al. (1991) stated that a better way to estimate losses from parasitism is to compare the number of hosts that enter the stage susceptible to parasitism to the number that subsequently become parasitized. Most data on the degree of parasitism of the citrus leafminer are expressed as the parameter 'percent parasitism', and are obtained from faunistic surveys (Bautista et al., 1996; Cave, 1996; Pefia et al., 1996). Such data are often based on the number of parasitoids observed once or twice a year in a single host stage (Hoy et al., 1997). The number and timing of samples taken for these types of studies are inadequate to properly assess the role of a parasitoid or a group of parasitoids as mortality factors (Van Driesche, 1983). Rather, total parasitism impact should be calculated as all host deaths resulting from the presence of the parasitoids, not just hosts utilized for parasitoid reproduction (Van Driesche, 1983). For instance, feeding on the host by parasitoids should also be considered as a source of mortality and part of the impact of parasitoids.

-

NATURAL MORTALITY FACTORS ACTING O N CITRUS LEAFMINER

329

Biological control theory and practice have traditionally emphasized the role of specialist natural enemies whose dynamics are tightly linked to those of a target pest (Doutt, 1964; Beddington et al., 1978; Hassel, 1978). However, increased attention has recently been directed to the role of generalist predators as regulators of insect hervibore populations in agricultural ecosystems. There is increasing evidence indicating that generalist predators can reduce pest populations in agroecosystems (Riechert and Lockley, 1984; Chiverton, 1986; Nyffeler and Benz, 1987; Young and Edwards, 1990; Wise, 1993; Rosenheim et al., 1993). Therefore, the investigation of the natural enemy complex associated with P. citrella and its effect on P. citrella populations should include all possible natural mortality factors (i.e. both specialists and generalists). The present study was conducted with the following objectives: (1) to measure the impact of native and exotic natural enemies on f! citrella in Florida, and (2) to assess their effect on temporal population changes of the P. citrella from 1994 through 1998.

Materials and methods Seasonal nzortality survey (1994-1996) The seasonal mortality of f! citrella was studied from 1994 through 1996 at a 0.2-ha insecticide-free lime orchard maintained at the University of Florida Tropical Research and Education Center in Homestead, Florida. Five sets of newly flushed leaves (n = 12.20 f 3.35 leaves per unit) were collected weekly from 5 randomly selected trees. Each set of leaves was placed in a plastic bag, transported to a laboratory where all leaves within a flush were examined starting with the first apical leaf to the terminal leaf at base of the flush (Knapp et al., 1996). Each leaf was placed under a dissecting microscope to locate all P. citrella stages in the adaxial and abaxial leaf surfaces, and identify sources of mortality. Data on parasitism was classified according to the presence or absence of host and parasitoid. If no parasitoid stages were observed, the P. citrella was classified as non-parasitized. If the P. citrella stage was necrotic or flaccid with symptoms of parasitoid or predator feeding, the P. cirrella was recorded as dead. Seasonal m o r t a l i ~survey (1997-1998) Another seasonal mortality survey was carried on for a year starting August 1997 to July 1998 in six lime orchards in Homestead, Florida, to single out the different natural mortality factors observed from the 1994 to 1996 survey. In all the orchards, the trees were 4.5 m tall and had canopies of

Table I. Identification of predation marks from different predatory arthropods including host-feeding of parasitoids Species

Method of predation

Symptom of predation

lacewing

-puncture the mine

-

ant

- slit open the mine

still visible -empty mine

hunting spiders

-

dead prey larval form

Prey stages attacked larval stage larval stage

to pull out the prey

puncture the mine

- slit

ectoparasitoids

open the mine -puncture the mine

-

necrotic marks from incomplete feeding

larval and pupal stage

- empty

mine with the prey's crumpled skin from complete feeding

larval and pupal stage

- empty

larval and pupal stage larval stage

mine - necrotic marks from incomplete feeding

Note: Predation symptom shared by two or more predators is classified as inactive incomplete mine with unknown mortality.

approximately 5.0 m in diameter. The density of P. citrella at each orchard was determined by collecting 10 terminals (30-cm-long with 15.0 f 1.3 leaves per terminal) at random from the middle regions of 5 trees every 2 weeks. Each terminal was placed in a plastic bag, transported to a laboratory, and examined under a microscope to determine the cause of mortality of P. citrella. Presence of parasitism was recorded and the parasitoids were identified and counted separately. Predator attacks were counted and identified based on the descriptions listed in Table 1. Predation showing empty larval mines and necrotic marks on larvae is included in the inactive incomplete mines with unknown mortality since these predation marks are shared by ant and hunting spiders and by ectoparasitoids and hunting spiders, respectively (Table 1). Only recently dead mines on newly flush leaves were accounted for the mortality data for all the sampling dates.

Mortality estimates When these estimates were available, mortality (MI) was summarized for 1994 as follows:

where Hp is the number of hosts with parasitoid eggs, larva or pupa, Hd is the number of hosts killed by a parasitoid or by a predator including the unknown

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

33 1

mortality divided by H,, the total number of mines per leaf. A second parameter was added when the mortality (M2) was evaluated from January 1995 through December 1996 and was summarized as:

The second equation takes into account inactive incomplete mines (I,) as a mortality factor caused by the action of ectoparasitoids and predators. Other parameters were included for the 1997 to 1998 mortality estimates and was summarized as follows:

The third equation takes into account the action of introduced parasitoid (Hpi), native parasitoid (H,,), lacewing predation (HL), and spider predation (H,) separately. The inactive incomplete mines (I,) include the action of other predators and the host feeding from parasitoids. Statistics

&I

Regression analysis using the general linear models procedure of SAS (SAS Institute, 1987) was used to determine the relationship between P. citrella stages and number of parasitized individuals and number of dead individuals, respectively. The number of individuals parasitized or dead in the same stage (dependent variable) was regressed on the number of individuals in each stage (independent variable) on each sample date. For this analysis, data for each stage and sampling date were transformed using log transformation [i.e. loglo (X + I)]. Regression analysis for the total number of P. citrella and total parasitized individuals or total number of dead individuals was done using untransformed numbers. Mean weekly percent and the standard error were computed for parasitized individuals and dead individuals before and after the establishment of the introduced parasitoid, A. citricola.

C

Results Seasonal citrus leafminer population trends

The seasonal population pattern from 1994 to 1996 and 1997 to 1998 is shown in Figure 1 and Figure 7, respectively. In all years, the general seasonal pattern of P: citrella populations shows an abrupt decline from late fall (November to mid December) and through the winter (late December to March). Population

333

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

Table 2. Parameters of linear regression equations between log (total parasitized f! citrella + 1) and log (total population of f! citrella + 1) from 1994 through 1996, Homestead, Florida

Year

f! citrella stages

I instar larvae

a

b

df

F

P

r2

-

I1 instar larvae

-0.07 -

I11 instar larvae

prepupae

Pupae

-0.39 -0.18 0.31 -0.17 0.01 -0.04 -0.59 0.04 0.93

* regression parameters followed by an asterisk are significantly different from zero, according to t-test. - indicates correlation not significant a t p = 0.05.

1-

peaks were observed in the middle of spring (April to May), through summer (June to September), and early fall (mid September to October). The numbers of eggs deposited were lowest between November and March during the years of this study (Figure 2). The lower egg deposition during winter is related to reduction of leaf flushes and low temperatures (Pefia, 1998). The pattern of seasonal abundance of instars 1-11 followed the pattern of eggs during 1994 to 1996. The population trends of prepupae and pupae were also similar to the other stages, but the population levels were very low for both stages. Dynamics of parasitism of CLM in relation to instar and life stage

Table 2 shows the linear regression analyses between the total parasitized P. citrella and the total population of P. citrella. The results of the regression analyses indicate a more consistent effect of parasitism in later stages of P. citrella than in earlier stages. The coefficient of determination (r2) values indicated that most of the total variation in counts of parasitized instar I and I1 was not explained by the regression model during the three years of evaluation

334

D.M. AMALIN, J.E. PENA, R.E. DUNCAN. H.W. BROWNING AND R. MCSORLEY

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

20

0 M A M J

335

n4-i-?%??+=4

J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

Figure 3. Dynamics of parasitism of I! citrella observed in different stages and larval instars from 1994 through 1996. The vertical line indicates when A. citricola began to be recorded.

-

(Table 2). The stronger relationships for parasitized instar 111, prepupa and pupa may indicate that parasitism may increase or is reduced from one year to the next during the three years of the study. Thus, the population trends of parasitized first instar and second instar larvae (Figure 3) were similar to the trends of the total first and second instar populations between 1994 and November 1995 (Figure 2). The numbers of parasitized individuals of these two instars were reduced after the exotic parasitoid A. citricola began to be recovered in November 1995. More third instar larvae were parasitized in 1994 than in 1995, and this number increased again during 1996. Parasitism of the prepupal stages showed strikingly different peaks from 1994 through 1996. Pre-pupal parasitism increased dramatically after recovery of A. citricola (Figure 3). Dynamics of natural enemy feeding in relation to instar and life stage The regression analyses between the total dead R citrella and the total population of R citrella showed a more consistent relationship in earlier stages than later stages (Table 3). This result is the reverse of the parasitism effect (Table 2). First-instar larvae were killed either through host feeding by the

parasitoids or predation attacks by the generalist predators during the spring and summer of 1994 and also during mid spring through summer of 1995. Mortality of this instar was seldom observed after recovery of A. citricola (Figure 4). Destruction of second instar larvae was commonly observed in 1994 and 1995 and reduced during 1996. Mortality of third instar larvae increased dramatically after recovery of A. citricola at the end of 1995 and through 1996. Sporadic prepupal and pupal mortality increases were observed in 1994 and 1995. Prepupal and pupal host mortalities were fairly minor until the winter of 1996. Mortality for each instar and stage is consistently positively associated with density, but becomes very weakly positive for prepupae and pupae (Table 3). The coefficient of determination (r2) values indicated that a significant proportion of the total variation in counts of dead individuals for instar I and I1 during the study was better explained by the regression model than for instar 111, prepupae and pupae (Table 3). Therefore, coefficient of determination values for dead individuals of instar I11 did not indicate that a significant proportion of dead individuals in these stages occurred during the last two years of the study (Table 3). Correlations for dead prepupae were similar to those for dead pupae during 1994. Dead instar I and dead 111 instar were similar in slope during 1994, but slopes for dead instar I1 remained similar between years 1994 and 1995 so, mortality had about the same variation for this instar during the first two years of the study. The stronger relationship between dead individuals and total population counts early in the study compared with the last year is probably because of changes in level of host feeding. The mean weekly percent parasitized and dead I? citrellu before and after the establishment of A. citricola is shown in Figure 5. Before the first collection of the introduced parasitoid, A. citricola, the highest average percentage of individuals parasitized by ectoparasitoids every week was observed to be in I11 instar (13.47) followed by prepupa (11.31). The percentage of I? citrella prepupae and pupae attacked by indigenous parasitoids (8.55 and 7.01, respectively) was lower than that of the other immature stages. However, after the first recovery of A. citricola, the percentage of pupae that were parasitized rose to 56.27 followed by 14.01 for prepupae and 10.8 1 for 111 instar. Also percent parasitism of I1 and I instars was reduced. Before the introduction of A. citricola, the highest percentages of dead individuals were observed in I1 instar (17.75) and I11 instar larvae (15.24). After the introduction of the introduced parasitoid, the percentage killed increased for I11 instar larvae (21.36), and for I1 instar larvae (31.5), but were reduced to 1.93 for I instar larvae.

a

337

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

Table 3. Regression analysis between log (total dead P. citrella + I ) and log (total population of P. citrella + 1) from 1994 through 1996, Homestead, Florida

Year

P. citrella stages

a

b

df

F

P

r2

-

a

1994 1995 1996 1994

I instar larvae

I1 instar larvae

1995 L

1996 1994 1995 1996 1994 1995 1996 1994 1995 1996

I11 instar larvae

prepupae

pupae

* regression parameters followed by an asterisk are significantly different from zero, according to t-test. - indicates correlation not significant a t p = 0.05. Roles in host regulation by generalist predators and host-spec@c parasitoids In this study there was a lack of correlation between total parasitization of l? citrella by indigenous parasitoids with increases of host density before the establishment of A. citricola (Table 4, Figure 6A). On the other hand, the degree of parasitism by indigenous parasitoids and the introduced parasitoid, A. citricola, revealed a significant relationship between the number of parasitized hosts and density of l? citrella (Table 4, Figure 6B). However, if the effect of feeding and the presence of incomplete inactive mines are considered to be mortality factors, the degree of dependence between l? citrella density and parasitoid-affected hosts is highly significant both before and after the introduction of A. citricola (Table 4, Figure 6A and 6B). The degree of parasitization by A. citricola does not show a high correlation with l? citrella density as expressed in the linear regression equation (Table 4). However, an improved correlation exists between the total l? citrella density and the degree of parasitization by the indigenous parasitoids after establishment of A. citricola. Moreover, there is a high degree of correlation between killed

Figure 4. Dynamics of feeding by natural enemies observed on different immature stages and larval instars from 1994 through 1996. The vertical line indicates when A. citricola began to be recorded.

hosts by the indigenous natural enemies and host density. The high degree of correlation is not surprising, particularly since parasitism causes a rather small proportion of the total mortality off! citrella. In 1997 to 1998, we were able to further separate the different mortality factors with an emphasis on the most important ones: the introduced endoparasitoid, A. citricola, native ectoparasitoids (i.e. Pnigalio minio), various species of hunting spiders, Chiracanthium inclusum, Hibana velox, and Trachelas volutus, green lacewing, Chrysoperla rujilabris, and the unknown mortality (Figure 7). The population of the f! citrella attacked by the hunting spiders showing the crumpled skin of the prey after complete feeding (Figure 7E) followed the population trend off! citrella (Figure 7A). A similar trend was observed from the action of lacewings (Figure 7D). The action of the hunting spiders showed a relatively high relationship with the population of l? citrella (Table 4). An even higher relationship was obtained between the unknown mortality factors and population of the f! citrella (Table 4). The unknown mortality consisted of predation from host-feeding parasitoids and attacks from other predators (i.e. ants, mirid bugs), predation from other spiders showing empty mines (i.e. salticids) and necrotic marks (i.e. sac

total parasitization by indigenous parasitoids and introduced parasitoid total mortality from parasitism by indigenous parasitoids and introduced parasitoid and predation by predators and host-feeding by indigenous parasitoids total parasitization by introduced parasitoid

total mortality from predation by predators and host-feeding by indigenous parasitoids total mortality from predation by hunting spiders total mortality from predation by lacewing total mortality from unknown factors

l? citrella density

l? citrella density

Year

Equation parameters a b

- indicates correlation not significant at p = 0.05.

* regression parameters followed by an asterisk are significantly different from zero, according to t-test.

l? citrella density l? citrellu density l? citrella density

l? citrella density

l? citrella density

total parasitization by indigenous parasitoids

l? citrella density

Variables Y

P

Table 4. Parameters of linear regression between different mortality factors ( Y variable) and l? citrella density ( X variable)

r2

25

7

Indigenous Natural Enemies ( I N E ~

a,

5

INE + A. citricola

60

C

0

LT

m

$

40

T

I

11

111

Prepupae

-1 Pupae

Larval instar

Figure 5. Percentage of F? citrella individuals parasitized or killed by natural enemies before (upper figure) and after (lower figure) introduction of the exotic parasitoid, A. citricola.

spiders) similar to attacks by host-feeding parasitoids. On the other hand, the population of A. citricola and P. minio seems to show a lower relationship with the population of P. citrella probably because of their low level of occurrence during these years. There was a lack of correlation between total parasitization of P. citrella by A. citricola with increases of host density (Table 4, Figure 8). On the other hand, a positive significant correlation was observed between parasitization by P minio and the population of P. citrella (Table 4, Figure 8). A significant correlation was also obtained between predation from the spiders and green lacewing with the population of P. citrella (Table 4, Figure 8). Mortality estimates The mortality estimates from 1994 to 1998 caused by different natural enemies is shown in Figure 9. In 1994, 37% of the total population of P. citrella was controlled by the natural enemies, 34% of which was caused by predation from the generalist predators and host feeding by the ectoparasitoids. Only 3% was caused by parasitism. In 1995 and 1996, the mortality

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

34 1

Before A. citricola Established o ---

Total P. citrella mortality (parasitized, dead & inactive incomplete) Total P. citrella mortality regression Parasitized P. citrella Parasitism regression

Total P. citrella Population per Five Shoots -

Parasitism from indigenous parasitoids Indigenous parasitoid regression 0 Ageniaspis citricola parasitism A. citricola parasitism regression A Dead + inactive incomplete mines --- Dead + inactive incomplete mines regression o

--

0

50

100

150

200

After A. citricola Established

250

300

350

400

Total P. citrella Population per Five Shoots

Figure 6. ( A ) Correlation between total F! citrella density (x) and degree of parasitization and mortality (y) before and after the recovery of A. citricola. (B) Correlation between total F! citrella density (x) and the degree of parasitism and mortality after the recovery of A. citricola.

D.M. AMALIN, J.E. PENA, R.E. DUNCAN, H.W. BROWNING AND R. MCSORLEY

A

Citrus Leafminer

B

JL c

A. citricola

-P. minio

F

incomplete mines

t

Hunting spiders

D

Green lacewing

-

4 S 0 N D J

F M A

M J J

1997

Months

1998

Months

Figure 7. Seasonal trends of (A) P. citrella, ( B ) A. citricola, ( C ) P. minio, ( D ) lacewing,

( E ) hunting spiders, and (F) incomplete mines.

estimate increased to 52%. The introduced parasitoid, A. citricola, contributed 4% to the mortality estimates. The indigenous parasitoids gave a 3% mortality estimate. A high proportion of the mortality estimate was caused by predation from the different predators and host feeding by the ectoparasitoids, which was 45% of the total P: citrella population. In 1997 to 1998 total mortality estimate increased to 57%. During these years, the proportion of the mortality estimates contributed by each natural enemy are as follows: 5% by A. citricola, 3% by P: minio, 10% by C. rujilabris, 15% by the various species of sac spiders, and 24% from inactive incomplete mines (i.e. unknown mortality factors) (Figure 9).

343

NATURAL MORTALlTY FACTORS ACTING ON CITRUS LEAFMINER

V parasitism from A. cilricola - A. citricola parasitism regression parasitism from P. minio - P. minio parasitism regression W spider predation spider predation regression lacewing predation lacewing predation regression

0

-

0

2

4

6 8 10 12 Total P. citrella population

14

16

18

Figure 8. Correlation between total P. citrella density ( X ) and degree of parasitization by A. citricola, P. minio,and predation from lacewing and hunting spiders (Y).

Discussion The changes in parasitism and mortality of different l? citrella larval instars, prepupa, and pupa could be the result of interspecific competiton. For instance, Peiia et al. (1996) observed that the parasitoid, Pnigalio minio, was the most abundant species (68-88%) during all seasons from 1993 through 1995, whereas levels of abundance of the parasitoids Cirrospilus sp., Closterocerus sp., Zagrammosoma multilineatum (Ashmead), Elasmus tischeriae Howard and Horismenus sp., varied erratically (0-13%). It is possible that some indigenous species could not coexist with the highly specific A. citricola (Hoy and Nguyen, 1997) and their populations were reduced to even lower levels. For instance, l? minio prefers to parasitize the late l? citrella larval instars and prepupa, and to feed on second to early third instars (Duncan and Peiia, 2000). The introduction and establishment of the introduced parasitoid, which oviposits in the eggs of l? citrella and emerges from the prepupae, may have affected the habits and abundance of l? minio. Most studies on host-parasitoid population dynamics have focused on discrete generation and synchronized interactions, and have been conducted

344

D.M. AMALIN, J.E. PENA, R.E. DUNCAN, H.W. BROWNING AND R. MCSORLEY

i incomplete mines

indigenous parasitoid

E

CI V

0

d!

'

incomplete mines

.-

%

fE

-

$ m

z

p

introduced parasitoid

indigenous parasitoid

introduced parasitoid indigenous pararitoid

0.0

0.1

0.2

0.3

0.4

0.5

P. citrella mortality estimates Figure 9. I? citrella mortality estimates caused by different natural mortality factors from 1994 to 1998.

NATURAL MORTALITY FACTORS ACTING ON CITRUS LEAFMINER

345

with the assumption that effective parasitoids are specialists, and have neglected the large category of broadly polyphagous parasitoids (Hassell, 1986). The latter appear to have a very different dynamic relationship with their hosts than host specific parasitoids as has been shown in this study. The generalist parasitoids (i.e. P. minio) not only parasitized the larvae and prepupae of P. citrella, but also exhibited host feeding, which causes death of various instars (Duncan and Peiia, 2000). Since A. citricola is a koinobiont endoparasitoid and does not kill its host until the last larval instar or prepupa, we infer that host-lulling by ectoparasitoids was a more important source of mortality than the actual parasitization of the host and a significant factor responsible for reducing populations of P. citrella in south Florida. Moreover, when the effects of parasitism by the indigenous parasitoids and by the introduced parasitoid are analyzed separately in relation to the increments in host density, the degree of parasitization of P. citrella by A. citricola was not host density dependent nor was the degree of parasitism by the indigenous parasitoids. However, the proportion of the hosts killed by indigenous parasitoids and predators is much more significant than parasitism alone. Similar results have been observed in Nicaragua by Llana (1996), where indigenous generalist parasitoids have caused severe mortality of P. citrella without the introduction of exotic parasitoids. Likewise in Texas, relatively high level of parasitism from indigenous parasitoids and predation attacks from native predators have been documented, indicating a need to conserve this native natural enemy complex (Legaspi et al., 2000). The current data show that the diversity of the natural enemy complex of P. cirrella in a lime orchard contributed significantly to the overall management of P. citrella in the field as shown by the mortality estimates for five years from the different natural mortality factors. A high level of mortality estimate was recorded from predation by the generalist predators and probably host feeding by the ectoparasitoids. On the other hand, the mortality estimate from the introduced host specific parasitoid, A. citricola, was relatively low. These observations are consistent with the regression data, which indicate a more consistent relationship for parasitism in later stages of P. citrella than in earlier stages; however, mortality relationships from predation and host feeding from parasitoids were more consistent in earlier stages of P. citrella than in later stages. This suggests that combined predation and host feeding may be very important in the mortality of these younger stages of P. citrella. Also, the fact that there were no increase in slopes of regression equations developed from data collected later, when A. cirricola was introduced, seems to suggest that this parasitoid may have a smaller effect than the predators and host-feeding parasitoids. Nevertheless, even low contributions to the mortality of pests by any natural enemy should not be ignored in

the overall pest management. The objective of all biological control must be the reduction of population levels of the pest, and the assemblage of natural enemies acting together usually gives a more meaningful reduction in pest population. Therefore, conservation of all of these natural enemies in lime orchards should be given serious attention.

Acknowledgements ?

We thank S. Subramanian for providing technical assistance during the investigation. F. Consoli, J. Jacas, and R. Van Driesche reviewed earlier drafts of the manuscript and provided helpful suggestions. We also thank K. Portier for helping in the statistical analysis. This research was partially supported by a grant from the Florida Citrus Production Research Advisory Council. Florida Agricultural Experiment Station Journal Series R-06203.

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