Competitor-Free Space Mediates Non-Target Impact of an Introduced Biological Control Agent

Ecological Entomology (2009), 34, 107–113 DOI: 10.1111/j.1365-2311.2008.01046.x

Competitor-free space mediates non-target impact of an introduced biological control agent

RUSSELL H. MESSING 1 and XIN-GENG WANG 2 1 Department of Plant and Environmental Protection Science, University of Hawaii at Manoa, Kapaa, Hawaii, U.S.A. and 2 Department of Entomology, University of California, Riverside, California, U.S.A.

Abstract. 1. Enemy-free space has been shown to mediate host shifts in herbivores, but this has not previously been documented in parasitoids. Also, natural enemies shown to maintain host shifts have always been from higher trophic levels, rather than competitors. 2. In Hawaii, an Australian parasitoid (Diachasmimorpha tryoni) of medflies that loses competition contests to a subsequently introduced Asian parasitoid (Fopius arisanus ) has shifted its realised host range to attack non-target gall flies on lantana. 3. The present study demonstrates experimentally that D. tryoni reproduction is: (i) lower on medflies in coffee when F. arisanus is present than when it is absent; (ii) higher in gall flies on lantana than on medflies in coffee, when F. arisanus is present; and (iii) higher in medflies on coffee than in gall flies on lantana, when F. arisanus is absent. This meets Berdegue et al. ’s (Ecological Entomology , 21 , 203 – 217, 1996) three conditions to confirm the importance of enemy-free space. 4. In the field, F. arisanus is abundant on medflies, whereas D. tryoni is rare on medflies, but is the dominant parasitoid of lantana gall flies. 5. Competitor-free space is thus shown to be a key mechanism maintaining an apparent host shift by an introduced biocontrol agent onto a non-target species. Key words. Biological control , enemy-free space , intrinsic competition , medﬂ y , non- target impact , parasitoid .

Introduction Of the numerous parasitoids introduced against tephritid fruit fly pests in the islands, several have been documented to attack Biological control of invasive species using introduced natural ene- beneficial non-target species. Lethal intrinsic competition (i.e. mies has long been considered an environmentally safe and sus- between larvae of different parasitoid species within the same tainable form of arthropod pest management. However, recent host individual) has also been shown to occur ( Wang & Messing, studies have documented that imported predators and parasitoids 2003a ). sometimes attack non-target species, and occasionally lead to eco- The present study focuses on a complex tephritid host – parasitoid logical disruption ( Boettner et al. , 2000; Louda et al., 2003). To system in Hawaii (Fig. 1 ), in which competition between two date, little attention has been paid to the mechanisms underlying introduced biological control agents for a common resource this change in the realised host range of introduced natural enemies. appears to have led to a host shift in the inferior competitor. The While studies have shown the importance of enemy-free space in larval parasitoid Diachasmimorpha tryoni (Cameron), collected promoting host shifts of herbivores, where the shift involves escape in Australia in 1913 on its host Bactrocera tryoni (Froggatt), was from higher trophic level consumers ( Mulatu et al. , 2004; Murphy, brought to Hawaii for control of the Mediterranean fruit fly [med- 2004 ), neither the factors underlying parasitoid shifts among their fly, Ceratitis capitata (Wiedemann)]. It effectively reduced the phytophagous hosts, nor the possible role of competitors in the abundance of this pest on its major host crop, coffee (Pemberton process have previously been well documented. & Willard, 1918; Willard & Mason, 1937 ). Subsequently, the Hawaii has been a focal point in the debate about non-target egg-attacking parasitoid Fopius arisanus (Sonan) was introduced impacts of biological control agents ( Messing & Wright, 2006 ). as a biocontrol agent for the invasive Oriental fruit fly, Bactrocera dorsalis (Hendel), and was reported to displace D. tryoni as the Correspondence: Russell H. Messing, University of Hawaii at Manoa, dominant parasitoid of medfly ( Clancy, 1950; Bess et al., 1961). Kauai Research Center, 7370 Kuamoo Rd, Kapaa, HI 96746. E-mail: Soon thereafter, D. tryoni was reported to attack a non-target [email protected] beneficial tephritid, the lantana gall fly (Eutreta xanthochaeta

F. arisanus D. tryoni general concept of ‘enemy-free space,’ where competition can egg parasitoid larval parasitoid be recognised to exert strong selection pressure on an inferior SE Asia Australia competitor. It was previously determined that F. arisanus prevails in intrinsic competition against larval parasitoids (including D. tryoni ) within multi-parasitised host larvae, through physiological suppression of parasitoid egg development ( Wang & Messing, B. dorsalis C. capitata E. xanthochaeta 2003a). To determine whether the non-target gall flies provide Oriental fruit fly medfly lantana gall fly competitor-free space, the present study experimentally tested Asia Africa Mexico the three hypotheses of Berdegue et al. (1996) . In addition, we obtained and reared samples from the field to test the hypothesis that D. tryoni would be relatively more abundant in gall fly infested lantana than in medfly infested coffee. Both laboratory Lantana camera and field results indicate that competitor-free space is a key Coffee and weed other fruits C & S. America mechanism maintaining the acquisition of a new host by an introduced biological control agent. Fig. 1. Partial food web showing origin, host stage attacked, and rela- tionships among a suite of exotic insects, all of which interact within Hawaiian agro-ecosystems. Materials and methods

Field sites Aldrich), which itself was introduced from Mexico to Hawaii for control of the noxious weed Lantana camera (Stone et al., 1992). The study was conducted on the island of Kauai, Hawaii. All The target (medfly) and non-target (gall fly) hosts differ greatly collections of wasps and flies for experimental colonies and all in their ecological habitats: medflies infest fruit tissues of a wide field surveys were conducted at the 1375-ha Kauai Coffee range of host plants, while the gall fly is specific to stem tissues Plantation (elevation 122 m), and at Kokee State Park (elevation of lantana. Adult gall flies lay eggs inside the growing tips of 500 – 1500 m). Both sites are located on the western side of the newly sprouted lantana shoots, and the larvae form green sphe- island, approximately 20 km apart, separated by sugarcane roid galls on stems, each gall containing a single larva. fields and pasture. The coffee plantation contains the greatest There has been no recorded occurrence of D. tryoni attack- reservoir of naturally occurring C. capitata in Hawaii ( Vargas ing any gall-fly species in Australia, its native region. Eutreta et al. , 1995 ), while E. xanthochaeta is mostly abundant in the xanthochaeta was never established in Australia for weed Kokee State Park area ( Duan et al. , 1998 ). control ( Julien & Griffiths, 1998 ), thus parasitism of this species by D. tryoni in Hawaii represents a new host association. Diachasmimorpha tryoni was established in Hawaii in 1913, Study organisms and E. xanthochaeta has been widely established here since 1902, but D. tryoni was not recorded as parasitising the gall All laboratory tests and rearing of flies and parasitoids were fly until 1950, 1 year after the introduction of F. arisanus conducted at the University of Hawaii Kauai Agricultural (Clancy, 1950). We cannot be certain that parasitism of Research Center (KARC), Kauai, under laboratory conditions E. xanthochaeta by D. tryoni did not occur prior to 1950, only (22 ± 2 °C, 60 – 90% RH). Ceratitis capitata and F. arisanus that there are no published records of parasitism in a host in- were collected from the coffee plantation during September to sect that was deliberately released and monitored by State December, 2005. Ripe coffee fruits were collected and incu- entomologists. bated on wet tissue paper in Petri dishes in the laboratory. The Berdegue et al. (1996) suggested three tests to confirm the dishes were placed inside a screened fibreglass box importance of enemy-free space in maintaining an organism’s (45 × 3 0 × 15 cm) containing 2 cm of sand on the bottom for acquisition of a new host. First, the enemy’s impact on reducing fly pupation. Puparia were collected and placed into screened the organism’s fitness should be demonstrated in the original Plexiglas cages (30 × 3 0 × 30 cm), and newly emerged adult host or habitat. Second, the existence of a refuge should be veri- C. capitata and F. arisanus were separately maintained at ap- fied by showing increased fitness of the organism in the alter- proximately 50:50 sex ratios in cages with water and honey/ nate host or habitat; and third, the positive link between enemy yeast provided ad libitum. One to two week old female flies and impact and refuge should be confirmed by documenting de- wasps from the holding cages (sexually mature, and assumed to creased fitness of the organism in the alternate host or habitat have mated) were used for all tests. when enemies are absent. Conceptually, these tests are exactly Diachasmimorpha tryoni and E. xanthochaeta were collected the same whether one is examining a herbivore feeding on a from lantana plants in the Kokee State Park area during March plant host, or a parasitoid feeding on an insect host. The logic is to September 2005. Lantana twigs containing mature galls were also equally valid for organisms killed due to intrinsic competi- cut and inserted into containers (150 ml) filled with water, and tion, as for those killed by predators or parasitoids. Hence, we placed in screened cages (45 × 4 5 × 60 cm) in the laboratory. consider ‘competition-free space’ to be a subset of the more Newly emerged D. tryoni and E. xanthochaeta were collected

© 2008 The Authors Journal compilation © 2008 The Royal Entomological Society, Ecological Entomology, 34, 107–113 Competitor-free space mediates non-target impact 109 into screened plastic cages with water and honey/yeast provided medflies, for 2 days. Most of the released flies and wasps were ad libitum . One to two week old female flies and wasps (held found dead by the end of a 2-day exposure period in the cages with an approximately equal number of males since emergence) (if not, they were removed after the exposure). After 5– 6 days, were used to establish laboratory populations of both species. when medflies developed to the third instar, all fly-infested Eutreta xanthochaeta was maintained on lantana plants in 10 cm fruits in each cage were harvested. Pre-sampling showed that diameter pots in the greenhouse (natural temperature and light the above exposures resulted in 0 –3 larvae per fruit, with 70 – conditions) at KARC. A group of 10 –15 lantana plants (each 90% parasitism of the larvae by F. arisanus . To reduce variation 30 – 50 cm high with three to five branches) was moved into a among replicates, all fruits in each category (i.e. containing un- screened cage (1 × 1 × 1 m) and exposed to female E. xan- parasitised or F. arisanus -parasitised larvae) were mixed before thochaeta flies (fly density and exposure time varied, depending they were assigned to treatment 1 or 2. The actual number of on the availability of flies). Diachasmimorpha tryoni was main- host larvae and percentage parasitism by F. arisanus in each tained on these reared E. xanthochaeta in the laboratory. Five to replicate was determined based on final rearing and dissection ten lantana plants containing 20 –40 young galls (3 –5 mm diam- following the tests. The 10 exposed coffee fruits produced a eter, harbouring second- to third-instar E. xanthochaeta larvae) mean of 8.7 medfly puparia and 6.3 F. arisanus wasps. were placed in the large cage and exposed to 10 female D. try- In accordance with medfly densities in treatments 1 and 2, oni (exposure time depending on gall density). All exposed two lantana plants containing a combined total of 6– 12 galls galls were reared directly on the plants until wasps or flies were provided for each wasp in treatments 3 and 4. All lantana emerged. 6- to 10-day-old mated female D. tryoni were used for plants used for this test were first infested with E. xanthochaeta all tests. in the laboratory, by exposing 10 plants to 25 female E. xanthochaeta flies in a large cage for 2 days. Half of these plants were immediately placed in another cage and exposed to 30 fe- Experimental test of enemy-free space hypotheses male F. arisanus for 2 days. After 7 – 9 days, when the galls were 3 – 5 mm in diameter, the plants were used for testing in treat- The experiment consisted of four treatments: ( 1 ) a single ments 3 and 4. The mean (± 1 SE) host density was 8.5 ± 0.52, adult female D. tryoni was released into a screened plastic cage 8.8 ± 0.51, 9.0 ± 0.31, and 8.4 ± 0.27 for treatments 1, 2, 3, (30 × 3 0 × 30 cm) containing 10 coffee fruits on wet tissue pa- and 4, respectively; there was no significant difference among = = per in a Petri dish (with a 15-cm long coffee twig included for the four treatments (F 3,76 0.46, P 0.71). realism). The Petri dish was raised on another vial to the same The exposed coffee fruits and lantana galls were reared un- height as the twig; both vials were placed close together in the til all flies and wasps emerged, and the number and sex of middle of the cage. The 10 coffee fruits were infested with third- each were then recorded. After 90 days, all unemerged fly pu- instar medfly larvae (see below). ( 2 ) Identical to treatment 1, paria and lantana galls were dissected to determine the num- except that the 10 medfly-infested coffee fruits were previously bers of individuals that had developed into adults, but which exposed to F. arisanus during the egg stage of the hosts; ( 3 ) a died before emergence, the number of live individuals (still in single female D. tryoni was released into a screened cage the larval stage, but in diapause), and the number of dead (40 × 4 0 × 60 cm) containing two lantana plants, each harbour- hosts. A sample of 20 D. tryoni adult females (1 – 3 days post- ing four to six young galls caused by E. xanthochaeta . The plants emergence) reared from each host ( C. capitata and E. xan- were not previously exposed to F. arisanus ; ( 4 ) identical to thochaeta) were measured for body size (length of ovipositor, treatment 3, but the two lantana plants were previously exposed forewing, and hind tibia) through a stereomicroscope equipped to F. arisanus following oviposition by E. xanthochaeta (i.e. with an ocular micrometer. All measured wasps were then during the egg stage of the gall fly). Each treatment lasted for dissected to determine the number of mature eggs in the ova- 48 h, and each was repeated 20 times. ries. Additional samples of 43 D. tryoni reared from E. xan- When D. tryoni deposits an egg into a host larva previously thochaeta and 83 D. tryoni reared from C. capitata were parasitised by F. arisanus during the egg stage, F. arisanus has monitored to record the developmental time from oviposition already developed into a first-instar larva within the host ( Wang until adult emergence. & Messing, 2003a ). To obtain coffee fruits containing unparasi- tised medfly larvae or larvae previously parasitised by F. arisanus, medfly and F. arisanus were reared in field cages in a Field sampling coffee plantation at KARC. Coffee branches harbouring imma- ture fruits (uninfested by wild fruit flies) were selected and Field sampling was conducted in the coffee plantation from placed individually in cylindrical cages (20 × 30 cm) con- September 2005 to January 2006 (the harvest season of coffee), structed of wire covered by a fine nylon mesh sleeve (with mesh and in Kokee Park from June to December 2005. At 1 –2-week size sufficiently small to exclude all naturally occurring flies intervals, 300 – 500 ripe coffee fruits and 100 – 200 mature lan- and parasitoids). In each cage 10 –12 fruits were kept (all others tana galls were collected randomly from the fields and reared in fruits removed), and when the fruits matured, 10 female med- the laboratory (see rearing methods above). A total of 10 sam- flies (from the field collection, above) were released inside the ples was collected at each location. All emerged wasps and flies cage for 2 days. Half of these cages were selected at random, were counted and identified to species. Voucher specimens are and 10 female F. arisanus (from field collection, above) were deposited in the insect collection of the University of Hawaii released into each cage immediately following the exposure to Kauai Agricultural Research Center.

Data analyses There was no significant difference in the number of emerged D. tryoni in the presence or absence of F. arisanus in the lantana < = To test each of the three predictions for the enemy-free space habitat ( F 1,38 0.01, P 0.99). No F. arisanus wasps emerged hypothesis, the reproductive success of D. tryoni was sequen- from the lantana galls in treatment 4, indicating that F. arisanus tially compared between two of the four treatments using one- did not attack E. xanthochaeta. As a result, the number of way anova . In addition, the mean number of emerged host flies emerged D. tryoni wasps was higher in the lantana habitat than and D. tryoni , sex ratio of the emerged wasps, and percentage of in the coffee habitat in the presence of F. arisanus ( Table 1 : = < unemerged (dead) hosts were compared using two-way anova treatment 2 vs 4; F1,38 73, P 0.0001). among the four different treatments, while body size and devel- In the absence of F. arisanus , D. tryoni successfully produced opmental time of female wasps reared from E. xanthochaeta more offspring in the coffee habitat than in the lantana habitat = < and C. capitata were compared using one-way anova . Due to ( Table 1 : treatment 1 vs 3; F 1,38 5.5, P 0.05). This suggests the large variation in developmental time among wasps reared that D. tryoni was more successful in parasitising hosts in fruits from the same treatment (particularly from E. xanthochaeta ), than in the novel habitat of galls in the absence of competition data on males and females were pooled for this analysis. All with F. arisanus . Thus, all three predictions for enemy-free data analyses were performed using JMP 4.1 (SAS, Cary, North space were fulfilled in these experiments. Carolina). Proportional data were arcsin square root transformed Diachasmimorpha tryoni reared from E. xanthochaeta were before the tests. larger than those reared from C. capitata in terms of the length = < = of hind tibia ( F1,38 68.9, P 0.0001), ovipositor (F 1,38 36.1, < = < P 0.0001), and forewing (F 1,38 43.8, P 0.0001) ( Table 2 ). Results These larger female D. tryoni had a greater number of mature = < eggs in the ovaries ( F1,38 76.1, P 0.0001) ( Table 2 ). The mean number of emerged host flies and parasitoids, the sex However, the developmental rate from egg to adult for D. tryoni ratio of emerged wasps, and the percentage of unemerged (dead) was substantially slower on the gall fly than on medfly ( Fig. 2 ). hosts are presented in Table 1 . Two-way anova showed that host Furthermore, final dissections showed that 21.4% of D. tryoni species, presence or absence of the competitively superior F. on E. xanthochaeta went into diapause as mature larvae (i.e. did arisanus (competition), and the interaction of both factors af- not develop to adults after 90 days), while less than 0.5% of D. fected the number of emerged D. tryoni (host: F = 15.3, d.f. = 1 , tryoni larvae went into diapause on medfly. P = 0.009; competition: F = 43.5, d.f. = 1, P < 0.001; In the field on Kauai Island, the composition of the parasitoid host × competition: F = 43.4, d.f. = 1, P < 0.001). Host species fauna on target and non-target hosts reflected the outcome pre- also affected the number of emerged flies (host: F = 54.7, dicted by laboratory experiments. In commercial coffee planta- d.f. = 1, P < 0.001; competition: F = 1.6, d.f. = 1, P = 0.217; tions heavily infested with fruit flies, the egg-attacking F. host × competition: F = 1.5, d.f. = 1, P = 0.216). The percentage arisanus was the dominant parasitoid, while all larval-attacking = = of female D. tryoni (F 3,64 2.07, P 0.11) and the number of parasitoid species combined (including D. tryoni, as well as = = dead hosts (F 3,76 2.57, P 0.06) were not significantly affected Diachasmimorpha longicaudata and Fopius vandenboshi ) ac- by host species, nor the presence or absence of competition. counted for only 2.6% of relative parasitoid abundance. In con- More D. tryoni offspring were produced in the absence of F. trast, in stands of lantana infested with E. xanthochaeta , D. arisanus than in the presence of F. arisanus in the coffee habitat tryoni was the dominant parasitoid attacking the gall fly, while = < ( Table 1 : treatment 1 vs 2; F 1,38 95, P 0.0001). In treatment not a single F. arisanus was recovered ( Fig. 3 ). 2, approximately 90% of the C. capitata larvae were previously parasitised by F. arisanus, based on the total number of wasps and flies emerged. As a result, D. tryoni had little chance to suc- Discussion cessfully produce offspring in the presence of F. arisanus . 11 out of 20 replicates did not produce a single D. tryoni The concept of enemy-free space is considered an important offspring. factor shaping the ecological niche of phytophagous insects

Table 1. Reproductive success of individual female Diachasmimorpha tryoni parasitoids attacking coffee fruits containing target (Ceratitis capitata ) larvae, or lantana galls containing non-target (Eutreta xanthochaeta ) larvae, in the presence or absence of the competitively superior parasitoid Fopius arisanus .

Treatment * No. D. tryoni % female No. Host ﬂ y speciesF. arisanus No. ﬂ ies emerged emerged D. tryoni % dead hosts

1 C. capitata Absent 1.7 ± 0.37 4.85 ± 0.43 52.4 ± 6.1 23.4 ± 3.8 2 C. capitata Present 0.8 ± 0.17 0.50 ± 0.14 72.2 ± 14.7 13.5 ± 2.7 3 E. xanthochaeta Absent 3.8 ± 0.40 3.55 ± 0.36 50.4 ± 5.6 17.7 ± 3.1 4 E. xanthochaeta Present 3.8 ± 0.38 3.55 ± 0.33 45.5 ± 3.9 12.4 ± 2.6

* Values are mean ± 1 SE; there were 20 replicates for each treatment.

Table 2. Body size and mature egg load of adult female Diachasmimorpha tryoni parasitoids reared from the target host (Ceratitis capitata ) and from a non-target host (Eutreta xanthochaeta ) .

Host speciesn Hind tibia (mm) Ovipositor (mm) Forewing (mm) Egg load

E. xanthochaeta 20 1.38 ± 0.03 a 4.87 ± 0.13 a 4.54 ± 0.10 a 76.5 ± 2.5 a C. capitata 20 0.97 ± 0.03 b 3.76 ± 0.13 b 3.55 ± 0.11 b 47.3 ± 2.4 b

Mean ( ± 1 SE) values in each column followed by the same letter are not signiﬁ cantly different (one-way anova , P > 0.05).

(Jeffries & Lawton, 1984 ). The term has been widely used, par- which is consistent not only with our own field surveys, but also ticularly in the insect herbivore literature (e.g. Feder, 1995 ), and with several other studies. These studies have documented the remains a key consideration among the factors thought to con- dominance of the egg-attacking parasitoid in coffee plantations tribute to addition of host lineages and thus, speciation and di- and other medfly host crops throughout the Hawaiian Islands versity ( Heard et al. , 2006 ). However, as pointed out by (Vargas et al. , 1993, 2001; Duan et al. , 1998 ). Berdegue et al. (1996) , there was inconsistency in the literature While D. tryoni is very susceptible to competition from F. as to how enemy-free space was defined, and how best to deter- arisanus in fruits, and has been widely displaced in crops mine its relative importance in maintaining the acquisition of throughout the islands, it finds a complete refuge from this com- new hosts. The authors proposed a set of three null hypotheses petition in the galls of E. xanthochaeta on lantana. In addition to that should be tested and rejected sequentially if enemy-free our experimental results, additional surveys and rearing of lan- space is to be considered an important factor in host shifts. As tana galls in our laboratory ( Duan & Messing, 1996; Duan et al. , pointed out in Berdegue et al. (1996), only 10 systems out of 41 1998) and by others ( Wong et al., 1991) have repeatedly yielded that they examined actually tested all three hypotheses suffi- significant levels of parasitism of E. xanthochaeta by D. tryoni , ciently to demonstrate the importance of enemy-free space. but never a single case of successful parasitism by F. arisanus . Since then, the rigor of their approach has been acknowledged In the behavioural repertoire of F. arisanus adult females lead- and more commonly used to demonstrate examples of enemy- ing up to oviposition, it appears that only soft fruit tissues are free space in a variety of ecosystems (e.g., Mulatu et al. , 2004; recognised as suitable host habitats ( Wang & Messing, 2003b; Murphy, 2004 ). However, to date, enemy-free space has not Wang et al. , 2004; Rousse et al. , 2007 ). been documented to impact the host range of insect parasitoids, A parasitoid’s fitness depends on many factors, such as nor have competitors been shown to be responsible in the same searching efficiency, host availability and host suitability under manner as consumers. different conditions. It is difficult to quantify an exact fitness Our laboratory and field data confirm that refuge from com- advantage for D. tryoni in acquiring the new host, because cof- petition is an important factor maintaining host use by an intro- fee and lantana habitats are extremely different. In the labora- duced parasitoid of a beneficial non-target species. The tory test, we controlled host density at approximately the same experimental results show substantially reduced reproductive level in the two habitats, provided each wasp with a surplus of success of D. tryoni on medfly in the presence of F. arisanus , hosts, then measured the number of offspring produced per wasp. While this outcome primarily measures host suitability, it does, to some extent, integrate the wasps’ searching behaviour 50

F. arisanus D. tryoni Other parasitoid species 100 40

80 30

60 20 40

Developmental time (days) 10 20

0 Percentage of parasitoids recovered 0 E. xanthochaeta C. capitata Lantana patches Coffee fields Host species Fig. 3. Parasitoid recovery from natural populations of Ceratitis capi- Fig. 2. Developmental time (males and females combined) from egg to tata in cultivated coffee ﬁ elds, and Eutreta xanthochaeta in wild lantana adult emergence for the parasitoid Diachasmimorpha tryoni reared from patches, on the island of Kauai, Hawaii. From the left: open bars, Fopius Ceratitis capitata ( n = 83) and Eutreta xanthochaeta ( n = 43) arisanus ; ﬁ lled bars, Diachasmimorpha tryoni ; grey bars, other parasit- = < ( F1,124 116.1, P 0.001). oid species.

(host finding and host acceptance) within the confines of the host in Hawaii for the past 60 years. At the same time, although cage into the measurement of reproductive output. Other studies the sampling level has been less extensive, there are also field of enemy-free space in herbivores have similarly used indirect and laboratory data that are consistent with what we consider the measures of fitness ( Mulatu et al. , 2004; Murphy, 2004 ). absolute nature of the refuge from F. arisanus on the new host, Utilising the number of offspring as an indirect measure of fit- E. xanthochaeta on lantana ( Duan & Messing, 1996; Duan et al. , ness in the experiment, indicated that there is a cost to using the 1997, 1998; Wang & Messing, 2003b, 2004 ). Resolving the lantana gall fly refuge: D. tryoni that parasitised lantana gall flies question of whether heterogeneity (as shown by Heard et al. , had lower reproductive success than those that parasitised med- 2006) or consistency (as shown in our system) in enemy-free flies. This would seem to show that the selective advantage of space is the more common pattern, and whether it is trophic- competitor escape is substantial enough in itself to outweigh the level dependent, will require many more datasets for both her- disadvantage of using an inferior host. However, we recognise bivores and parasitoids on broader spatial and temporal scales. that there are additional factors involved in the interpretation of The concept of enemy-free space, which has previously been overall fitness. The experimental D. tryoni females were reared shown to be effective in maintaining plant host shifts in phy- from a common host ( E. xanthochaeta ) and were therefore ap- tophagous insects, appears to be equally valuable in relation to proximately the same size in both treatments. However, lantana parasitoids shifting among arthropod hosts. The functional gall fly larvae are, on average, larger than medfly larvae, and par- equivalence of lethal intrinsic competition to predation (or para- asitoid offspring from gall flies are larger and thus have poten- sitism) suggests that the characterisation of ‘enemy’ in enemy- tially higher fecundity in subsequent generations. This comes at free space can be expanded to include those competitors that the cost of a remarkably slower developmental rate and a substan- exert strong selection pressure, even if they do not actually con- tially increased rate of diapause induction in wasps that attack sume the inferior competitor. Use of the competition-free space gall flies. Overall fitness is likely to vary, and to be a complex concept at higher trophic levels, and exploration of intra-guild function of fecundity, rate of development, and other diverse fac- dynamics such as these may be heuristically constructive in tors [such as exposure to predators and competing herbivores like helping to evaluate the non-target risks of biological control. the lantana tortricid moth (Epinotia lantana Busck)], in the widely Biological pest control is not only the foundation for inte- varying habitats in which lantana is found throughout the grated pest management in agro-ecosystems, but is increasingly Hawaiian Islands. While a long-term examination of the compre- viewed as an important tool for conservation in natural areas hensive fitness of D. tryoni attacking the two hosts under a variety (Hoddle, 2004). However, the practice of biocontrol is under of field conditions was beyond the scope of the present study, the considerable critical scrutiny because of the ongoing debate components of fitness that were measured in the laboratory (i.e. about the frequency and intensity of non-target impacts. Moving lower fecundity, slower developmental rate) point to significant beyond a mere documentation of non-target effects, and begin- costs incurred by the parasitoid when ovipositing into the novel, ning to understand the selective pressures that foster them may non-target gall fly. If, in some situations, bottom-up (nutritional) help make biocontrol more predictive, and may increase confi- factors complement the top-down escape from F. arisanus, it does dence in the use of this critically important tool to mitigate im- not negate the importance of competitor-free space, which theo- pacts of destructive invasive species. retically may facilitate a host shift even when the adoption of a new host itself leads to increased enemy pressure under some Acknowledgements field conditions in diverse environments ( Heard et al. , 2006 ). Enemy-free space for some phytophagous insects has been We thank Terri Moats for assistance in collection, rearing, and shown to operate as a geographic and temporal mosaic ( Heard maintaining parasitoid colonies. We also thank Peter McEvoy et al. , 2006 ), and these authors point out that a host shift does not and his lab group for constructive reviews of this manuscript. necessarily lead to consistent enemy-free space. Admittedly, our This research was supported in part by a grant from the USDA – field data were restricted to a single site of each type (coffee and CSREES – TSTAR program (Tropical and Subtropical lantana) during a single year. However, in contrast to the system Agricultural Research). studied by Heard et al. (2006), historical data on tephritid parasitism in Hawaii indicate that while the magnitude of the selective advantage of the enemy-free host may vary, the direction has References been consistent in both space and time. Numerous field studies by investigators sampling on different islands over the past Berdegue , M. , Trumble , J.T. , Hare , J.D. & Redak , R.A. (1996 ) Is it 60 years have repeatedly documented the dominance of F. arisa- enemy-free space? The evidence for terrestrial insects and freshwater nus over D. tryoni and other larval– pupal parasitoids in parasit- arthropods . Ecological Entomology , 21 , 203 – 217 . ism of fruit-infesting tephritids in Hawaii (Willard & Mason, Bess , H.A. , van den Bosch , R. & Haramoto , F.H. ( 1961 ) Fruit ﬂ y para- 1937; van den Bosch et al., 1951; Bess et al., 1961; Haramoto & sites and their activities in Hawaii. Proceedings of the Hawaii Ento- mological Society , 17 , 367 – 578 . Bess, 1970; Wong et al., 1984; Vargas et al., 1993; Purcell et al. , Boettner , G.H. , Elkington , J.S. & Boettner , C.J. ( 2000 ) Effects of a bio- 1998). These field data, combined with laboratory experiments logical control introduction on three nontarget native species of Sa- documenting the extent and mechanism of competition ( van den turniid moths . Conservation Biology , 14 , 1798 – 1806 . Bosch & Haramoto, 1953; Wang & Messing, 2002, 2003a; Wang van den Bosch , R. , Bess , H.A. & Haramoto , F.H. ( 1951 ) Status of orien- et al. , 2003 ) leave little doubt that D. tryoni has been subject to tal fruit ﬂ y parasites in Hawaii . Journal of Economic Entomology , 44 , continuous and intense selection pressure on its original medfly 753 – 759 .

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Biological Control , 1 , 2 – 7 . tions between augmentatively released Diachasmimorpha longicaudata and a complex of opiine parasitoids in a commercial guava Accepted 16 June 2008 orchard . Biocontrol Science and Technology , 8 , 139 – 151 . First published online 29 September 2008