Biological Control 35 (2005) 55–67 www.elsevier.com/locate/ybcon
Does fundamental host range match ecological host range? A retrospective case study of a Lygus plant bug parasitoid T. Haye a, H. Goulet b, P.G. Mason b, U. Kuhlmann a,¤ a
b
CABI Bioscience Centre, Agricultural Pest Research, Rue des Grillons 1, CH-2800 Delémont, Switzerland Agriculture and Agri-Food Canada, Research Centre, K.W. Neatby Building, Ottawa, Ont., Canada K1A 0C6 Received 15 March 2005; accepted 21 June 2005 Available online 10 August 2005
Abstract Using the retrospective case study of Peristenus digoneutis (Hymenoptera: Braconidae) introduced in the United States for biological control of native Lygus plant bugs (Hemiptera: Miridae), laboratory and Weld studies were conducted in the area of origin to evaluate whether the fundamental host range of P. digoneutis matches its ecological host range. Furthermore, it was determined whether these approaches would have been indicative of the post-introduction host range of P. digoneutis in North America [Day, W.H., 1999. Host preference of introduced and native parasites (Hymenoptera: Braconidae) of phytophagous plant bugs (Hemiptera: Miridae) in alfalfa-grass Welds in the north-eastern USA, BioControl 44, 249–261.]. Seven non-target mirid species were selected to deWne the fundamental host range of P. digoneutis in the area of origin in Europe. Laboratory choice and no-choice tests demonstrated that all selected non-target species were attacked by P. digoneutis and were largely suitable for parasitoid development. To conWrm the validity of the fundamental host range, the ecological host range of P. digoneutis in the area of origin was investigated. Peristenus digoneutis was reared from 10 hosts, including three Lygus species and seven non-target hosts from the subfamily Mirinae. Despite the fact that laboratory tests demonstrated a high parasitism level in non-targets, ecological assessments in both North America (Day, 1999) and Europe suggest a much lower impact of P. digoneutis on non-target mirids, with low levels of parasitism (below 1% in Europe). Therefore, ecological host range studies in the area of origin provide useful supplementary data for interpreting pre-release laboratory host range testing. 2005 Elsevier Inc. All rights reserved. Keywords: Biological control; Fundamental host range; Ecological host range; Non-target eVects; Risk assessment; Lygus plant bugs; Parasitoids; Braconidae; Peristenus digoneutis
1. Introduction Within the last twenty years, concerns regarding the safety of arthropod biological control using invertebrates have increasingly been discussed (e.g., Howarth, 1983, 1991; SimberloV and Stiling, 1996; Follett et al., 2000; Lynch et al., 2001; Stiling, 2004). It has been stressed that classical biological control could have major environmental costs if introduced natural enemies colonize and disrupt
*
Corresponding author. Fax: +41 32 421 4871. E-mail address:
[email protected] (U. Kuhlmann).
1049-9644/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2005.06.008
native systems (Hawkins and Marino, 1997). The main areas of concern include the irreversibility of exotic introductions, the dispersal of non-indigenous natural enemies to new habitats, and the potential host range expansion of the agent to include native or beneWcial insects, thereby causing harm to such non-target hosts (non-target eVects), (e.g., Howarth, 1983, 1991; Secord and Kareiva, 1996; SimberloV and Stiling, 1996; Follett et al., 2000; Lockwood, 2000; Michaud, 2002; Kuris, 2003; Louda et al., 2003a; Stiling, 2004). However, despite the relatively large number of exotic natural enemies introduced for biological control of insects and mites (Greathead, 1995; Wratten and Gurr, 2000), negative environmental eVects from such releases
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T. Haye et al. / Biological Control 35 (2005) 55–67
have rarely been reported (Onstad and McManus, 1996; van Lenteren et al., 2003). Many publications focusing on non-target eVects of exotic natural enemies consider target speciWcity as the key to safety in biological control (Nechols et al., 1992; Onstad and McManus, 1996; Secord and Kareiva, 1996; Strand and Obrycki, 1996; Sands, 1997; Keller, 1999; Knight, 2001; van Lenteren et al., 2003). Host speciWcity can be deWned as the degree to which a species restricts its diet (Nechols et al., 1992). In the context of a natural enemy, this can refer to the species host range, broadly deWned as the set of species that can support development of a parasitoid or serve as prey for a predator (Strand and Obrycki, 1996). In addition to collecting available information from the literature (De Nardo and Hopper, 2004; Sands and van Driesche, 2004), a Wrst essential step in host range assessment is to conduct laboratory tests to investigate whether non-target species are attacked under a variety of test conditions (van Driesche and Hoddle, 1997; van Lenteren et al., 2003). The set of species that can support development of a parasitoid or serve as prey for a predator—observed under laboratory conditions exclusively—is deWned as the fundamental (syn. physiological) host range of a potential agent (Onstad and McManus, 1996). In contrast, the ecological host range is deWned as the current and evolving set of host species actually used for successful reproduction in the Weld (Nechols et al., 1992; Onstad and McManus, 1996). Estimating a species’ host range is often linked with various practical problems, such as lack of knowledge on the ecology of non-targets hosts (host plants, habitats, behavior etc.), their phylogentic relatedness to the target host, suitable rearing protocols, availability of desired non-target hosts or large numbers of potential non-target species to be tested (van Driesche, 2004). Therefore, several diVerent approaches have been used in the past by biological control practitioners to assess host speciWcity of biological control agents. These approaches primarily included reviews of scientiWc literature (De Nardo and Hopper, 2004), laboratory tests (e.g., Sands and Coombs, 1999; Porter, 1979; Babendreier et al., 2003) or Weld surveys in the area of origin (Fuester et al., 2001; Haye and Kenis, 2004). However, each approach has inherent strengths and weaknesses (De Nardo and Hopper, 2004; van Driesche and Reardon, 2004). In particular, many studies have shown that the fundamental host range of a biological control agent is often greater than its ecological host range (Cameron and Walker, 1997; Morehead and Feener, 2000; Froud and Stevens, 2003); this is likely due to diYculty in accurately reproducing the factors that inXuence host searching and assessment behaviour of a parasitoid in its natural environment (Nechols et al., 1992; Sands, 1993). Furthermore, it has been stated that laboratory observations should be combined with Weld observations to provide a
basis for correctly interpreting fundamental host range estimations (Onstad and McManus, 1996; Hopper, 2001; Kuhlmann and Mason, 2003). Here, we present a retrospective case study on Peristenus digoneutis (Hymenoptera: Braconidae), a European parasitoid of Lygus spp. that was introduced in the early 1980s in the United States for biological control of the native plant bug Lygus lineolaris (Palisot de Beauvois) (Day et al., 1990, 2003). Laboratory and Weld studies were conducted in the area of origin of the parasitoid to evaluate whether the fundamental host range matches the ecological host range of P. digoneutis in Europe, and determine whether these approaches would have been indicative for the post-introduction host range of P. digoneutis in North America, as reported by Day (1999).
2. Material and methods 2.1. Fundamental host range 2.1.1. Selection of non-target hosts In accordance with Kuhlmann and Mason (2003), the selection of non-target hosts for laboratory testing was based on phylogenetic criteria, availability, spatial and temporal overlap of potential non-target hosts and Lygus hosts in their natural habitats in the area of investigation (Schleswig-Holstein, northern Germany). According to the maximum Wt cladogram of Lygus and its outgroup taxa (Schwartz and Foottit, 1998), two Mirini species, Lygocoris pabulinus (L.) and Liocoris tripustulatus (Fabricius), were chosen as test candidates following a centrifugal phylogenetic approach (Wapshere, 1974). Another candidate chosen from the tribe Mirini (to which Lygus belongs) was the potato bug, Closterotomus norwegicus (Gmelin), which is the most abundant mirid found in spring in northern Germany (Afscharpour, 1960) and occurs at the same time and habitat as Lygus species. To include less closely related mirid species into the testing procedure, four grass bugs from the tribe Stenodemini were selected, including Leptopterna dolobrata L., Stenodema calcarata (Fallén), Notostira elongata (GeoVroy), and Megaloceraea recticornis (GeoVroy). In the present study, L. rugulipennis Poppius represented the target host instead of the congeneric North American Lygus species. To investigate if variations in acceptance and parasitoid development occur when diVerent Lygus target hosts are oVered, Lygus maritimus Wagner, which in contrast to the former species occurs primarily in coastal habitats, was included in the testing procedures. 2.1.2. Source and rearing of parasitoids, Lygus hosts, and potential non-target hosts Peristenus adults were reared from parasitized nymphs of the Wrst and second generation of
T. Haye et al. / Biological Control 35 (2005) 55–67
L. rugulipennis collected in clover and camomile habitats in northern Germany. Parasitoids were kept in a subterranean insectary at temperatures of 15–18 °C and provided with honey and water. Before each test, parasitoids were allowed to adapt to the laboratory temperature (25 °C) for at least 1 h. Nymphs of L. rugulipennis, L. maritimus, and L. tripustulatus were reared from overwintered adults, and nymphs of L. pabulinus were reared from newly emerged adults from the spring generation; for all species above, the adults were collected in their natural habitats and brought to the laboratory for egg laying. Adults were kept at 20 °C and 16 h photoperiod and provided with lettuce and sprouting potatoes as oviposition substrates. Nymphs were produced following a combination of rearing methods described for L. lineolaris by Stevenson and Roberts (1973) and Snodgrass and McWilliams (1992). To establish a culture of S. calcarata, grass ears in which adults had previously oviposited before were collected in the Weld to obtain freshly emerged nymphs. To obtain nymphs of univoltine non-target mirids that overwinter in the egg stage, such as C. norwegicus, L. dolobrata, and M. recticornis, Wrst and second instar nymphs were Weld-collected in early spring. Small instar nymphs of N. elongata were collected in early July when the second nymphal generation started to emerge. In the very early period of nymphal emergence, the risk that nymphs have already been parasitized in the Weld is generally low. However, as an additional control, subsamples of small non-target nymphs were always reared and dissected for parasitism. Nymphs of Lygus, Closterotomus, Lygocoris, and Liocoris that were attacked during exposure to P. digoneutis females in laboratory tests were reared individually in small plastic vials to investigate whether observed oviposition by female parasitoids was successful and whether hosts were suitable for parasitoid development. Although nymphs of most of the species tested were provided with Romaine lettuce and potato sprouts as a food source, grass bug nymphs were placed on grass ears or leaves instead. A thin layer of moistened vermiculite covered the bottom of the vials to oVer emerging parasitoid larvae a pupation site. 2.1.3. Small arena no-choice test The aim of the test was to determine whether P. digoneutis accepts non-target nymphs consistently and whether non-target nymphs are suitable hosts for parasitoid development. Three day old, mated, naïve females were Wrst exposed to a single second or third instar nymph of the target L. rugulipennis. As P. digoneutis has no pre-oviposition period (Haye, 2004), females that did not react to Lygus nymphs were presumably unWt for use and thus, were excluded from the testing procedure. In the subsequent no-choice tests, females that were presumed ready for oviposition were individually placed into a clear plastic vial of 30 £ 55 mm each containing a
57
small instar nymph of a non-target host (Fig. 1). Each parasitoid was given a maximum time period of 20 min to Wnd and parasitize the nymph. The same procedure was repeated 24 h later, but oVering a L. rugulipennis nymph in order to exclude false-negatives in case females had not reacted to the non-target host (control, Fig. 1). When the parasitoid was observed to insert the ovipositor into the nymph, the host was recorded as attacked (“Attacks on hosts”, Fig. 1). A host was further counted as accepted when either a parasitoid larva was found in the dissection or a parasitoid cocoon was found in the rearing vial (“Host acceptance”, Fig. 1). Encapsulated eggs or larvae were never observed in dissections of attacked mirid hosts and have never been reported in the literature and thus, they did not inXuence the measure of host acceptance. A mirid host was classiWed as suitable when parasitoid larvae successfully completed their development and formed a cocoon outside their hosts (“Host suitability”, Fig. 1). 2.1.4. Small arena behavioral choice test To investigate whether the behavioral threshold for attacks on non-target hosts can be changed in the presence of the target host (“Host preference”, Fig. 1), choice tests were conducted in which parasitoids were oVered Lygus nymphs and non-target nymphs simultaneously (Fig. 1). For these tests, six to eight day old experienced female parasitoids (n D 20/non-target species) were used. Parasitoid experience was acquired in the no-choice tests (as outlined in Fig. 1). Individual female wasps were oVered three second or third instar Lygus nymphs and three second or third instar non-target nymphs at the same time in a Petri dish (diameter 5 cm). Following observed attack, potentially parasitized nymphs were immediately removed with a mouth aspirator and replaced by new, non-parasitized ones to maintain a constant number of each mirid species at all times. Tests lasted for 5 min, and the number of attacks on each mirid species was recorded. 2.1.5. Statistical analysis The -square test after McNemar was used to analyze the data sets obtained from each mirid species tested in small arena no-choice tests. For comparing the levels of host acceptance, only data obtained from parasitoids which had attacked Lygus and non-target nymphs in the no-choice tests were used. Consequently, the number of replicates was automatically reduced. Some of the attacked nymphs died and dried out during the rearing process and thus, could not be dissected for parasitism. In these cases the complete test series was excluded from the analysis (Fig. 1) and consequently, the number of replicates was further reduced. Data sets of small arena behavioral choice test were analyzed using the Wilcoxon pairedsample test. All statistical analyses were carried out with the SPSS 10.0 software package (SPSS Inc., 1999).
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T. Haye et al. / Biological Control 35 (2005) 55–67
Fig. 1. General procedure for assessing the fundamental host range of P. digoneutis.
2.2. Ecological host range 2.2.1. Selection of non-target hosts According to Wagner (1952), approximately 2000 species belonging to the family Miridae occur in the Palaearctic region (307 in Germany) and thus, the number of potential non-target hosts of P. digoneutis is immense. The aim of collections in various habitats in northern Germany (e.g., stinging nettle stands, fallow Welds, and grasslands) was to obtain samples from a broad range of common and rare mirid species considered as potential non-target hosts of P. digoneutis. In order to investigate whether these parasitoids are actually speciWc to the subfamily Mirinae, collections were also focused on other mirid subfamilies, such as Bryocorinae, Orthotylinae, and Phylinae. 2.2.2. Source and rearing of potential non-target hosts Potential non-target mirid nymphs were collected from various host plants (Table 1) in natural or agricultural habitats using a standard sweep net. As parasitoid larvae are known to emerge from late nymphal instars or rarely from teneral adults (Loan, 1980), mirid nymphs
were collected exclusively in the fourth or Wfth instar. From each collection, subsamples of 20–50 nymphs were taken randomly and dissected to assess percent parasitism. However, mirid species that were found in low numbers were exclusively used for rearing out parasitoids, as dissection provides no information pertaining to parasitoid species. The rearing system for samples up to 50 nymphs consisted of 1.2 L plastic containers Wtted with removable Petri dishes on the bottom. The Petri dishes were Wlled with moist vermiculite and separated from the rest of the container by a round piece of gauze (width 1.20 £ 1.38 mm) which allowed larval parasitoids gain access to the Petri dish for pupation (Drea et al., 1973). Larger samples of a maximum of 500 nymphs were kept in plastic buckets with the bottoms removed and replaced with gauze. Plastic funnels terminating in vermiculite-Wlled Petri dishes were attached to the bottom of the buckets in order to collect emerging parasitoid larvae that fall through the gauze. Mirids belonging to the genus’ Lygus, Adelphocoris, Closterotomus, and Calocoris were fed with organically grown beans and lettuce. For all other mirid species the host plants they were
Table 1 Ecological host range of P. digoneutis: Mirid species, host plants, and details regarding collection and rearing of mirid nymphs collected in Schleswig–Holstein, northern Germany to assess presence and parasitism by P. digoneutis in target and non-target mirids No. of No. of No. of nymphs taken mirid adults cocoons into rearing reared received
No. of No. of sites No. of sites parasitoids sampled with P. digoneutis emerged present
Parasitoid species composition (%)
Overall % Parasitism % Parasitism parasitism by P. digoneutis by P. digoneutis (%) in the Weld in the laboratory
Host Plantsa
Bryocorinae Dicyphus globulifer (Fallén)
Sv
274
100
69
38
1
0
0
100
0
35.5
0
—
Ml
568
336
59
35
11
0
0
97
3
14.9
0
—
1114
466
155
86
13
1
2
72
26
25
0.3
—
U
186
120
25
21
8
1
5
91
4
17.2
0.8
—
A
14
6
2
2
2
0
0
100
0
25.0
0
—
15,149
7639
1828
1357
29
9
4
89
7
19.3
0.7
100
U
4217
1197
839
409
31
4
1
79
20
41.2
0.6
73
U
1955
1127
141
74
27
2
3
54
43
11.1
0.3
21
Mr
1243
877
80
62
6
2
10
86
4
8.4
0.6
96
488 39,851
363 22,154
67 7296
65 5556
13 20
8 19
34 58
52 37
14 5
15.6 24.8
5.3 14.4
— 91
A Q
97 23
64 11
4 8
1 8
4 2
0 0
0 0
100 100
0 0
5.9 42.1
0 0
— —
G
911
351
252
200
9
0
0
85
15
41.8
0
—
G
3285
1218
529
375
23
0
0
70
30
30.3
0
G
37
25
6
4
1
0
0
100
0
19.4
0
G
2450
1772
75
43
14
0
0
91
9
4.1
0
11
G
7872
4014
757
603
23
0
0
82
18
15.9
0
83
G
1970
1038
458
269
25
1
