South American invasive plants in the genus Ludwigia (Onagraceae) degrade many riparian and aquatic ecosystems worldwide. Biological control may aid in the management of these exotic weeds, but data on the host specificity of Ludwigia natural enemies is limited. The biology and host range of Sudauleutes bosqi Hustache (Coleoptera: Curculionidae), an herbivore of Ludwigia spp. in South America, was studied to determine its suitability as a biological control agent for 3 exotic Ludwigia spp. (targets) in the US. Weevils maintained at 25 °C (± 1 °C) and a 14:10 h (L:D) photoperiod developed through 7 life stages, with a generation time from egg to adult of 17.6 (± 1.2) d when reared on the target weed Ludwigia hexapetala (Hook. & Arn.) Zardini, Gu & P. H. Raven (Onagraceae). There was no difference in mean body length between females (2.6 ± 0.1 mm) and males (2.5 ± 0.1 mm). No-choice and multiple-choice host range tests were conducted using 3 exotic Ludwigia spp. and 8 native US plant species. Sudauleutes bosqi larvae completed development on the 3 target weeds and 4 native plant species, and oviposition occurred on all but 1 of the plant species that supported larval development. In multiple-choice tests, S. bosqi oviposited on 9 of 11 plant species tested. Results indicate that host selection and development of S. bosqi is not limited to target weeds but also includes valued non-target species. Therefore, S. bosqi is not sufficiently host-specific for further consideration as a biological control agent of exotic Ludwigia spp. in the US and additional testing is not warranted.
Las plantas invasoras sudamericanas del género Ludwigia (Onagraceae) degradan muchos ecosistemas ribereños y acuáticos en todo el mun-do. El control biológico puede ayudar en el manejo de estas malas hierbas exóticas, pero los datos sobre la especificidad de hospedero de los enemigos naturales de Ludwigia son limitados. Se estudió la biología y el rango de hospederos de Sudauleutes bosqi Hustache (Coleoptera: Curculionidae), un herbívoro de Ludwigia spp. en América del Sur para determinar su sostenebilidad como agente de control biológico de 3 especies exóticas de Ludwigia spp. (objetivos) en los EE.UU. Los gorgojos mantenidos a 25 °C (± 1 °C) con un fotoperíodo de 14:10 h (L:D) se desarrollaron a lo largo de 7 estadios de vida, con un tiempo de generación de huevo a adulto de 17,6 (± 1,2) dias cuando se criaron en la maleza objetivo Ludwigia hexapetala (Hook. & Arn.) Zardini, Gu & PH Raven (Onagraceae). No hubo diferencia en la longitud corporal media entre las hembras (2,6 ± 0,1 mm) y los machos (2,5 ± 0,1 mm). Se realizaron pruebas de variedad de hospederos de opción múltiple y de elección múltiple utilizando 3 especies exóticas de Ludwigia spp. y 8 especies de plantas nativas de EE. UU. Las larvas de Sudauleutes bosqi completaron el desarrollo en las 3 malezas objetivo y las 4 especies de plantas nativas, y la oviposición sucedió en todas menos 1 de las especies de plantas que apoyaron el desarrollo de las larvas. En las pruebas de opción múltiple, S. bosqi ovipositó en 9 de las 11 especies de plantas analizadas. Los resultados indican que la selección de hospederos y el desarrollo de S. bosqi no se limita a las malas hierbas objetivo, sino que también incluyen valiosas especies no objetivo. Por lo tanto, S. bosqi no es lo suficientemente específico para el hospedero como para ser considerado como un agente de control biológico de especies exóticas de Ludwigia en los EE. UU. y no se garantizan pruebas adicionales.
Ludwigia species (Onagraceae) were introduced from Central and South America to locations worldwide as ornamentals in the mid-nineteenth century (Wagner et al. 2007; Grewell et al. 2016a). A select group from the largely aquatic Ludwigia section Jussiaea (Hoch et al. 2015) have invaded both aquatic and riparian ecosystems (Thouvenot et al. 2013; Kim et al. 2019) and now are considered among the most aggressive weeds in the world (Cronk & Fuller 2001). This is particularly evident in the western and southeastern coastal regions of the US where 4 Ludwigia taxa have naturalized in aquatic systems (Grewell et al. 2016a): Ludwigia hexapetala (Hook. & Arn.) Zardini, Gu & P. H. Raven, Ludwigia peploides (Kunth) P. H. Raven subsp. peploides, Ludwigia peploides (Kunth) P. H. Raven subsp. montevidensis (Spreng.) P. H. Raven, and Ludwigia grandiflora (Michx.) Greuter & Burdet (all Onagraceae). These species form dense mats that impact ecological processes in aquatic ecosystems, including the displacement of desired wildlife and vegetation (Stiers et al. 2011; Thouvenot et al. 2013; Grewell et al. 2016b, 2019; Khanna et al. 2018). They also impede navigation and interfere with recreational activities, irrigation, drainage, and agricultural production (Thouvenot et al. 2013; Grewell et al. 2016a). The invasive potential of these taxa often is attributed to habitat eutrophication, adaptation through hybridization, phenotypic plasticity, vegetative and sexual modes of reproduction, and a general lack of specialized herbivores in the introduced range that regulate plant population growth (Grewell et al. 2016a, b; Reddy et al. 2021).
Management of exotic Ludwigia spp. in the US has relied on physical and chemical methods (Thouvenot et al. 2013); however these options often provide short term control and require repeated annual treatments (Sarat et al. 2015, 2018; Grewell et al. 2016a), which also are costly. For example, in the US, the Division of Boating and Waterways, Sacramento, California, USA, spends $7 million per yr to control invasive plants, including Ludwigia spp., in the Sacramento–San Joaquin River Delta in northern California (Brusati 2009). In addition, L. hexapetala and L. peploides produce viable seeds with a high capacity for germination under a wide range of temperatures (Gillard et al. 2017a, b) resulting in persistent seedbanks that require long-term management programs (Grewell et al. 2019). Additional tools are needed, specifically in environmentally sensitive systems where herbicide use is limited or not permitted (Grewell et al. 2016a). One sustainable and long-term alternative under consideration since the 1970s is the use of natural enemies to control exotic Ludwigia spp. in the US (i.e., biological control) (Cordo & DeLoach 1982a, b). The first foreign explorations for natural enemies of Ludwigia spp. were conducted by Cordo and DeLoach (1982a, b) in Argentina where they reported 5 beetle species. A more recent and comprehensive survey was conducted by Hernández and Cabrera Walsh (2014) in Argentina, which enumerated 19 insect species across 6 feeding guilds that feed on L. hexapetala. Among the described species, the defoliating weevil Sudauleutes bosqi Hustache (= Auleutes bosqi) (Coleoptera: Curculionidae) (Colonnelli 2004), was observed commonly in both surveys feeding on Ludwigia spp. (Cordo & DeLoach 1982a; Hernández & Cabrera Walsh 2014).
Little is known concerning the life history and host specificity of S. bosqi. Adults are small, reddish-brown, and feed on the surface of leaves, removing about 14.3 mm2 of foliar surface area per d (Cordo & DeLoach 1982a). The larvae prefer to feed on young apical leaves, and pupation occurs within a spherical cocoon at the base of the plant (Hernández & Cabrera Walsh 2014). Moreover, based on field observations, Cordo & DeLoach (1982a) suggested S. bosqi was a possible candidate biological control agent of exotic Ludwigia spp. in the US and that its host range probably was limited to the genus Ludwigia in Argentina. However, aside from reporting the basic life history of S. bosqi adults, neither study investigated the biology and physiological host range of S. bosqi. Formal host specificity testing is needed to quantify the diet breadth of S. bosqi in relation to the diverse native Ludwigia species in the US (Reddy et al. 2021). Filling this knowledge gap, coupled with the renewed interest in Ludwigia biological control in the last decades (Reddy et al. 2021), led to surveys in Argentina and Uruguay in 2019, with a specific focus on collecting and colonizing S. bosqi for the present study.
Therefore, the primary objective of this research was to test the hypothesis that S. bosqi is host specific to plants within the Ludwigia section Jussiaea, which is required for a suitable biological control agent in the US given that there are no native representatives of the Jussiaea (Reddy et al. 2021). To accomplish this goal, no-choice and multiple-choice host range tests were conducted with an initial suite of 11 plant species that represented 3 exotic Ludwigia targets (L. hexapetala, L. peploides subsp. peploides, and L. peploides subsp. montevidensis) and 8 native US taxa. In addition, biological characteristics of S. bosqi were investigated to aid in interpreting herbivore performance across host plants, and thus supplement existing knowledge for this species.
Materials and Methods
ORIGIN AND REARING OF SUDAULEUTES BOSQI
Sudauleutes bosqi were collected from L. hexapetala plants on the edge of Laguna del Diario (34.8969904°S, 55.0033590°W), Uruguay during Mar 2019. The nascent S. bosqi colony was exported from Uruguay under scientific collection permit N° 9/2019 supplied by the Dirección Nacional de Medio Ambiente and imported under USDA APHIS-PPQ permit #P526P-19-03070 to a USDA-ARS containment facility in Albany, California, USA. Species identity was confirmed by the USDA-ARS Systematic Entomology Laboratory at the Smithsonian Institution, National Museum of Natural History, Washington, DC, USA, based on specimens in the US National Collection, and identified by Enzo Colonnelli. Vouchers were deposited in that institution. The colony was maintained on a laboratory benchtop under ambient temperature (20–25 °C), lighting, and humidity conditions. Adults were kept in a cylindrical 947 mL plastic containers (14.5 cm H × 11.5 cm D) with a piece of fine mesh cloth integrated into the lid to allow air circulation and prevent condensation. Approximately 15 adults per container fed and reproduced on a bouquet composed of 3 excised L. hexapetala stems (15 cm long) inserted into a plastic floral water tube. Stems were changed weekly. Between feedings, water was added to floral tubes as needed to maintain plant turgor. Periodically, older bouquets harboring eggs were retained and reared to augment colony numbers. The colony was reared exclusively on L. hexapetala, originally collected from the Sacramento–San Joaquin River Delta in northern California (38.002453°N, 121.568594°W). All subsequent biology and host range experiments were conducted in an environmental chamber set to constant 25 °C (± 1 °C), with a 14:10 h (L:D) photoperiod.
EGG DEVELOPMENT OF SUDAULEUTES BOSQI
Fresh bouquets of L. hexapetala stems were provided to all adult colony containers described above. Plant material was removed after 24 h and all eggs were collected. Individual eggs were cut from foliage, mixed with individuals from other colony containers, and randomly spread across 15 replicate Petri dishes (90 mm diam; Fisher Scientific, Waltham, Massachusetts, USA). Each Petri dish contained 10 eggs that were placed carefully on sterile filter paper (Whatman No. 2; Fisher Scientific, Waltham, Massachusetts, USA) moistened with water pri- or to sealing the dish with Parafilm® (Bemis Company, Inc., Neenah, Wisconsin, USA) to avoid desiccation. Replicated Petri dishes were arranged in a completely randomized design in a chamber and their position was rotated daily when egg hatching was monitored. Water was added as needed to keep the filter paper moist. Mean development time (d) and egg viability (larval hatching proportion: larvae hatched divided by eggs monitored) per Petri dish were calculated. Additionally, egg size was measured from 20 randomly selected eggs that originated from different parental females. Eggs were measured from pole to pole across the long side using a dissecting microscope (Olympus Corporation, Shinjuku, Tokyo, Japan) equipped with an ocular micrometer (Nikon, Minato, Tokyo, Japan).
LARVAL DEVELOPMENT OF SUDAULEUTES BOSQI
Twenty neonate larvae (≤ 24 h old) from the egg development study were collected at random and transferred individually using a fine brush onto the young leaves of a L. hexapetala stem (10 cm long) inserted into a floral water tube situated within an enclosed cylindrical 237 mL plastic container (4.0 cm H × 11.5 cm D). Fresh stems were provided weekly and water in the floral tube was replenished 3 times per wk. Larvae were monitored daily and developmental stage (visualized by the presence of exuviae) was recorded until adult metamorphosis. On the d each molt occurred, head capsule size was measured at the widest point (genae) using a dissecting microscope (Olympus Corporation, Shinjuku, Tokyo, Japan) equipped with an ocular micrometer (Nikon, Minato, Tokyo, Japan). Subsequently, the number of instars, head capsule size (mm) of each instar, and development time (d) of each stage were calculated. Total development time from egg to adult was calculated by adding mean egg, larval, and pupal development times. Finally, the length of 30 randomly selected adults from the colony (15 females and 15 males) was measured from the most forward part of the head (at the frons between the eyes) to the last abdominal segment. Because it was not possible to separate the sexes using morphological characters, females were identified by conducting 48 h oviposition tests; adults were placed singly in filter paper-lined Petri dishes containing a L. hexapetala leaf, with wet cotton wrapped around the petiole base. Water was added to the cotton after 24 h to prevent wilting. After 48 h, foliage was checked for the presence of eggs to differentiate females from males.
HOST RANGE EXPERIMENTS: TEST PLANTS
The test plant list was comprised of 11 taxa from the Onagraceae: 3 exotic Ludwigia targets (L. hexapetala, L. peploides subsp. peploides, and L. peploides subsp. montevidensis), 7 native taxa (Ludwigia polycarpa Short & Peter, Ludwigia repens J. R. Forst., Ludwigia palustris (L.) Elliott, Epilobium ciliatum Raf. subsp. ciliatum, Epilobium canum (Greene) P. H. Raven, Clarkia amoena (Lehm.) A. Nelson & J. F. Macbr., and Oenothera elata Kunth subsp. hookeri (Torr. & A. Gray) W. Dietr. & W. L. Wagner [all Onagraceae]) and Ludwigia decurrens Walter (Onagraceae), a congener that is sympatric with the target weeds and native to eastern-central US. Ludwigia decurrens is non-native to California where it established around 2011 as a noxious weed in rice fields (Kelch 2015). The native test species (non-targets) were selected based on their phylogenetic relationship to the 3 target species (Reddy et al. 2021). All test species were used in both no-choice and multiple-choice host range experiments. Plants were propagated over time in a greenhouse under controlled temperature (20–32 °C), a 14:10 h (L:D) photoperiod, and ambient humidity conditions. They were incorporated into host-range tests as available, and always included L. hexapetala as the control.
NO-CHOICE DEVELOPMENT AND OVIPOSITION TESTS
Four neonate larvae (≤ 24 h old) randomly were assigned a host plant species and transferred with a fine brush onto the young leaves of a 10 cm long stem (experimental unit) inserted into a floral water tube. Five replicate stems were placed individually in a cylindrical 473 mL plastic container (7.5 cm H ' 11.5 cm D) (4 neonates × 5 replicate stems = 20 larvae per test plant species; n = 11 plant species). Larvae were transferred to fresh stems of their assigned test plant species twice per wk. Water in the floral tubes was replenished 3 times per wk during which time the larvae were observed under a dissecting microscope (Olympus Corporation, Shinjuku, Tokyo, Japan) to record survival and developmental stage. Larval survival rate (proportion) and mean development time from first instar to adult (n = 4 larvae per replicate) were calculated for each replicate stem.
The resulting adults from the no-choice development tests were collected and grouped by emergence date. Following the colony rearing methods described above, adults were kept in a rearing container for 1 wk to allow sexual maturation and mating, and they were fed the plant species from which they emerged. Females were identified by conducting 48 h oviposition tests described above, then each was paired with 1 male and placed in a 473 mL plastic container together with a bouquet of 2 to 3 stems (10 cm long) of the plant species on which the female was reared. This process was repeated until 5 replicate females per test plant were evaluated. Some test plant species had less replicates evaluated because the number of emerged females dictated the number of replicates. Adult males (1–4 wk old) from the colony were used if there were not enough males from the experiments described above. Eggs were collected from each bouquet after 8 to 10 d and counted under a dissecting microscope (Olympus Corporation, Shinjuku, Tokyo, Japan). Eggs from each female then were placed on moistened filter paper as described above and egg viability (hatching) was monitored daily until all eggs hatched or became shriveled (indicating mortality). Water was added to the filter paper as needed during monitoring. Subsequently, the number of eggs oviposited and egg viability (larval hatching proportion: larvae hatched divided by eggs oviposited) was calculated for each replicate female.
MULTIPLE-CHOICE OVIPOSITION TESTS
Experiments were conducted using colony adults (1–4 wk old). Gravid females were identified by conducting 48 h oviposition tests as described above. Five adult pairs were placed in a plastic container (36 cm L × 28 cm W × 24 cm H) together with 3 to 5 bouquets (1 bouquet per test plant). Five replicate bouquets per plant species were assessed (5 replicates × 11 plant species = 55 bouquets total). Each bouquet was composed of 2 stems (15 cm long) from a single test plant species inserted into a floral water tube as a potential source for feeding and oviposition. The side walls of the container were modified with a piece of fine mesh cloth to allow air circulation and prevent condensation within the container. Adults were collected and returned to the colony after an oviposition period of 4 d and eggs oviposited on each bouquet were counted. The presence of feeding damage was noted, but not quantified. The experimental setup was repeated over time in 3 separate trials where a different set of plant species, including L. hexapetala as the control, was tested (i.e., 5, 5, and 3 plant species per trial).
DATA ANALYSES
Data were tested for normality using Shapiro-Wilk tests. Larval hatch and survival data (proportion) were arcsine square-root transformed; eggs per female, eggs per plant, and adult body length data were square-root transformed; and mean larval development data were log10 transformed to normalize results prior to analyses. One-way ANOVAs were then used to compare body length between female and male adults and to compare larval survival, mean larval development time, eggs per female, and egg viability (larval hatching proportion per female) among plant species in no-choice tests. A linear mixed model was used to test for the effect of test species on oviposition (eggs per plant) in multiple-choice tests. Post-hoc pairwise comparisons between test species were made with Tukey's HSD (α = 0.05). Plant species on which larvae failed to survive or females did not oviposit were omitted from the analyses. All analyses were conducted using JMP® PRO, version 15 (SAS 2019).
Results
LIFE HISTORY OF SUDAULEUTES BOSQI
Sudauleutes bosqi completes 7 stages during development: egg, 3 larval instars, prepupa, pupa, and adult. Generation time from egg to adult was 17.6 ± 1.2 d (range 17.3–21.3 d; n = 12; hereafter means are reported with ± 1 SD) when feeding on L. hexapetala and reared at 25 °C.
Females oviposited eggs singly below the epidermis of the leaf lamina into holes chewed by the female, usually on the margins but occasionally in the interior of the leaf. Eggs were light yellow and slightly oval in shape with symmetrical round poles. Mean length was 0.5 ± 0.04 mm (range 0.4–0.6 mm; n = 20). Mean development time from oviposition to hatch was 3.3 ± 0.4 d (range 2.7–3.8 d; n = 15), and an average proportion of 0.9 ± 0.1 (range 0.6–1; n = 15) of those eggs were viable.
Neonate larvae were transparent yellow with a black head. Average development time of the first, second, and third larval instars were 2.6 ± 1.1 d (range 2–6.5 d; n = 19), 2.3 ± 1 d (range 1–5.5 d; n = 19), and 2.6 ± 1.1 d (range 2–5.5 d; n = 11), with an average head capsule size of 0.2 ± 0.01 mm (n = 20), 0.4 ± 0.03 mm (n = 19), and 0.5 ± 0.03 mm (n = 19), respectively. Larvae ceased feeding at the end of the third larval stage, the body became opaque yellow and moved to the base of stems (i.e., bouquet). The larvae then formed a spherical pupal case that was attached to a moist surface, typically where the stems meet the lid of the plastic water tube but occasionally on a substrate at the bottom of the rearing container. Because not all larvae built a pupal case, it was possible to measure the duration of the prepupal stage for some individuals. Mean prepupal and pupal periods were 1.5 ± 0 d (n = 8) and 6.6 ± 1.3 d (range 5–10 d; n = 12). Total larval development time (neonate to adult) was 14.3 ± 1.2 d (range 14–18 d; n = 12).
Table 1.
Larval survival and development (first instar to adult), oviposition, and egg viability of Sudauleutes bosqi on exotic Ludwigia and native test plant species in no-choice host range tests. Mean ± 1 SE (n).
Newly emerged adults were tan and turned reddish brown after ≤ 24 h. Adults were observed feeding on apical and older leaves of L. hexapetala. There was no difference (F = 1.94; df = 1,28; P = 0.174) between average body length of females (2.6 ± 0.1 mm; range 2.4–2.9 mm; n = 15) and males (2.5 ± 0.1 mm; range 2.4–2.6 mm; n = 15).
NO-CHOICE HOST RANGE TESTS
Sudauleutes bosqi larvae did not survive on 4 native plant species (E. canum, E. ciliatum subsp. ciliatum, O. elata subsp. hookeri, and L. decurrens), but successfully completed development on the remaining 7 plant species tested: 3 exotic Ludwigia targets and 4 native species (Table 1). However, larval survival proportion did not differ among plant species that supported complete development (F = 1.75; df = 6,43; P = 0.132). In contrast, mean larval development time differed across plant species (F = 12.01; df = 6,20; P < 0.0001), which ranged from 15.75 (L. peploides subsp. montevidensis) to 24.00 d (L. repens). Development was faster on L. peploides subsp. montevidensis than on L. hexapetala, L. repens, and L. palustris (Tukey's HSD test, P ≤ 0.05). Development was slower on the native L. repens than on the 3 Ludwigia target weeds (L. hexapetala, L. peploides subsp. peploides, L. peploides subsp. montevidensis) and on C. amoena (Tukey's HSD test, P ≤ 0.05). Within the 3 Ludwigia target weeds, larval development time differed between L. hexapetala and L. peploides subsp. montevidensis (P = 0.046), but not between L. hexapetala and L. peploides subsp. peploides (P = 0.216) or between L. peploides subsp. peploides and L. peploides subsp. montevidensis (P = 0.956).
Oviposition was monitored on the 7 test plant species that supported complete larval development (Table 1). However, L. polycarpa and C. amoena were excluded from the analyses because there was only 1 replicate (of 5) each where 1 and 4 eggs were oviposited (larval hatching proportion of 1.00 and 0.14), respectively. Ludwigia palustris also was excluded because no oviposition occurred on this test plant. For the remaining 4 test plant species, no difference in total eggs per female (F = 1.22; df = 3,12; P = 0.346) or larval hatching proportion (F = 2.73; df = 3,12; P = 0.090) was observed among plant species.
MULTIPLE-CHOICE HOST RANGE TESTS
Of the 11 plant species tested, S. bosqi did not oviposit on L. decurrens and E. canum (Table 2). The number of eggs oviposited per plant differed among test plant species (F = 22.95; df = 8,42.36; P < 0.0001). The highest number of eggs were oviposited on L. hexapetala, which differed from L. palustris, C. amoena, E. ciliatum subsp. ciliatum, and O. elata subsp. hookeri (Tukey's HSD test, P ≤ 0.05). Clarkia amoena, E. ciliatum subsp. ciliatum, and O. elata subsp. hookeri received the lowest number of eggs and they differed from L. peploides subsp. peploides, L. peploides subsp. montevidensis, L. polycarpa, and L. repens (Tukey's HSD test, P ≤ 0.05). Oviposition did not differ among the 3 Ludwigia target weeds (Tukey's HSD test, P > 0.05). Adult feeding damage was observed on all test plant species, except E. canum and L. decurrens, and minimally on C. amoena, E. ciliatum subsp. ciliatum, and O. elata subsp. hookeri.
Discussion
Life history characteristics and host range of S. bosqi were examined as part of a risk assessment to determine its potential as a candidate biological control agent in the US. Particular interest in this species was placed as previous reports prioritized this species for consideration (Cordo & DeLoach 1982a). Weevils have been particularly successful in controlling invasive plants throughout the world, including Eichhornia crassipes (Mart.) Solms (water hyacinth; Commelinales: Pontederiaceae), Salvinia molesta D.S. Mitch. (giant salvinia; Salviniales: Salviniaceae), Pistia stratiotes L. (water lettuce; Alismatales: Araceae), and Carduus thistles (Carduus acanthoides L. and Carduus nutans L.; Asterales: Asteraceae) (O'Brien 1995; Julien & Griffiths 1999; Kok 2001; Herrick & Kok 2010). Beyond biological control, data reported here also have relevance to general life history characteristics of S. bosqi, because no information on its biology has been reported until now, except for adult body length (about 2.5 mm) (Cordo & DeLoach 1982a). Results from this study show that there is no difference in length between S. bosqi females (2.6 mm) and males (2.5 mm). Sudauleutes bosqi belongs to the minute seed weevil subfamily Ceutorhynchinae (Colonnelli 2004) and its life history is similar to that of 2 confamilials and biological control agents, Euhrychiopsis lecontei Dietz (milfoil weevil) and Rhinoncomimus latipes Korotyaev (mile-a-minute weevil) (both Coleoptera: Curculionidae). The size of S. bosqi adults and egg length (about 0.5 mm) were similar to E. lecontei, which are 2 to 3 mm and 0.5 mm, respectively (MAISRC 2021). Generation time of S. bosqi (about 17 d) is 3 and 5 to 6 d faster than E. lecontei (Mazzei et al. 1999) and R. latipes, respectively, at 25 °C (Hough-Goldstein et al. 2016). The fast generation time of S. bosqi greatly facilitated its rearing and experimentation during host specificity testing. The early larval instars are not difficult to rear. Higher mortality was observed, however, between the late third instar and pupal stage, possibly due to pupal requirements for a moist environment that was difficult to maintain in the laboratory setting (Cordo & DeLoach 1982a).
Table 2.
Eggs oviposited by Sudauleutes bosqi females on exotic Ludwigia and native test plant species in multiple-choice host range tests. Mean number of eggs ± 1 SE (n).
We found no evidence to support the hypothesis that S. bosqi is host specific to species within the Ludwigia section Jussiaea, herein represented by the target weeds L. hexapetala, L. peploides subsp. peploides, and L. peploides subsp. montevidensis. Under no-choice conditions, S. bosqi larvae fed and completed development on 7 of the 11 plant species tested: all 3 target weeds and 4 native species (L. polycarpa, L. repens, L. palustris, and C. amoena). There was no difference in S. bosqi survival among host species, but development time varied across species with higher levels of variability among the native species (Table 1). Interestingly, no development occurred on the closely related L. decurrens as compared to more distantly related hosts. We hypothesize that unlike other Ludwigia species, L. decurrens is not part of the host range of S. bosqi because the growth habitat requirements of this plant preclude any association between S. bosqi and L. decurrens. In its native range, L. decurrens is found in habitats similar to that of other Ludwigia species from the Macrocarpon section and S. bosqi has not been found on these plant species (ADS personal observation). In contrast, Ludwigia species associated with S. bosqui (L. hexapetala and L. peploides spp.) have growth habits similar to that of L. repens and L. palustris, which also are part of the host range of S. bosqi.
While larval survival and development provides important insights to host specificity, comparing adult fitness between individuals reared on different species can reveal sublethal effects of suboptimal hosts. Oviposition patterns separated test plant species that supported complete development into 2 groups: the 3 target weeds versus the 4 native species (Table 1). The number of eggs oviposited per female ranged from 24 to 36 on the target weeds but was consistently lower (0–9.5 eggs per female) on 4 native plant species (L. polycarpa, L. repens, C. amoena, and L. palustris). The combined effect of limited oviposition (L. palustris) and replication (L. polycarpa and C. amoena) in no-choice tests precluded comparing oviposition between a greater sample of native plant species and the target weeds. Nevertheless, the number of eggs oviposited and subsequent viability of these eggs did not differ between the 3 target weeds and the native L. repens (Table 1), suggesting no apparent decrease in fitness over a generation of feeding exclusively on the test plant species. It is surmised from development and oviposition data that several native plants included in this study are likely to support sustained S. bosqi populations for more than the 1 generation.
Whereas S. bosqi larvae may lack host specificity, females can restrict host use through selective oviposition. Therefore, multiple-choice tests were conducted to provide insights to the herbivore's ovipositional host plant selection preferences. Herein, however, S. bosqi females did not demonstrate a strong ovipositional preference for species in the Jussiaea section of Ludwigia over native conspecifics (Table 2). Oviposition occurred on all but 2 (L. decurrens and E. canum) of the 11 plant species tested. Females oviposited the most eggs on the 3 target weeds and 2 native plant species (L. polycarpa and L. repens), but oviposition did not differ between these 5 species. In contrast, females oviposited significantly fewer eggs on a separate group of 4 native plant species (L. palustris, C. amoena, E. ciliatum subsp. ciliatum, and O. elata subsp. hookeri), but oviposition did not differ between these species as well. Sudauleutes bosqi also oviposited on plant species that do not support development. The weevil oviposited on E. ciliatum subsp. ciliatum and O. elata subsp. hookeri, yet larval development tests showed that larvae cannot complete development on these species. These data suggest that S. bosqi females can oviposit broadly among hosts that range from optimal to unacceptable suitability for larval survival.
Collectively, these data indicate that S. bosqi is not a specialist of the Jussiaea section but rather an oligophagous herbivore of Ludwigia spp., and possibly related species (e.g., C. amoena). Sudauleutes bosqi did not distinguish Ludwigia spp. from C. amoena during development, but preferred Ludwigia spp. to C. amoena in multiple-choice oviposition tests. The findings are consistent with field observations of S. bosqi by Cordo and DeLoach (1982a) and Hernández and Cabrera Walsh (2014), who recorded adults feeding on Ludwigia peploides (Kunth) P. H. Raven, L. hexapetala, L. grandiflora, Ludwigia elegans (Cambess.) H. Hara, and Ludwigia leptocarpa (Nutt.) H. Hara. These data also indicate that the physiological host range of S. bosqi does not mirror the phylogenetic relationship of the Ludwigia species and their more distant relatives (i.e., development/oviposition on L. polycarpa, L. repens, L. palustris, and C. amoena but not on L. decurrens) (Reddy et al. 2021).
Although our results demonstrate that S. bosqi is not a suitable biological control agent for invasive Ludwigia spp. in the US, these should not be extended to presume S. bosqi is equally unsuitable for biological control in other parts of the world where exotic Ludwigia spp. also are problematic. The data reported herein are the first to quantify larval developmental parameters of S. bosqi, which are critical for estimating the host range of this herbivore. Sudauleutes bosqi may still be considered for introduction elsewhere and these data can guide future host range testing as well as facilitate the rearing and handling of these weevils in general.
Acknowledgments
The authors thank the staff from the Fundación para el Estudio de Especies Invasivas (FuEDEI), Argentina, and Pablo Calistro at the Instituto Nacional de Investigación Agropecuaria (INIA), Uruguay, for assistance in foreign surveys, and the anonymous reviewers for useful comments on an earlier version of the manuscript. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. USDA is an equal opportunity employer and provider.
