Abstract The evolutionary constraints on host plant preferences of herbivorous insects are a question of theoretical and practical importance. Since larvae have limited mobility, natural selection pressures should promote host plant choices by adults that optimise larval survival. Such optimal host utilisation is predicted to reinforce the evolution of specialisation. This concept is expressed and tested as the ‘preference‐performance hypothesis’ (PPH), which predicts a positive relationship between adult preference and larval performance. Through a series of choice and no‐choice experiments, the preference‐performance relationship was examined for the oligophagous tortoise beetle, Cassida rubiginosa , used for biocontrol of thistle weeds. It was hypothesised that adult host plant choices based on olfaction, final location, feeding intensity and oviposition are adaptive and reflect the optimal hosts for larval performance (survival). In accordance with the PPH, there was a positive relationship between all measures of adult preference and larval performance. Adult preference based on olfactometer experiments showed a significant but weak correlation, explaining 27.6% of the variation in larval survival. The strongest correlation was found for adult preference measured as the amount of feeding, explaining 72.5% of the variation in larval survival. Adult preference based on location at the end of the experiment showed a similarly strong correlation, explaining 69.4% of the variation in larval survival. Adult preference based on oviposition and the number of hatched larvae was moderately correlated with larval survival, explaining 46.8% and 48.7% of larval survival, respectively. While there is a significant positive relationship between preference and performance, adult host preference is only a moderate predictor of larval performance. The adult beetle showed substantial flexibility in host plant selection, and equal performance across most Cardueae hosts. This is likely due to the larvae being more sensitive to physical and chemical defences, and possibly an evolutionary lag in the host range between the life stages. Any expansion of the realised host range will be limited by the ability of the larva to adapt to suboptimal hosts.

The leaf-feeding beetle, Cassida rubiginosa, was introduced into New Zealand from Europe in 2007 as a biological control agent for Cirsium arvense (Californian thistle) and is now widely distributed. To determine the extent to which geographically variable climate might explain its observed sporadic impact, we developed a climate-niche model for each of the species using CLIMEX. The models reveal that the climate throughout most of New Zealand is variably suitable for both species, although everywhere relatively less suitable for the beetle. However, we found no evidence that this climate suitability bias favouring the thistle explains the sporadic impact of the beetle. First, 64 geographically separate beetle populations classified by field observers according to their impact on the thistle ('none', n = 29; 'low', n = 22; 'high', n = 13) showed no relationship with the modelled climate suitability (Ecoclimatic Index) of their locations for either the beetle or the thistle. Second, the two models reveal a similar climate suitability bias favouring the thistle throughout much of the species' 'eco-climatic suitability space' in their native range (Eurasia) where the beetle is a widespread natural enemy of the thistle. Hypothetical explanations for the sporadic impact of the beetle in New Zealand are explored, including lack of suitable overwintering habitat, predation, and incompatible thistle control operations. We conclude that in general, climate suitability, while a necessary condition, is alone insufficient for the success of a weed biological control program, and that non-climatic habitat factors may often explain spatially patchy impacts of natural enemies such as C. rubiginosa.

Silybum marianum (L.) Gaertner (variegated thistle, milk thistle) is one of the worst weeds of hill country pasture in the east coast regions of New Zealand. As part of an initial investigation into the potential for biocontrol, a survey of the phytophagous insects associated with S. marianum in New Zealand was carried out at 16 populations from 29 November to 16 December 2021. Phytophagous associations were determined by collecting insects on the plant and by seedhead dissections. The species collected were grouped into three categories: 'specialists', 'generalists', and 'tourists' that comprised 27.4%, 66.3%, and 6.3% of the total number of specimens collected, respectively. The specialist group was almost entirely comprised of the seedhead weevil, Rhinocyllus conicus (Fr & ouml;lich). While adults of R. conicus were commonly collected on variegated thistle, only 6.5% of seedheads contained larvae of the weevil, indicating that S. marianum is not a common developmental host. Generalist species commonly collected on S. marianum included: Nysius caledoniae Distant, Rhypodes clavicornis (F.), Closterotomus norwegicus (Gmelin) and Nezara virdula (L.). Overall, the field surveys document low levels of attack from generalist herbivores and minimal attack by the specialist weevil, R. conicus. Vacant niches are present on the weed in New Zealand, providing a strong case for introducing specialised biocontrol agents.

Silybum marianum (L.) Gaertner (variegated thistle, milk thistle) is one of the worst weeds of hill country pasture in the east coast regions of New Zealand. A recent field survey in New Zealand found low levels of attack from generalist herbivores and minimal attack by the specialist weevil, Rhinocyllus conicus (Frolich). In the native range of Europe there are at least 41 insects and 12 fungi sufficiently specialised to be considered for biocontrol. The literature search revealed some potential biocontrol agents not known during the historical biocontrol surveys for S. marianum. These include the specialised biotypes of the tephritid fly, Terellia fuscicornis Loew, and the seedhead weevil, Larinus latus Herbst. The literature search also revealed the discovery of the stem-galling cynpid wasps, Aulacidea freesei Nieves-Aldrey and Phanacis zwoelferi Nieves-Aldrey, that are likely specific to S. marianum. The discovery of the smut fungus, Microbotryum silybum Vanky & Berner, host specific to Silybum species, is another promising biocontrol candidate. A shortlist of nine priority agents (eight insects and one fungal pathogen) were selected based on reported affinity for S. marianum, evidence for impact on the weed, and niches attacked. In the case of S. marianum, there are sufficiently specialised agents that attack several niches, including seeds, stems, and the root or root collar. Testing for complementary agents that attack different niches is likely to improve the chance of successful biocontrol. It is recommended that S. marianum be considered a priority target for classical biocontrol in New Zealand.

This editorial summarises the first 75 years of the New Zealand Plant Protection Society. The structure of the Society and approaches to plant protection have changed over the years but the essence of the original vision to bring together industry, government, and university researchers, to ‘pool and exchange information’, is still relevant today.

BACKGROUND:Predicting the host range of biocontrol agents is important for the safe and effective implementation of biocontrol of weeds. In this study, we examined the phylogenetic pattern of host selection and acceptance by the biocontrol beetle, Cassida rubiginosa. The beetle was released in New Zealand for control of Cirsium arvense, its primary host plant, but has potential to attack many Cardueae (thistles and knapweeds) species. We conducted a series of no-choice and choice experiments and modelled the responses of Cassida rubiginosa in relation to phylogenetic distance from Cirsium arvense. RESULTS:The olfactory recognition (single odour) and preference (two odours) of the beetle showed a significant phylogenetic relationship. These relationships showed a high degree of correlation with 66.9% of the variation in olfactory recognition and 82.8% of the variation in olfactory preference explained by phylogeny. Where the beetle could contact plants, under no-choice conditions there was no phylogenetic pattern to host plant acceptance. However, under choice conditions, phylogenetic distance was a strong predictor of feeding and oviposition preference. These relationships showed a high degree of correlation, with 63.4% of the variation in feeding preference, and 89.0% of the variation in oviposition preference, explained by phylogeny. CONCLUSIONS:As far as we are aware, this is the first demonstration of an herbivorous insect that exhibits a phylogenetic pattern to olfactory host plant selection. Host plant utilisation by Cassida rubiginosa in New Zealand will be mostly restricted to Cirsium and Carduus species, with minimal potential for impact on other Cardueae weeds. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The thistle biocontrol beetle, Cassida rubiginosa is established in New Zealand, but often not sufficiently abundant to achieve control of the weed, Cirsium arvense (Californian thistle). Mass production of the beetle could enhance biocontrol efforts through supplemental and inundative releases. We carried out an initial test of a semiartificial diet (containing host plant material) designed for laboratory mass production of the beetle. Larval survival rates were tested on diets with three different concentrations of preservatives (full, half, and no preservative), and compared to a positive control (leaf disc of Cirsium arvense), and a negative control (water). Only larvae on the leaf disc developed to the adult stage. Of the diets, the longest survival time was on the full preservative diet, with a mean mortality time of 8.8 ± 0.6 days, and a maximum survival time of 21 days. Although no larvae completed development on the diets, some important progress was achieved: (1) Neonate larvae were mobile on the diet; (2) larvae fed on the diet; and (3) there was adequate control of microbial contamination without being acutely toxic to the larvae. Further development of a diet for Cassida rubiginosa should focus on nutritional components for larval development.

The leaf-feeding beetle, Cassida rubiginosa, is an oligophagous biocontrol agent capable of feeding on most species in the tribe Cardueae (thistles and knapweeds). The beetle was released in New Zealand in 2007, primarily to control Cirsium arvense (Californian thistle), with the recognition that it had potential to control multiple thistle weeds. The objective of this study was to test the impact of different densities of Cassida rubiginosa larvae (0, 50, 100, or 200 per plant) on the growth and reproductive performance of the annual thistle weed, Carduus pycnocephalus (slender winged thistle). Since the effectiveness of biocontrol agents is often enhanced when plants are stressed, different levels of growth constraint were imposed by growing the weed in different pot sizes (0.5, 1, 5, and 12 litres). We hypothesised that feeding damage by Cassida rubiginosa larvae would have a greater impact on the weed when grown in smaller pots, since root growth would be constrained, and the weed’s ability to compensate for feeding damage would be restricted. Contrary to our hypothesis, pot size had no effect on feeding damage by Cassida rubiginosa on Carduus pycnocephalus. As expected, most measures of plant performance increased with larger pot sizes, including plant height, biomass, and the number of seedheads per plant. The results of this study indicate that Cassida rubiginosa is unlikely to contribute to the control of Carduus pycnocephalus. Additional oligophagous biocontrol agents targeting the rosette stage and seed production should be considered for release in New Zealand.

The gall fly, Urophora stylata, was released in New Zealand in 1998 as a biocontrol agent for the thistle weed, Cirsium vulgare (Scotch thistle). In the summer of 2018, a survey was conducted to assess the field host range of the biocontrol agent in New Zealand. A random selection of 18 pasture populations under sheep and/or beef production, where C. vulgare was present, was surveyed to quantify the attack intensity (gall size relative to seedhead size) on C. vulgare, and the presence of attack on other thistle weeds within the same population. At each location, seedheads were collected from C. vulgare and all other thistle species (Cardueae) present, which included Cirsium arvense (Californian thistle), Cirsium palustre (marsh thistle), Carduus nutans (nodding thistle), and an Arctium species (burdock). In addition to attack on C. vulgare, the gall fly was recorded on C. arvense (at six locations) and C. palustre (at one location). The probability of the presence of attack on C. arvense was positively correlated with the attack intensity on C. vulgare, suggesting that attack on C. arvense is a ‘spill-over effect’ occurring where seedheads of C. vulgare are in limited supply.

The gall fly, Urophora stylata F. (Diptera: Tephritidae), was released in New Zealand in 1998 as a biocontrol agent against the thistle weed, Cirsium vulgare (Savi) Tenore. A survey of 20 randomly selected pasture populations of C. vulgare was conducted to assess the impact of the gall fly on seed production. Attack by the biocontrol agent reduced the number of seeds per seedhead by 47%, individual seed weight by 21%, and seed germination rate by 30%. Where the biocontrol agent was present, population seed reduction ranged from 11 to 61%. The impact of the gall fly significantly increased from southern to more northern latitudes of C. vulgare populations in New Zealand. This study represents the first post-release assessment of this biocontrol agent and indicates that U. stylata can have a significant impact on the seed production of C. vulgare, especially in northern New Zealand where seedhead attack is greatest.

The pasture weed, Cirsium arvense (Californian thistle), is notorious for its ability to tolerate defoliation by herbivores, mowing, or herbicides, which is facilitated by the growth of adventitious shoots from its extensive clonal underground root system. In an outdoor potted-plant experiment, we examined the tolerance of 36 unique genotypes of C. arvense to defoliation by establishing pairs of clonal replicates that were assigned to a clipped, or unclipped treatment. Three clipping treatments were applied, and the final height, number of shoots, and biomass, were measured to compare the fitness between the clipped and unclipped clones. The majority of genotypes were negatively affected by clipping and showed a reduction in most final fitness measurements. However, some genotypes were equivalent or even greater than their unclipped counterparts indicating a large genetic range for tolerance to defoliation. The mean range in tolerance ratios (clipped/ unclipped) was 0.17 to 1.3 for shoot height, 0.26 to 1.2 for shoot density, and 0.6 to 1.2 for biomass (where a ratio of 1 indicates equivalence to the unclipped state). Since repeated defoliation is recommended for control of this weed, selection for more tolerant genotypes is possible, which may have management implications.

Numerous adaptations are gained in light of a symbiotic lifestyle. Here, we investigated the obligate partnership between tortoise leaf beetles (Chrysomelidae: Cassidinae) and their pectinolytic Stammera symbionts to detail how changes to the bacterium's streamlined metabolic range can shape the digestive physiology and ecological opportunity of its herbivorous host. Comparative genomics of 13 Stammera strains revealed high functional conservation, highlighted by the universal presence of polygalacturonase, a primary pectinase targeting nature's most abundant pectic class, homogalacturonan (HG). Despite this conservation, we unexpectedly discovered a disparate distribution for rhamnogalacturonan lyase, a secondary pectinase hydrolyzing the pectic heteropolymer, rhamnogalacturonan I (RG-I). Consistent with the annotation of rhamnogalacturonan lyase in Stammera, cassidines are able to depolymerize RG-I relative to beetles whose symbionts lack the gene. Given the omnipresence of HG and RG-I in foliage, Stammera that encode pectinases targeting both substrates allow their hosts to overcome a greater diversity of plant cell wall polysaccharides and maximize access to the nutritionally rich cytosol. Possibly facilitated by their symbionts' expanded digestive range, cassidines additionally endowed with rhamnogalacturonan lyase appear to utilize a broader diversity of angiosperms than those beetles whose symbionts solely supplement polygalacturonase. Our findings highlight how symbiont metabolic diversity, in concert with host adaptations, may serve as a potential source of evolutionary innovations for herbivorous lineages.

Cirsium arvense (Californian thistle) is a problematic weed in agricultural systems throughout New Zealand and the rust fungus Puccinia punctiformis is a potential biological control agent for this weed. Puccinia punctiformis can systemically infect thistles but the movement of the pathogen in planta is not fully understood. This research determined the level of infection in planta caused by P. punctiformis at a single time point. The concentration of P. punctiformis DNA in planta was determined to ascertain the location of the fungus within naturally field-infected C. arvense. Quantitative polymerase chain reaction was undertaken on above-ground symptomatic and asymptomatic C. arvense tissue at various locations within leaves (top, middle and bottom) and the main stem. All C. arvense shoots had detectable amounts of P. punctiformis but the concentration was 100× greater in symptomatic compared with asymptomatic shoots. In general, the concentration of fungus progressed up the leaves with a significant effect between locations (P<0.001). Puccinia punctiformis was found in planta but broadscale disease of C. arvense does not occur and the reason for this is unknown.

Intensification of pastoral farming in the temperate world has seen a dramatic shift from the botanically diverse native grasslands (e.g. tussock grasslands of New Zealand, prairie and Pacific grasslands of North America, pampas of South America and steppes of Europe) where many grasses, forbs and shrubs coexisted to botanically depauperate systems. In the extreme, such as in high-intensity flatland dairy farming, high farm production targets may be achieved with a pasture composed of as few as two species such as Lolium perenne and Trifolium repens. In less-intensive farm systems, such as sheep or beef cattle grazing on hill lands, a botanically more diverse pasture is common and often acceptable. Weed control is central to the establishment, maintenance and sustained productivity of these managed pastures. In particular, it is necessary to control non-palatable, injurious and poisonous species that would otherwise reduce livestock carrying capacity or impose other costs on the farm system such as those associated with reduced animal product quality, health and welfare (Fig. 1). These costs can be substantial (Jones et al., 2000; Sinden et al., 2004; Bourdôt et al., 2007a; Kaye-Blake et al., 2010).

The folivorous beetle, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), was released in New Zealand in 2007 as a biocontrol agent against the pasture weed, Cirsium arvense (L.) Scop. The impact of the beetle on shoot population density and spread was assessed over two years on isolated experimental plots within an established population of the weed. Four folivory treatments were imposed by applying 0, 5, 10 or 20 larvae per shoot. Folivory in the 10 and 20 larvae per shoot treatments caused C. arvense population declines of 29% and 75%, respectively, although this effect was not consistent between the two years. Shoot spread was reduced in both years where 10 or 20 larvae per shoot were applied. This study represents the first post-release assessment of this biocontrol agent in New Zealand, and indicates that average densities of ≥ ten larvae per shoot can reduce population density and spread of C. arvense.

The leaf-feeding beetle, Cassida rubiginosa Müller, was introduced to New Zealand in 2007 as a biological control agent against the pasture weed, Cirsium arvense (L.) Scop. (Californian thistle). The beetle is now established in most regions, but outbreak populations have only been observed in one region (Wairarapa). The overwintering habitat type might affect the survival of the beetle and its ability to achieve outbreak populations. We tested the overwintering survival of C. rubiginosa in three different habitats: radiata pine forest, native forest, and pasture. In autumn 2016, adult diapausing C. rubiginosa were placed in overwintering cages established in the three habitats, and in spring their survival was assessed. Temperature and relative humidity was recorded in each habitat, and predatory spiders were also sampled. The total recovery rate of overwintered beetles was greatest in the native forest (42 ± 7.9%), followed by ungrazed pasture (32.5 ± 7.5%), and pine forest (7.5 ± 4.2%). The total percentage recovered was not significantly different between the native forest and pasture habitats, but both of these habitats had greater recovery rates than the pine habitat. The native forest provided the best insulation against temperature and humidity extremes on a daily basis, and across the winter season, and is the likely explanation for the greater recovery rates in that habitat. This study has provided initial evidence that the particular overwintering habitat may contribute to the success of this biocontrol agent, and suggests that native forest is likely to provide a better habitat than pine forest.

The folivorous beetle Cassida rubiginosa was introduced to New Zealand to control the weed, Californian thistle (Cirsium arvense). Although Californian thistle is the primary host, many other thistles are accepted hosts. The objective of this study was to test if the beetle can reduce the fitness of marsh thistle (Cirsium palustre). A potted plant experiment was established with four treatments (0, 50, 100, and 200 larvae/plant). Plant growth (width, height, and number of branches) and reproductive performance (number of flowers, seeds, seed weight and percent germination) parameters were measured. No significant differences were found for any of the measured parameters, except percent germination. Higher larval densities (100 and 200) resulted in approximately 10% less germination compared with lower densities (0 and 50). Under these experimental conditions, C. rubiginosa had minimal impact on the performance of marsh thistle. For the beetle to have an impact, it would likely need to attack smaller, non-bolting rosettes, or be combined with additional stressors that might be encountered in a natural field population.

Pectin, an integral component of the plant cell wall, is a recalcitrant substrate against enzymatic challenges by most animals. In characterizing the source of a leaf beetle’s (Cassida rubiginosa) pectin-degrading phenotype, we demonstrate its dependency on an extracellular bacterium housed in specialized organs connected to the foregut. Despite possessing the smallest genome (0.27 Mb) of any organism not subsisting within a host cell, the symbiont nonetheless retained a functional pectinolytic metabolism targeting the polysaccharide’s two most abundant classes: homogalacturonan and rhamnogalacturonan I. Comparative transcriptomics revealed pectinase expression to be enriched in the symbiotic organs, consistent with enzymatic buildup in these structures following immunostaining with pectinase-targeting antibodies. Symbiont elimination results in a drastically reduced host survivorship and a diminished capacity to degrade pectin. Collectively, our findings highlight symbiosis as a strategy for an herbivore to metabolize one of nature’s most complex polysaccharides and a universal component of plant tissues.

Defoliation has frequently been proposed as a means of controlling Cirsium arvense (L.) Scop. (Californian thistle, Canada thistle, creeping thistle, perennial thistle), an economically damaging pastoral weed in temperate regions of the world, but its optimization has remained obscure. We developed a matrix model for the population dynamics of C. arvense in sheep-grazed pasture in New Zealand that accounts for the effects of aerial shoot defoliation on a population's photosynthetic opportunity and consequential overwintered root biomass, enabling mowing regimes varying in the seasonal timing and frequency of defoliation to be compared. The model showed that the long-term population dynamics of the weed is influenced by both the timing and frequency of mowing; a single-yearly mowing, regardless of time of year, resulted in stasis or population growth, while in contrast, 14 of 21 possible twice-yearly monthly mowing regimes, mainly those with mowing in late spring, summer, and early autumn, resulted in population decline. Population decline was greatest (with population density halving each year) with twice-yearly mowing either in late spring and late summer, early summer and late summer, or early summer and early autumn. Our results indicate that mowing can be effective in reducing populations of C. arvense in pasture in the long term if conducted twice each year when the initial mowing is conducted in mid spring followed by a subsequent mowing from mid summer to early autumn. These mowing regimes reduce the photosynthetic opportunity of the C. arvense population and hence its ability to form the overwintering creeping roots upon which population growth depends.