SHORT COMMUNICATION

 

Comparative utilisation of exotic and native Solanum (Solanaceae) species by Chnootriba hirta (Thunberg) (Coccinelidae), a native herbivorous ladybird

 

 

Terence Olckers

School of Agriculture and Science, University of KwaZulu-Natal, Scottsville, South Africa

Correspondence

 

 

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ABSTRACT

Chnootriba hirta (Thunberg) feeds on the foliage of several native Solanum species in South Africa, with occasional associations with introduced congeners. In comparing the beetle's performance and preferences across three exotic and one native Solanum species, this study produced an unusual result. During adult and larval no-choice tests, the exotic Solanum americanum Miller proved the most suitable host plant, followed by the native S. dasyphyllum Schumacher and Thonning, with the exotic cultivated S. lycopersicum Linnaeus (tomato) proving marginally suitable. In contrast, the exotic S. viarum Dunal, an invasive weed in the southern USA, did not support feeding and development. During adult choice tests, C. hirta did not discriminate between S. americanum and S. dasyphyllum, but avoided S. lycopersicum. These differences in host-plant suitability may relate, in part, to the presence of glandular leaf trichomes, renowned anti-herbivore defences, on S. viarum and S. lycopersicum, but not on S. americanum or S. dasyphyllum. Since C. hirta, a broadly oligophagous herbivore within the native Solanum insect community, cannot exploit S. viarum, the plant may well expand its range in South Africa due to an escape from insect herbivory.

Keywords: Epilachnini, host-plant suitability, insect herbivory, Solanum americanum, Solanum viarum

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Chnootriba hirta (Thunberg) (Coccinelidae: Epilachnini), typically misidentified as Epilachna hirta (Thunberg) and Henosepilachna hirta (Thunberg) (see Tomaszewska and Szawaryn 2016), feeds on the foliage of several native and exotic Solanum species (Solanaceae) in its native southern African range (Olckers and Hulley 1989a, 1989b, 1991; Olckers et al. 1995). Known as the hairy potato ladybird, the beetle constitutes a minor pest of cultivated Solanum species, notably potatoes, tomatoes, and eggplant (Prinsloo and Uys 2015). However, utilisation of invasive Solanum species in the field is sporadic and damage levels are largely inconsequential (Olckers and Hulley 1989a, 1989b, 1991; Hill et al. 1993), a likely consequence of feeding preferences or anti-herbivore defences.

In this study, the performance and preferences of C. hirta were compared between three exotic and one native Solanum species, which are common and co-occur in the KwaZulu-Natal Midlands region of South Africa. These were the native S. dasyphyllum Schumacher and Thonning, the weedy S. americanum Miller and S. viarum Dunal, and the cultivated S. lycopersicum Linnaeus (tomato). Solanum dasyphyllum (previously called S. cf. acanthoideum E. Meyer) served as the control plant because, besides its coexistence with other test species in the field, it falls within the known host range of the beetle (Olckers and Hulley 1991; Olckers et al. 2024). Although C. hirta feeds on a wide range of native Solanum species across several South African provinces (Olckers and Hulley 1989a, 1989b, 1991; Olckers et al. 1995, 2024), there is no evidence of preferences among native hosts.

Native to the Neotropics and often misidentified as S. nigrum Linnaeus or S. nodiflorum Jacquin (SANBI 2024), S. americanum is widely naturalised around the world and regarded as a minor weed in South Africa (Wells et al. 1986). Also of Neotropical origin, S. viarum is a major invasive species in the southern USA (Mullahey et al. 1993), but has minor weed status in South Africa (Welman 2003). Since an earlier survey revealed low insect herbivore diversity and abundance on S. viarum relative to S. dasyphyllum in the KwaZulu-Natal Midlands (Olckers et al. 2024), the aim of this study was to confirm whether this is due to host-plant preferences or effective anti-herbivore defences.

Adults and egg batches of C. hirta were collected on S. americanum plants growing in a paddock at the Ukulinga Research Farm (29°39'42" S, 30°24'14" E) of the University of KwaZulu-Natal (UKZN) and transferred to the laboratory at the UKZN Pietermaritzburg campus. Adults were used in no-choice and choice feeding trials, while hatching first-instar larvae were used in larval no-choice tests. Leaves of the four Solanum species were obtained from plants growing in the UKZN Botanical Garden. The leaf petioles were wrapped in moist paper towelling to preserve freshness. All trials were conducted at ambient room temperature (ca. 20 °C).

During no-choice tests, 12 adults were individually confined with an excised leaf of the test-plant species inside a large glass petri dish (14 cm diameter). The percentage of leaf material consumed was recorded after 48 hours, by comparing before and after images of the leaves taken with an Apple iPhone XS. Feeding scores were allocated as follows: 0 = 0%; 1 = 1-10%; 2 = 11-20%; 3 = 21-30%; 4 = 31-40%; and 5 = 41-50% of leaf tissue consumed. Each beetle was exposed to all four test-plant species over the trial period. During subsequent choice tests, leaves of the three species that supported feeding in the no-choice tests (see below) were provided to each of the 12 beetles, in large flat plastic containers (32 x 22 x 5 cm). After 48 hours, the relative amount of feeding on each plant species was ranked between 0 and 2, where 0 = no feeding, 1 = moderate feeding, and 2 = considerable feeding (i.e., twice as much as moderate).

During larval no-choice trials, five newly hatched first-instar larvae were placed on an excised leaf of the test-plant species inside a glass Petri dish (14 cm diameter). The larvae were monitored for survival and development until adult emergence, with fresh leaves provided every 2-3 days. The time taken from first-instar hatching until adult emergence was calculated and the mass of each emerging adult was recorded using a Sartorius analytical balance (0.1 mg resolution). Each trial was replicated five times, ensuring 25 larvae per test-plant species. Comparative scores of host suitability were calculated for species that supported development to adulthood, using the formula of Maw (1976) as follows: ((mean adult mass x % survival to adulthood)/mean developmental time to adulthood).

Data were analysed using IBM SPSS version 29. Since the assumptions of normality and equality of variances were not satisfied, statistical analyses involved non-parametric tests and generalized linear modelling. Adult feeding scores across the test-plant species were compared using Kruskal-Wallis tests, with Mann-Whitney U-tests used in pairwise comparisons. The proportions of larvae that survived to adulthood were compared across test-plant species using a model with a binomial distribution and logit link function. Developmental times and adult masses were compared using models with a Tweedie distribution and log link function. All models were analysed with Wald chi-square statistics and corrected for over-dispersion or under-dispersion, using scale-weight variables. Where overall differences were significant (p < 0.05), pairwise comparisons were conducted using sequential Bonferroni tests (larval survival) or Fisher's least significant difference tests (development).

There were significant differences in adult feeding among the test-plant species during the no-choice trials (χ² = 28.769, df = 3, p < 0.001). Feeding on S. americanum and S. dasyphyllum was similar, but significantly lower on S. lycopersicum, with no feeding recorded on S. viarum (Figure 1a). This pattern was sustained during the choice trials (χ² = 20.736, df = 2, p < 0.001), except that S. lycopersicum did not support feeding (Figure 1b). Similarly, there were significant differences in percentage larval survival among the test-plant species during the no-choice trials (χ² = 18.999, df = 3, p < 0.001). Survival to adulthood on S. americanum and S. dasyphyllum was also similar, but significantly lower on S. lycopersicum, with no survival on S. viarum (Figure 2). There were significant differences in developmental times to adulthood among the three test-plant species that supported larval development (χ² = 60.193, df = 2, p < 0.001). Development was quickest on S. americanum, followed by S. lycopersicum and S. dasyphyllum (Figure 3a). However, there were no significant differences (χ² = 2.013, df = 3, p = 0.365) in adult mass among these three test-plant species (Figure 3b). Overall, S. americanum proved the most suitable larval host (score of 75.5), with S. dasyphyllum (55.2) and S. lycopersicum (39.2) scoring 73% and 52% as suitable, respectively.

 

 

 

 

 

 

The feeding behaviour and larval survival of phytophagous beetles associated with Solanum species can vary between different testing methodologies, notably excised leaves versus leaves on intact plants (e.g., Risch 1985; Olckers and Hulley 1994). However, since all test species were presented to C. hirta as excised leaves, the direction of the observed trends, rather than the actual amounts of feeding or larval survival, are expected to be accurate.

Although native insect herbivores may colonise non-indigenous plants, providing some biotic resistance that can reduce their invasive properties (e.g., Maron and Vilá 2001), this has rarely occurred on invasive Solanum species, which remain largely free of natural enemy pressure (Olckers and Hulley 1989a, 1989b, 1991; Hill et al. 1993). Within the native Solanum insect herbivore community, C. hirta has displayed a wide host range, with a higher likelihood of attacks on introduced congeners than other specialist insect herbivores. Indeed, there are host records of C. hirta on at least eight native Solanum species (Olckers and Hulley 1989a, 1989b, 1991; Olckers et al. 1995), three invasive alien species (Olckers and Hulley 1989a, 1989b, 1991; Hill et al. 1993) and three cultivated species (Prinsloo and Uys 2015), albeit with higher incidence and numbers on native species.

Previous surveys of invasive Solanum species in the Eastern Cape Province revealed the presence of C. hirta in 10%, 17% and 29% of samples collected on S. elaeagnifolium Cavanilles (Hill et al. 1993), S. mauritianum Scopoli (Olckers and Hulley 1989a) and S. sisymbriifolium Lamarck (Hill et al. 1993), respectively, indicating some potential to utilise these species. However, despite an earlier record on S. viarum (Olckers et al. 2024), C. hirta failed to feed or develop on this species and succumbed as first instars. In contrast, C. hirta readily fed and developed on S. americanum, performing 27% better than on the native S. dasyphyllum. Despite no significant feeding preferences for S. americanum or S. dasyphyllum, the adults and larvae used in these trials originated from S. americanum in the field, even though both species grew in very close proximity, further suggesting higher host-plant quality in S. americanum. Although C. hirta is known to feed on S. americanum in the field (Prinsloo and Uys 2015), it is unusual for an exotic plant to exhibit higher host-plant quality than congeneric native species.

The unsuitability of S. viarum as a host plant for C. hirta may relate, in part, to the presence of glandular leaf trichomes, which present a barrier to herbivory by native insects. While reporting the presence of glandular trichomes on S. viarum (= S. acanthoideum E. Meyer), Hill et al. (1997) demonstrated that similar trichomes on S. sisymbriifolium significantly reduced feeding by the specialist native beetle Conchyloctenia tigrina Olivier (Chrysomelidae). However, removal of the glandular exudate, while increasing feeding, did not render the plant as suitable as the beetle's native host, suggesting that other defences also influence the utilisation of exotic Solanum plants (Hill et al. 1997).

While glandular trichomes are absent on S. americanum and S. dasyphyllum, they are also present, albeit at lower density, on S. lycopersicum, thereby aligning with the reduced adult feeding and larval survival on tomato. This study supports the contention that the negligible levels of herbivory suffered by S. viarum in South Africa (Olckers et al. 2024) relate to anti-herbivore defences, which could also include secondary plant compounds, rather than host-plant preferences. Since C. hirta, a broadly oligophagous herbivore within the native Solanum insect community, cannot exploit S. viarum, the plant continues to persist under natural enemy release and may well expand its range in South Africa. Although C. hirta utilises the exotic S. americanum in the field, observations suggest that the levels of herbivory are moderate and similarly unlikely to affect the distribution of the plant.

 

ACKNOWLEDGEMENTS

The University of KwaZulu-Natal provided financial support through its research-productivity funding platform. The comments of two anonymous reviewers improved the manuscript.

 

AUTHOR CONTRIBUTIONS

TO: conceptualisation, investigation, data curation, formal analysis, writing the original draft.

 

ORCID ID

Terence Olckers: https://orcid.org/0000-0001-6750-8683

 

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Correspondence:
Terence Olckers
Email: Olckerst@ukzn.ac.za

Received: 1 August 2025
Accepted: 12 September 2025