Costas Zachariades

The nocturnal larvae of a specialist folivore perform better on Chromolaena odorata leaves from a shaded environment

Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment‐mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full‐sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) King & Robinson (Asteraceae). In addition, the performance was evaluated in two generations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full‐sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non‐structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but above‐ground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full‐sun foliage. Although immature survival and adult longevity did not differ between habitats, Maws host suitability index indicated that shaded leaves were more suitable for the growth and reproduction of P. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light‐mediated changes in plant traits and leaf characteristics can affect insect folivore performance.

Variation in host plant has no effect on the performance and fitness-related traits of the specialist herbivore Pareuchaetes insulata

The effect of host plant dissimilarity on insect preference and performance was tested using two morphological forms of Chromolaena odorata (L.) King & Robinson (Asteraceae) (one from Florida, USA, another from South Africa), and a specialist herbivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae) from Florida, USA, that was introduced as a biological control agent in South Africa. Although this insect did establish at one site, of some 21 sites at which over 800 000 individuals were released, its population level in the field has remained low after an initial outbreak in 2006. To explain the poor performance of P. insulata, we hypothesised that P. insulata larvae prefer Floridian C. odorata to the southern African C. odorata, which is morphologically and probably genetically distinct, and that larvae reared on Floridian C. odorata should have higher fitness and performance. We tested this by comparing insect performance metrics on each of the two plant forms in laboratory experiments. Apart from pupal mass, which was significantly greater on southern African C. odorata, P. insulata performance metrics were similar on both plant forms; there were no significant differences in total leaf area consumed, egg and larval development, immature survival rates, feeding index, host suitability index, growth index, and fecundity between the Floridian and southern African C. odorata plants. In sum, we could not demonstrate that differences in plant forms in C. odorata are responsible for the poor performance of P. insulata in South Africa.

Nitrogen fertilisation improves growth of Chromolaena odorata (Asteraceae) and the performance of the biological control agent, Pareuchaetes insulata (Erebidae)

ABSTRACT Recent studies have demonstrated, through their contrasting results, that relationships between nitrogen levels in host plants and phytophagous insect performance are not simple. This study examined the effect of varying fertilisation regimes on the invasive alien plant, Chromolaena odorata (L.) (Asteraceae) and the response of a specialist folivore (a biological control agent), Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae). C. odorata plants were treated with 3 different levels of fertilisation and plant characteristics were measured within 2–3 months of fertiliser application. Leaves from each of the three treatments were fed to newly hatched larvae until pupation in order to determine the effect of nitrogen fertilisation on herbivore performance metrics such as survival, development time, fecundity and longevity. High and medium fertilisation significantly increased foliar nitrogen concentrations, basal stem diameter, leaf length, shoot height and above-ground biomass of C. odorata plants relative to low fertilisation. When individuals of P. insulata were fed on leaves from medium- or high-fertilisation treatments, they developed faster, grew to a larger size (by 8%) and achieved higher fecundity (19–22%) than leaves from the low-fertilisation treatment. The results suggest that in mass-rearing, increased production of this biological control agent will occur in high- or medium-fertilised plants.

Genetic diversity in Pareuchaetes insulata and its implications for biological control of Chromolaena odorata

AbstractChromolaena odorata (L.) King & H.E. Robins. (Asteraceae) threatens several economic and environmental activities in the eastern subtropical regions of South Africa and is a target for biological control. Three populations of Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae) originating from Florida (USA), Cuba and Jamaica were released at 30 separate sites in KwaZulu-Natal, South Africa. Subsequent overlapping of two of these populations in the field and the likelihood of interbreeding posed a potential risk of establishment failure. To explore the genetic diversity of the different P. insulata populations and test for the existence of pre- and post-zygotic reproductive isolation between them, molecular analyses and cross-mating experiments were conducted. Mitochondrial DNA sequences revealed significant genetic similarity between them. Cross-mating trials between Floridian and Jamaican populations of P. insulata revealed no significant pre- and post-zygotic isolation, with no demonstrable differences in measured parameters between the ‘parental’ populations, the F1 ‘hybrids’ and the backcrosses. These results suggest that P. insulata populations released in South Africa are part of the same ‘parental’ population. Genetic incompatibility is, therefore, relatively unlikely to be the cause of any failure in establishment of P. insulata at any of the release sites.

Seasonal activity, habitat preferences and larval mortality of the leaf-mining fly Calycomyza eupatorivora (Agromyzidae), a biological control agent established on Chromolaena odorata (Asteraceae) in South Africa

The leaf-mining fly Calycomyza eupatorivora Spencer (Diptera: Agromyzidae) has become widely established in the eastern regions of South Africa, following its release for the biological control of the invasive shrub Chromolaena odorata (L.) King and Robinson. This study was conducted to gain some insight into the impact of C. eupatorivora populations, by assessing their seasonal activity, habitat preferences, levels of leaf exploitation and extent of larval mortality in the field over a 1-year period. Leaf mining intensity was poorly synchronised with leaf availability, with leaf exploitation peaking at the end of the growing season of C. odorata. Although significantly more mines were recovered on plants growing in shaded situations, the percentage of available leaves that were exploited for mining was not significantly different between plants growing in shaded versus open situations. Overall, the levels of leaf damage were trivial with mines recovered from <5% of available leaves that were sampled during the study. Besides leaf-quality requirements, these low levels of leaf exploitation may have been influenced by high larval mortality which varied between 60 and 83%, depending on whether lower or higher estimates were used. These evaluations have verified the results of earlier laboratory studies which suggested that the impact of C. eupatorivora on mature populations of C. odorata in South Africa will be negligible.

Is oviposition and larval damage by the leaf-mining fly Calycomyza eupatorivora (Agromyzidae) on its target weed, Chromolaena odorata (Asteraceae), restricted by leaf-quality preferences?

The leaf-mining fly Calycomyza eupatorivora Spencer (Diptera: Agromyzidae) was released in the eastern coastal regions of South Africa for the biological control of the invasive shrub Chromolaena odorata (L.) King and Robinson. Despite widespread establishment, its ability to inflict sufficient foliar damage has been questioned. This laboratory study was initiated to provide some insight into how increasing fly populations (represented by 1, 5 and 10 mating pairs per plant) are likely to influence leaf-mining intensity and the levels of damage. On average, individual plants exposed to five mating pairs displayed significantly more larval mines (248) than those exposed to single pairs (69), while plants exposed to 10 mating pairs were intermediate (125). Similarly, at densities of five mating pairs per plant, the percentages of available leaves that were exploited peaked at 36%, while the percentages of available leaf area that were removed by larval leaf mining peaked at 22%. The non-linear relationship between leaf mining and fly density and the high percentages of unexploited leaves suggest that leaf mining may be influenced by leaf quality, the nature of which is currently unconfirmed. These results also suggest that the levels of leaf exploitation by C. eupatorivora will be too low to have any meaningful impact in the field. Field evaluations, to be reported in a later contribution, have indeed confirmed that the impact of C. eupatorivora on mature populations of C. odorata is negligible.

Developmental and reproductive performance of a specialist herbivore depend on seasonality of, and light conditions experienced by, the host plant

Host plant phenology (as influenced by seasonality) and light-mediated changes in the phenotypic and phytochemical properties of leaves have been hypothesised to equivocally influence insect herbivore performance. Here, we examined the effects of seasonality, through host plant phenology (late growth-season = autumn vs flowering-season = winter) and light environment (shade vs full-sun habitat) on the leaf characteristics of the invasive alien plant, Chromolaena odorata. In addition, the performance of a specialist folivore, Pareuchaetes insulata, feeding on leaves obtained from both shaded and full-sun habitats during autumn and winter, was evaluated over two generations. Foliar nitrogen and magnesium contents were generally higher in shaded plants with much higher levels during winter. Leaf water content was higher in shaded and in autumn plants. Total non-structural carbohydrate (TNC) and phosphorus contents did not differ as a function of season, but were higher in shaded foliage compared to full-sun leaves. Leaf toughness was noticeably higher on plants growing in full-sun during winter. With the exception of shaded leaves in autumn that supported the best performance [fastest development, heaviest pupal mass, and highest growth rate and Host Suitability Index (HSI) score], full-sun foliage in autumn surprisingly also supported an improved performance of the moth compared to shaded or full-sun leaves in winter. Our findings suggest that shaded and autumn foliage are nutritionally more suitable for the growth and reproduction of P. insulata. However, the heavier pupal mass, increased number of eggs and higher HSI score in individuals that fed on full-sun foliage in autumn compared to their counterparts that fed on shaded or full-sun foliage in winter suggest that full-sun foliage during autumn is also a suitable food source for larvae of the moth. In sum, our study demonstrates that seasonal and light-modulated changes in leaf characteristics can affect insect folivore performance in ways that are not linear.

Laboratory studies on the biology and host range of Dichrorampha odorata (Lepidoptera: Tortricidae), a biological control agent for Chromolaena odorata (Asteraceae)

ABSTRACT Dichrorampha odorata (Lepidoptera: Tortricidae) is a moth from Jamaica whose larvae bore into, and kill, the shoot tips of the invasive alien plant, Chromolaena odorata (L.) King and Robinson (Asteraceae). This study reports aspects of the biology of D. odorata, and also determined the host specificity (larval and adult no-choice trials) of the moth. Adults were short lived (ranging from 2 to 7 days), with females laying a mean of 15.4 eggs. Eggs took 9 days to hatch, larvae 20–23 days to develop and the pupal stage lasted 11–12 days, giving an overall lifecycle period of 41–45 days. Larval no-choice tests using 34 asteraceous test species indicated that only C. odorata could sustain complete development of D. odorata to adulthood, although there was slight initial boring 14 test species (plus chromolaena). Results from the adult nochoice trials, in which seven test-plant species were exposed to D. odorata, were consistent with those from larval trials, with larval damage, pupae and adults of D. odorata recorded from only C. odorata. This confirmed that only C. odorata is a suitable host for D. odorata in South Africa. Permission has subsequently been granted for the release of D. odorata in South Africa, thus making it the first shoot-tip attacking agent to be released against C. odorata. It is hoped that in the field, high levels of damage by the moth will reduce the height and therefore competitiveness of C. odorata, thereby contributing to the success of biological control of this plant.

Which haystack? Climate matching to narrow the search for weed biological control agents.

Summary The shrub Chromolaena odorata (L.) King and Robinson (Asteraceae) is highly invasive in southeastern Africa and is the subject of a South African biological control programme. The biotype of C. odorata growing in South Africa differs in several respects from the more common type noted to be invasive elsewhere, including its apparent better adaptation to a cool climate. One challenge facing the biological control programme is the identification of agents that are both suited to develop on this host biotype and persist in the relatively cool conditions found in South Africa. C. odorata is native to the Americas, where it has a very extensive distribution spanning a wide range of climates. Two climate matching computer programmes (CLIMEX and FloraMap) were used to focus the agent search effort by identifying areas in the Americas that are climatically similar to the invaded region in southern Africa (SA). Several higher-latitude and higher-altitude areas in South and Central America were identified by both CLIMEX and FloraMap as being similar to the region invaded by C. odorata in South Africa. In many areas, the two models agreed, but in others, there were discrepancies, which are discussed. There was little overlap between the region from which the SA biotype is thought to have originated and climatically suitable/similar areas in the Americas indicated by either model.