Interaction BetweenUroplata girardi(Coleoptera: Chrysomelidae) andOphiomyia camarae(Diptera: Agromyzidae) on a Shared HostLantana camara(Verbenaceae)

Abstract

Multiple releases of insect agents intended to target a single plant pest species could result in competitive interactions that in turn might affect the community structure of the phytophagous insects. Two leaf-feeding biological control agents, Uroplata girardi Pic (Coleoptera: Chrysomelidae) and Ophiomyia camarae Spencer (Dipetera: Agromyzidae), were released against the weed Lantana camara L. (Verbenaceae) in South Africa in the 1970s and 2001, respectively. Since the population explosion of O. camarae in 2005, a decline of U. girardi populations had been observed in KwaZulu-Natal (KZN) humid coast, leading to speculation that negative interaction may be operating between the agents. The study therefore was conducted to determine the competitive effect of O. camarae on U. girardi. The study showed that 76% of O. camarae larval mines were formed on uninfested (clean) compared with only 24% formed on U. girardi-infested leaves, suggesting that the ßy chose to lay more eggs on clean leaves. Almost the same number of U. girardi larval mines was formed on both O. camarae-infested and clean leaves, indicating that U. girardi females in this case oviposited indiscriminately on the two types of leaves. The survival of U. girardi was 53.8% when reared on clean leaves compared with only 14.6% survival on O. camarae-infested leaves. At the end of the sampling period, densities of U. girardi was over two times higher in single-species than in combinedspecies treatment. Releasing both agents together did not signiÞcantly affect O. camarae densities during the sampling period. In the Þeld, O. camarae densities increased rapidly from spring to autumn, whereas those of U. girardi remained consistently low during the same period. The bias toward oviposition on clean leaves in O. camarae enables its larvae to avoid unfavorable encounters with U. girardi larvae, thus enhancing its development and survival. The apparent inability of U. girardi to distinguish between suitable and unsuitable leaves for oviposition could compromise the Þtness of this beetle, and this could explain the suppression of U. girardi populations during summer when O. camarae populations begin to increase rapidly. This study provides evidence for an asymmetric interaction between two introduced agents, and therefore highlights the importance of conducting interaction studies on agents with extensive niche overlap before their release into the environment.

Co-existence Between Two Leaf-Feeding Biological Control Agents of Lantana camara Alters Their Herbivory Under Semi-field Conditions

“…Therefore, it is likely that the exclusion trials by cages might have prevented other factors that inflict damage on the uncaged plants such as exposure to wind, sun, dryness, and raindrop intensity. Our results together with those reported by April et al (2011) support the notion that the decline in the population density of U. girardi in KZN Province (Heystek 2006) is due to co-infestation by both agents which has a negative effect on U. girardi and a minor effect on O. camarae populations. April et al (2011) found that colonization of L. camara by both U. girardi and O. camarae reduced the population density of U. girardi, with the infestation level by the beetle averaging 9% per plant when combined with O. camarae, compared with 29% per plant when confined alone.…”

“…Our results together with those reported by April et al (2011) support the notion that the decline in the population density of U. girardi in KZN Province (Heystek 2006) is due to co-infestation by both agents which has a negative effect on U. girardi and a minor effect on O. camarae populations. April et al (2011) found that colonization of L. camara by both U. girardi and O. camarae reduced the population density of U. girardi, with the infestation level by the beetle averaging 9% per plant when combined with O. camarae, compared with 29% per plant when confined alone. Similarly, field surveys in KZN indicated that U. girardi abundance levels ranged from 25 to 50% of infested leaves per plant prior to O. camarae release (Baars and Heystek 2003), and this declined to 2 to 7.5% four years after the release of the leafminer (April et al 2011).…”

“…However, the combination of U. girardi and O. camarae reduced leaf density by only 29%, which was much lower than what U. girardi does alone, and this is an indication of a negative interspecific interaction between the two species. However, given that O. camarae chooses to oviposit on clean rather than U. girardi-infested leaves (April et al 2011), it is likely that under field conditions the fly would disperse from U. girardiinfested leaves and oviposit on cleaner ones, thus mitigating the effect of interspecific competition. This is consistent with other studies that have found that agromyzid leaf miners often prefer undamaged leaves (Faeth 1985) for oviposition and this strategy also serves as a post-fertilization investment in some insect species (Faeth 1985).…”

“…Prior to O. camarae introduction into South Africa, Baars and Heystek (2003) had described the populations of U. Girardi in the KwaZulu-Natal coastal region as very abundant, translating to over 50% of larva-infested leaves per plant. However, since the release of O. camarae and its subsequent boom in the same region, the populations of U. girardi have declined (Heystek 2006;April et al 2011), indicating the possibility of a negative interaction between the two agents. Although U. girardi and O. camarae continue to co-exist and cause visible damage on leaf veins of L. camara along the coastal regions, population levels of U. girardi remain low (April et al 2011), and that the combined effect of the two agents may be less than additive.…”

“…However, since the release of O. camarae and its subsequent boom in the same region, the populations of U. girardi have declined (Heystek 2006;April et al 2011), indicating the possibility of a negative interaction between the two agents. Although U. girardi and O. camarae continue to co-exist and cause visible damage on leaf veins of L. camara along the coastal regions, population levels of U. girardi remain low (April et al 2011), and that the combined effect of the two agents may be less than additive. Therefore, the study investigated the extent to which co-infestation of U. girardi and O. camara reduced their impact under semi-field conditions.…”