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Expanding populations of Dioscorea bulbifera, a trellising invasive vine of Afro-Asian provenance, have become widely established in the southeastern United States. Its clambering habit enables the weed to grow over, smother and displace native vegetation while producing vast quantities of vegetative propagules (bulbils) and reducing biodiversity of different ecosystems in Florida. A specialized foliage-feeding beetle, Lilioceris cheni from Nepal and China, has been released in USA for biological control of D. bulbifera. To determine the beetle’s potential to curb the vine’s ability to overtop native vegetation and to suppress propagule production, beetle restricted (insecticide treated) and unrestricted (beetle inoculated) sites were compared at five localities in Florida. Dioscorea bulbifera cover, L. cheni population density, and herbivore damage were documented at 6-week intervals, with bulbil density and biomass measured annually, for 5 years. Results from beetle unrestricted treatment revealed that high L. cheni feeding damage reduced vine cover over native vegetation, and decreased bulbil density and biomass. Spillover of L. cheni populations from beetle unrestricted into restricted treatment areas after exhaustion of D. bulbifera vines resulted in coalescing effects between treatments, so the beetle restricted treatment also showed some reduction in vine cover and bulbil density but individual bulbil biomass remained unchanged. These results show that L. cheni has the ability to suppress the invasive attributes of D. bulbifera in its adventive range.

A four year common garden study was initiated using once-cut Melaleuca quinquenervia trees that were subsequently subjected to a full factorial of treatments that included reduced versus unrestricted herbivory from biological control agents, a mechanical treatment (trees were not cut or were cut every 6 m), and an irrigation treatment (trees were irrigated or not). Repeated cutting reduced the total tree biomass by 76.4%, herbivory alone reduced total biomass by 58.7%, and the combination of cutting and herbivory reduced total biomass by 80.1%. Unrestricted herbivory reduced the seed biomass per tree by 93.9% in uncut trees while repeated cutting eliminated all seed production regardless of herbivory. Uncut trees subjected to unrestricted herbivory allocated an average of 8.8% of their biomass to reproductive tissue, while those exposed to restricted herbivory allocated an average of 15.8%. Unrestricted herbivory by biological control agents on uncut trees also reduced the total number of seed capsule clusters by 79.2%, the number of capsules per cluster by up to 20.8%, and the individual seed weight by up to 58.9%. Repeated cutting did reduce the amount of damage to regrowth by one biological control agent, Oxyops vitiosa (Coleoptera: Curculionidae), but did not reduce the abundance another, Boreioglycaspis melaleucae (Hemiptera: Psyllidae). Trees that were only cut once at the initiation of the study exhibited vigorous regrowth, including increased height and seed production. The mortality of trees was lower compared with other studies, highlighting the influence of localized biotic and abiotic factors on the cumulative suppressive effects of cutting and herbivory on M. quinquenervia.

The severity of the effects of herbivory on plant fitness can be moderated by the ability of plants to compensate for biomass loss. Compensation is an important component of the ecological fitness in many plants, and has been shown to reduce the effects of pests on agricultural plant yields. It can also reduce the effectiveness of biocontrol through introduced herbivores in controlling weedy invasive plants. This study used a modeling approach to predict the effect of different levels of foliage herbivory by biological control agents introduced to control the invasive tree Melaleuca quinquennervia (melaleuca) in Florida. It is assumed in the model that melaleuca can optimally change its carbon and nitrogen allocation strategies in order to compensate for the effects of herbivory. The model includes reallocation of more resources to production and maintenance of photosynthetic tissues at the expense of roots. This compensation is shown to buffer the severity of the defoliation effect, but the model predicts a limit on the maximum herbivory that melaleuca can tolerate and survive. The model also shows that the level of available limiting nutrient (e.g., soil nitrogen) may play an important role in a melaleuca’s ability to compensate for herbivory. This study has management implications for the best ways to maximize the level of damage using biological control or other means of defoliation.

Successful biological control programs can have landscape-level effects on the management of intractable arthropod pests and weeds, improving ecosystem services and reducing both management costs and the widespread use of pesticides. However, biotic resistance can prevent biological control agents from establishing or limit their efficacy. We assessed the potential for biological control agents of the pest Lilioceris lilii, the lily leaf beetle, to attack L. cheni, a weed biological control agent for Dioscorea bulbifera, air potato. Both the suite of parasitoid biological control agents and L. cheni are contributing to the successful management of their respective targets. Thus, negative interactions between these species could potentially disrupt two effective biological control programs if range overlap occurs. Choice and no-choice tests were conducted with all three parasitoid species and the target and non-target beetles, and a phylogenetic tree was constructed to assess the relatedness of the Lilioceris species. The parasitoids displayed a clear preference for their host, L. lilii, and did not successfully parasitize L. cheni. Although interference between arthropod and weed biological control programs is not likely to be a common occurrence, practitioners in both subdisciplines should be cognizant of this possibility as new agents are developed.

The invasive tree Melaleuca quinquenervia experienced substantial declines in growth and reproduction in response to chronic herbivory by the defoliating weevil Oxyops vitiosa . Plants subjected to unrestricted defoliation replaced leaves that were more suitable for feeding by the next generation, a process envisioned by the Resource Regulation Hypothesis which posits that attack by one generation increases the amount of the preferred host resources for the next, resulting in a positive feedback loop for the herbivore. The production of juvenile replacement leaves stimulated additional bouts of oviposition and feeding by O. vitiosa , which ultimately produced positive effects for the herbivore with negative consequences for the plant. The addition of water resources to the plant prolonged the positive feedback loop such that more than twice as many insects were produced on irrigated versus non-irrigated trees. In a more simple, reassembled food web on M. quinquenervia , the lack of biotic constraints like parasitoids may have prevented the earlier termination of the feedback loop and thus increased the impact of the biological control agent on the target. The overall effectiveness of this classical biological control program can be attributed, in part, to the phenomenon of the target plant’s induced susceptible response to a herbivore.

Classical weed biological control agents, regardless of their effectiveness, may provide subsidies to predators and parasites. The chemically defended weevil Oxyops vitiosa Pascoe is a successful agent that was introduced to control the invasive tree Melaleuca quinquenervia. Two consecutive small plot experiments that lasted two and three years, respectively, examined the population dynamics of O. vitiosa while subjected to predation by Podisus mucronatus, a native generalist predator. During this time the estimated mean (±SE) percent predation of O. vitiosa larvae was 7.2 ± 1.7% per sample date in the two year study and 8.4 ± 0.8% in the three year study. There was no relationship between the number of larvae per tree and the number that were predated in either experiment. Consumer losses from predation did not cascade down to the producer level and influence any plant variable in either experiment. Time series analysis found no autoregressive processes for predation in either experiment while there were strong first through fourth-order auto-correlations for live larvae in both experiments, indicating the presence of strong trends in prey density. If longevity was a gauge of the relative importance of a predator subsidy, then any provided by O. vitiosa was negligible because predation was unlikely to increase over two consecutive sample periods despite increasing prey populations. The benign presence of sustained populations of a biological control agent provides a tailored counter argument to studies that imply inevitable and perilous linkages between introduced agents and community food webs.

The recruitment and mortality of Melaleuca quinquenervia seedlings were evaluated over a 3-year period in a seasonally inundated wetland in the western Everglades region. The mean (±SE) density of seedlings/saplings m−1 declined from 64.8 (±4.5) to 0.5 (±0.2) over the 3 years, a population reduction of 99.2%. Four distinct water regimes characterized this site: dry, dry to wet transition, flooded, and wet to dry transition. Seedling recruitment was highest in the dry to wet transition and lowest in the flooded water regime, while mortality was highest under flooded and dry water regimes. The mean estimate of population growth (λ) across water regimes was 0.64 ± 0.05 indicating negative population growth. Elimination of introduced insect herbivores using insecticides did not reduce mortality of recruited M. quinquenervia seedlings/saplings indicating that direct herbivory was not responsible for the decline in seedling density. On the other hand, a mean of only 0.2 (±0.03) viable seeds m−2 d−1 fell into the plots, an amount considerably lower than in previous studies. We submit that change in the invasion trajectory M. quinquenervia was most likely caused by reduced seed inputs from aerial seed banks depleted by insect herbivory rather than direct herbivory on seedlings. This may indicate a fundamental alteration of M. quinquenervia population dynamics ultimately resulting in a less invasive and, therefore, less ecologically damaging species.

Utilization of multiple natural enemy species in weed biocontrol programs has often been advocated. Herein, we tested the hypothesis that multiple guilds of natural enemies, rather than individual species, would result in greater levels of damage to target host plants. This was investigated by evaluating the herbivorous insects Oxyops vitiosa (weevil) and Boreioglycaspis melaleucae (psyllid) and rust fungus Puccinia psidii separately and together when attacking cut-stump coppices of the invasive tree Melaleuca quinquenervia (melaleuca). We felled 120 trees in each of three mature melaleuca stands and assigned 6–13 coppicing stumps to each of the four treatments: control (insects and rust fungus excluded); insect herbivory; rust-fungus-infection; and combination of insect herbivory and rust fungus infection. We repeated the experiment once in adjacent plots located within the same stands as above. Coppices were harvested annually for three years and evaluated for damage, height, branching, mortality, and biomass. Three treatments: insects, rust fungus, and their combination caused higher levels of coppice damage and mortality compared to controls; these three treatments also reduced plant height, branching, and biomass of surviving coppices. These impacts of insects and rust-fungus-combination were additive on cut-stump and coppice stem mortality, and reduction in height, branching, leaf biomass, and total biomass of coppices. The rust fungus and psyllid showed better ability to co-attack the same leaf tissues compared to the rust fungus and the weevil. In conclusion, overall effects of the three natural enemies led to markedly reduced performance of the invasive tree melaleuca.

Growth chamber experiments were conducted to examine environmental parameters affecting disease development by the indigenous pathogen Bipolaris sacchari isolate LJB-1L on the invasive weed Lygodium microphyllum (Old World climbing fern). An initial experiment examined three different temperature regimes (20/15, 25/20, 30/25 °C; light/dark cycle) and three different periods of leaf wetness (12, 24, 48 h). Disease incidence was significantly reduced by incubation of plants at the highest temperature regime (30/25 °C) and with the shortest period of leaf wetness (12 h). Subsequent experiments concentrated on the effect of temperature, as determination of an optimal temperature for disease development might determine the best time of year for field inoculations with this fungus. When the light cycle temperature was kept constant at 30 °C, a dark cycle temperature of 15 °C was more conducive for disease development than 25 °C based on disease incidence and area under the disease progress curve (AUDPC). A qualitative examination of conidia germination and appressorial formation in vivo indicated that these initial steps toward infection were probably not influenced by the dark cycle temperature. In vitro, temperature (range of 15–35 °C) did not significantly affect conidia germination, but hyphal growth was significantly affected, with optimal growth between 25 and 30 °C.

The invasive tree Melaleuca quinquenervia experienced substantial declines in growth and reproduction in response primarily to chronic herbivory by the defoliating weevil Oxyops vitiosa. Herbivory was mediated on individual trees using regular applications of the insecticide acephate during a 2-year period to minimize populations of O. vitiosa and later the phloem-feeding psyllid, Boreioglycaspis melaleucae. Half of the trees also received drip irrigation to evaluate the influence of water on plant responses to herbivory. Insecticide-protected trees grew in height nearly six times faster and became 108% taller. In contrast, unprotected trees produced only 19% as much woody biomass, 13% as much leaf biomass, and 0.9% as much seed as protected trees. The mean (±SE) of total above ground biomass was 873.1 ± 191.4 g in unsprayed trees and 3856.2 ± 491.5 g in sprayed trees. Unprotected trees produced more stem tips per unit of height, resulting in a more bushy structure. Greater amounts of water increased leaf biomass, but all other plant variables were influenced most by herbivory. The seed-fueled invasive capacity of melaleuca will probably be reduced in areas favorable to the biological control agents, especially O. vitiosa.

Invasion of native plant communities by the Australian paperbark tree (“melaleuca”), Melaleuca quinquenervia, complicates restoration of the Florida Everglades. Biological control, within the context of a comprehensive management program, offers a means to suppress regeneration of melaleuca after removal of existing trees and a mechanism to forestall reinvasion. To meet this need, a biological control program commenced in 1997 upon the release of an Australian weevil (Oxyops vitiosa [Pascoe] [Coleoptera: Curculionidae]). Release of a second biological control agent, the melaleuca psyllid (Boreioglycaspis melaleucae Moore), followed in February 2002 at field sites containing mixed age-class melaleuca stands or coppicing stumps. Each site was inoculated with 7000–10,000 adult psyllids, with one exception where 2000 nymphs were released on seedlings the following December. Psyllid populations established everywhere irrespective of colony source, site conditions, or the quantity released, although numbers released and, to a lesser degree, colony age influenced the numbers of colonies produced. Quantity included in the release was the major determinant of the resultant number of colonies, although the duration of their tenure in quarantine culture may have also influenced this. One site, comprised mainly of coppicing stumps, contained 3.3 million psyllids per ha within 3 months after release. Less than 1% of the coppices at a similar site harbored psyllid colonies 2 months after release (May 2002), but this rose to 75% in October then to 100% by December. The census population exceeded 715,000 adults and nearly 11 million nymphs by late January 2003. Psyllid populations dispersed 2.2–10.0 km/year, with the slower rates in dense, continuous melaleuca stands and faster rates in fragmented stands. Over 1 million psyllids had been redistributed to 100 locations as of December 2005. This species now occurs throughout much of the range of melaleuca in south Florida due to natural range expansion as well as anthropogenic dissemination.

We compared the effect of herbivory by two congeneric specialist weevils (Neochetina eichhorniae and Neochetina bruchi) on competition between their host Eichhornia crassipes and another floating aquatic plant (Pistia stratiotes) by experimentally manipulating plant densities, presence of either or both herbivore species, and nutrient levels. Measurements of E. crassipes yield per original plant included biomass, clonal expansion, and flower production. Without herbivory, intraspecific competition among E. crassipes individuals was 41 times more intense than interspecific competition in terms of biomass yield. Herbivory shifted competitive outcomes, bringing intraspecific competition in line with interspecific competition. Both weevil species reduced E. crassipes biomass and flowering, but N. bruchi also lessened clonal expansion (rosette density) whereas N. eichhorniae did not. Nutrient limitation reduced plant performance but did not alter the pattern of competitive interactions. However, low nutrients moderated differences between weevil species such that both seemed equally damaging. We conclude that while herbivory directly and indirectly affected plant performance by altering competition between these two invasive plant species, the competitive response varied depending upon the herbivore species and availability of nutrients. The influence of herbivores on competitive interactions may thus be an important consideration for evaluating the invasive potential of exotic plant species as well as for predicting the potential efficacy of biological control agents, but interactions between herbivory, competitors, and nutrient availability create complexities that must be considered.