Invasive floating water weeds – killing life and commerce

Weeds by definition are plants that grow in the wrong place. When their seeds or other plant parts are transported to other regions where their natural enemies are absent, they can multiply unhindered. Indigenous plants, especially those that are adapted for invading disturbed areas, can also become weeds. The first category is a particularly good target for classical biological control. Insects, mites and micro-organisms that feed on them are imported from their original area and released against the new invader. Against indigenous plants however, biological control is far less promising. By the end of 1980s, many of the water bodies in West Africa were invaded by alien plant species considered to be among the world’s worst aquatic weeds: water hyacinth Eichhornia crassipes, water lettuce Pistia stratiotes, and water fern Salvinia molesta. They were accidentally or deliberately introduced as ornamentals or for use in aquariums from their native range South America to many parts of the world where they have become invasive

Some biological parameters of Paracoccus marginatus Williams and Granara de Willink (Hemiptera: Pseudococcidae) and its natural enemy Acerophagus papayae Noyes et Schauff (Hymenoptera: Encyrtidae)

The papaya mealybug Paracoccus marginatus Williams and Granara De Willink (Hemiptera: Pseudococcidae) is an invasive insect species attacking diverse host plants causing enormous damage to crops including those of very high economic importance. Biological control of this mealybug is achieved through the field release of its natural enemy, Acerophagus papayae Noyes and Schauff (Hymenoptera: Encyrtidae). The study aims to determine the developmental time and mortality rate of the different stages of P. marginatus; daily fecundity and pre- oviposition, oviposition and post-oviposition times of a female P. marginatus and the total average number of emerging individuals of the hosts parasitized in 24 hours by a female and the total average development time of A. papayae. This study, which was conducted in the laboratory under conditions of 28 ± 2°C, 75 ± 5% RH, 12: 12 LD, determined the average daily fecundity of a female P. marginatus and the average number of offspring of a female A. papayae per day. The results obtained showed that a female P. marginatus has three periods of reproductive activity, namely the pre-oviposition, oviposition and post-oviposition periods, which are on average last 7.74 ± 1.26, 6.13 ± 3.02 and 7.45 ± 4.27 days respectively. During oviposition, a female P. marginatus lays an average of 25.262 ± 11.16 eggs per day, and an average total of 224.32 ± 29.99 eggs during her lifetime, which averages 18.44 ± 3.31 days. Also, during the development of the pre-imaginal stages of P. marginatus, significant mortality rates were noted in the first and second larval stages, which are 61.40 ± 0.05 and 52.8 ± 0.025 respectively, whereas in the third stage, this rate is 35.02 ± 0.03 for the female and zero for the male. The average total development time of the pest was 25.98 ± 4.47 days for the female and 29.70 ± 5.58 days for the male. In contrast, the progeny of its natural enemy A. papayae averaged 8.22 ± 4.676 males and 10.53 ± 4.43 females per day, giving an average total of 18.75 ± 9.087 offspring per day. During its lifetime (13.02 ± 0.08 days), a female A. papayae produced an average of 205.96 ± 9.87 offspring in an average development time of 13.82 ± 0.013 days. These results obtained on the progeny of a female and the development time of A. papayae species in relation to that of its host can be used to evaluate the effectiveness of the parasitoid in the management of the pest populations in Togo

Release, establishment, spread, and impact of the weevil Neohydronomus affinis (Coleoptera: Curculionidae) on water lettuce (Pistia stratiotes) in Benin, West Africa

The weevil Neohydronomus affinis Hustache (Coleoptera : Curculionidae) of Neotropical origin was imported into Benin in 1993 and released in 1995 for the biological control of water lettuce Pistia stratiotes L. (Araceae). After the release of N. affinis in water bodies linked to the Mono River system, southwestern Benin, larval mines characteristic of immature stages were observed in all seasons in the first year and the weevil was considered established. Two years later, it had spread 90 km from the release site to the Ouémé River, and by 2000 to Savalou, which is 250 km to the northwest. It also established in the north, in the watershed of the Niger River. At the two sites monitored, total plant biomass and weed cover fluctuated with seasonal factors but declined about ten-fold, which indicated successful biological control of water lettuce by the weevil

Measuring host finding capacity and arrestment of natural enemies of the cassava mealbug, Phenacoccus manihoti, in the field

In uninfested fields, 80 cassava tips were artificially infested with 0, 1, 2, 4, 8, 16, 32, or 64 third instars, and 20 or 100 eggs of cassava mealybug, Phenacoccus manihoti Matile‐Ferrero (Hom., Pseudococcidae). Another 80 uninfested tips served as a control. Tips were arranged in a circle of 28 m diameter, in the centre of which the following exotic natural enemies of P. manihoti were released: Apoanagyrus (Epidinocarsis) lopezi De Santis and A. diversicornis (Howard) (Hym., Encyrtidae), Hyperaspis notata (Mulsant) and Diomus hennesseyi Fürsch (Col., Coccinellidae), and others. This experiment was repeated six times. During the 4–14 days following release, all experimental tips were inspected at two‐hour intervals during each day and the presence of exotic as well as indigenous natural enemies, like Exochomus troberti Mulsant (Col., Coccinellidae), ants and spiders was noted. The experiment was repeated six times measured the aggregative response by the natural enemies to different host densities, achieved through host attractance and arrestment. All exotic natural enemies, except the males of Apoanagyrus spp., were fast attracted to the host colonies. As compared to the control tips, they concentrated on the infested tips about 50‐fold for the two Apoanagyrus spp. and 10 to 20‐fold for the exotic coccinellids. By contrast, non‐coevolved indigenous coccinellids, as well as generalist predators like ants and spiders were attracted to the infested tips only 2 to 5‐fold. A. lopezi responded best to different host densities, followed by A. diversicornis and the coccinellids, followed by ants and spiders. None of the parasitoids or predators was particularly attracted to egg masses. These results correspond closely to the known efficiencies of these natural enemies, A. lopezi standing out among all candidates. The results of such aggregation studies are compared with those of life‐table studies

Population regulation of a classical biological control agent: larval density dependence in Neochetina eichhorniae (Coleoptera: Curculionidae), a biological control agent of water hyacinth Eichhornia crassipes

The release of classical biological control agents has reduced the economic, environmental and social problems caused by water hyacinth, Eichhornia crassipes; however, additional control measures are needed in some locations. Water hyacinth plants were treated with different densities of eggs of the weevil Neochetina eichhorniae Warner, one of the main control agents, under different nutrient regimes in a controlled experiment. Plants were destructively sampled and the development of N. eichhorniae was assessed. The survival of first and second instars declined as larval density increased. Plant nutrient status did not directly affect the mortality rate of larvae, but at higher nutrient concentrations larvae developed faster and were larger at a given developmental stage. It is argued that the density dependence operating in N. eichhorniae occurs through an interaction between young larvae and leaf longevity. Consequently, events which disrupt water hyacinth leaf dynamics, e.g. frost or foliar herbicides, will have a disproportionately large effect on the control agents and may reduce the level of control of the host.DST-NRF Centre of Excellence for Invasion Biolog

Impact of two accidentally introduced Encarsia species (Hymenoptera: Aphelinidae) and other biotic and abiotic factors on the spiralling whitefly Aleurodicus dispersus (Russell) (Homoptera: Aleyrodidae) in Benin, West Africa

In early 1993, the spiralling whitefly, Aleurodicus dispersus (Russell), was observed in Benin for the first time, inflicting damage to ornamental and shade trees and cassava. The parasitoids Encarsia ?haitiensis Dozier and E. guadeloupae Viggiani were observed in the second half of 1993. They were known to have the same host in the Pacific region, and were thought to have been introduced accidentally. The impact of these parasitoids was quantified using four surveys from 1993 to 1995 (on 2541 trees in 537 localities) and by population studies on guava. In 1993, A. dispersus occurred mostly in towns in the southern part of Benin; penetration into farmland was observed later. E. ?haitiensis was more abundant and widespread than E. guadeloupae , and by 1995 it had been recovered from most (84%) of the infested localities. On guava trees, the annual peaks of A. dispersus population declined by ca. 80% between 1993 and 1996. During the same period parasitism rates increased. Econometric multiple regression analyses based on 996 infested trees demonstrated that A. dispersus population densities, the proportion of infested trees and damage scores all declined significantly with increasing duration of the presence of the parasitoids, indicating their impact. Other variables were also significantly related to A. dispersus levels

Economic impact of biological control of water hyacinth in southern Benin

A biological control program of water hyacinth was undertaken in Southern Benin between 1991 and 1993. It consisted of the release of three natural enemies, two weevil species and one moth, that feed exclusively on water hyacinth. In 1999, a survey of 365 men and women from 192 households in 24 villages in the target area, using participatory and quantitative methods, revealed that water hyacinth, although not eliminated, was perceived by the villagers as having been reduced from a serious pest to one of minor or moderate importance. According to their estimates of the impacts they perceived, at the peak of the infestation water hyacinth had reduced the yearly income of this population of about 200 000 by approximately US$84 million. Lost revenues for men were mostly in fishing, while women experienced lost revenues in trade, primarily food crops and fish. The reduction of water hyacinth cover through biological control was credited with an increase in income of US$30.5 million per year. The total cost of the control program is estimated at a present value of US$2.09 million. Assuming the benefits are to stay constant over the next 20 years, a most conservative assumption, the accumulated present value would be US$260 million, yielding a respectable benefit cost ratio of 124:1. This ratio is calculated for direct economic effects on the people of Southern Benin only, and does not take into account indirect benefits or the benefits of the project to other countries were the biological control agents were successfully introduced. Other effects mentioned, but not measured, included an increase of water quality and of human health