Small-scale dispersal of a biological control agent – Implications for more effective releases

Highlights

• Release of adults only and egg-laden plants only are both effective in establishing M. scutellaris populations.

• When released, female M. scutellaris tend to lay eggs before dispersing.

• Release of both adult M. scutellaris and egg-laden plants together increase propagule pressure.

• Release of adult M. scutellaris at high and low densities showed similar dispersal patterns.

Abstract

Eichhornia crassipes (Martius) Solms Laubach (Liliales: Pontederiaceae) was introduced to Florida in the 1880s as an ornamental and it once infested thousands of square kilometers across the state. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was developed as a classical biological control agent for this plant primarily because its free-living life stages allow it to better integrate with herbicides, which are currently used as the main control method for E. crassipes in Florida. Mass rearing and distribution programs can accelerate the benefits of biological control by augmenting natural dispersal, but an optimal release strategy must consider the entire system including the agent, the target weed, and the habitat. The effectiveness of various release strategies was evaluated using a tank experiment where single and multiple releases of either adult M. scutellaris only or E. crassipes infested with M. scutellaris eggs were compared to control treatments. The post-release dispersal capability of brachypterous M. scutellaris was evaluated using a linear transect of E. crassipes. Two density release treatments were tested and emerging nymphs were used as a proxy for female dispersal distances. All release treatments resulted in successful M. scutellaris population establishment and levels of M. scutellaris were not significantly different among them. The dispersal experiment indicated that adult females oviposit near the release point before dispersing. While the release experiment indicated that all treatments were similar, the continually fluctuating populations of E. crassipes makes establishment of populations difficult in the field. By releasing both adults and infested plants, additional propagule pressure can be attained from a single release event which can counter the tendency of adult M. scutellaris to disperse rapidly following release.

Introduction

Waterhyacinth (Eichhornia crassipes) (Martius) Solms Laubach (Liliales: Pontederiaceae) is a free-floating aquatic plant that has invaded fresh water bodies across the world, altering native habitats and outgrowing native vegetation (Little, 1965, Gopal, 1987, Schmitz et al., 1993, Center, 1994). This species was introduced to Florida in the 1880s as an ornamental (Klorer, 1909) where, because of the warm climate and nutrient rich waters, it once infested thousands of square kilometers across the state (Lugo et al., 1978, Reddy and DeBusk, 1984). Since the advent of synthetic herbicides, E. crassipes can now be effectively managed, but relying solely on herbicides requires repeated applications (Schmitz et al., 1993). This has been the experience in Florida, where E. crassipes is managed continually via herbicides by federal, state, and local agencies and costs can run into the hundreds of thousands of dollars annually (Gettys et al., 2014a).

Classical biological control programs in the U.S. utilizing monophagous insect herbivores have developed and deployed four species to increase suppression of this plant (Perkins, 1973, Center and Durden, 1981, Tipping et al., 2014b). The most numerous agent in Florida is Neochetina eichhorniae Warner (Coleoptera: Curculionidae), which is known to reduce the growth and reproduction of E. crassipes, but not significantly reduce coverage (Tipping et al., 2014a). It is difficult for biological control agents to build up to damaging densities because frequent herbicide applications can cause large fluctuations in E. crassipes populations over wide areas (Center et al., 1999). Despite these challenges, herbivory by biocontrol agents increases the effectiveness of herbicide treatments by allowing for reduced dosages without any loss of efficacy, plus retarding the rate of regrowth following applications and has thereby reduced the impact of this plant in Florida (Center et al., 1999, Gettys et al., 2014b, Tipping et al., 2014a, Tipping et al., 2017). While herbicide-managed areas tend to have less E. crassipes coverage, areas where the biological control agent populations are unperturbed by the constant boom and bust cycling of E. crassipes contain smaller plants that are physiologically stressed by the insects (Center et al., 1999).

The most recently released agent, Megamelus scutellaris Berg (Hemiptera: Delphacidae), was selected primarily because its free-living juvenile and adult life stages allow it to better integrate with herbicides (Tipping et al., 2011). Eggs are laid inside the petiole and lamina of E. crassipes. Once they emerge, M. scutellaris goes through five nymphal instars (Tipping et al., 2011). Generation time is ∼25 days outdoors in southern Florida. This species is multivoltine and multiple overlapping generations are observed in the laboratory and at established sites in Florida (Tipping et al., 2014a). While M. scutellaris can be very damaging to E. crassipes (Tipping et al., 2011, Sosa et al., 2007), to date it also has not substantially reduced surface coverage of the plant, which is the primary decision metric used by land managers (Tipping et al., 2014a).

Megamelus scutellaris occurs in both macropterous (flighted) and brachypterous (non-flighted) forms (Sosa et al., 2004), with the majority of insects produced for release being brachypterous. The dimorphism is likely density-dependent (Denno, 1994), but the exact mechanism triggering this phenomenon requires further study (Fitzgerald and Tipping, 2013). Other planthopper species that are wing-dimorphic are known for their macropters’ long-distance migrations (e.g. N. lugens, Denno and Peterson, 1995), while it is generally thought that brachypterous individuals do not disperse over longer distances (Kennedy, 1961, Denno, 1976). However, such smaller-scale dispersal may play an important role in the re-colonization of herbicide treated areas, as insects move from pockets of E. crassipes that escaped treatment into the expanding mat (Center et al., 1999).

Although there can be significant initial costs considering the long process before agent deployment, benefit-cost ratios of biological control tend to be high (Harris, 1991, Hill and Greathead, 2000, Culliney, 2005). Mass rearing and distribution programs can accelerate the benefits by increasing both the number of insects available for release and the number of release events, increasing propagule pressure and augmenting natural dispersal. A poor release strategy can potentially contribute to unsuccessful establishment of biological control agents (Grevstad, 1999). Therefore, optimizing a release strategy specific to a particular agent is but one way to decrease the time to establishment while increasing the total area covered.

The spread of M. scutellaris on the landscape is important because of its potential to integrate with the widespread herbicidal management of E. crassipes. By more efficiently building M. scutellaris numbers and increasing establishment, this species can be more effective in a shorter time span. The objectives of this study were to 1.) evaluate the effectiveness of various release strategies in establishing M. scutellaris populations, and 2.) determine the dispersal capability of brachypterous adult M. scutellaris post-release.