Biological Control of Mosquitos Through the Implementation of Flatworms

by Collin Rogers
BIOL/WATR 361, Spring 2015

Key taxa: Platyhelminthes, Turbellaria, Tricladida and Rhabdocoela

It is well known that predators can be effective at controlling populations of prey. It has also been observed that fish manipulate the food chain. Predation has also been observed closely with bobcat and snowshoe hare populations. Prey can be of many different life forms, from hares to fish. However, in this case, the prey are mosquito larvae. Mosquitos are pests to humans and vectors of various diseases. The fear of mosquitos in the past has been so overwhelming that crude oil has been sprayed in wetlands to kill larvae. In the early 1900’s the fear of malaria was so evident that wetlands were drained to control mosquito hatches. Even with today’s modern technology and pesticides mosquitos are still annoying pests to humans. You can spray yourself with bug spray, but there must be a chemical-free, more natural way. Considering all the evidence regarding the potential of biological control of mosquito populations, flatworms would not likely make effective biological control agents in a natural environment.

Among the many natural remedies, few are biological control agents. The phylum Platyhelminthes is a taxon containing flatworms. Within this phylum the class Turbellaria contains free living flatworms. The turbellarian flatworms are generally small (mm-cm) aquatic organisms that are typically benthic oriented. They feed on various food items. Some gain their nutrients through detritus while some are predators that consume small crustaceans, worms, insects, and even carcasses of larger dead animals.

Of the predatory Turbellarians the orders Tricladida and Rhabdocoela are taxa containing flatworms large enough to consume small invertebrates that are visible with the naked eye. Feeding strategies of these flatworms vary in the form of prey capture and consumption.

The primary form of triclad locomotion is crucial to their feeding strategy. Cilia on the underside of the flatworm facilitate movement with the aid of a mucus layer that is also secreted underneath the flatworm. Prey is often caught after it has become tangled within the mucus trail left by a traveling triclad. Prey capture with a mucus trail is most effective at capturing prey with multiple appendages, which become more entangled as they attempt to break free (Jennings, 1957). This can be crucial in capturing arthropods and insects.

The order Rhabdocoela consumes prey without the aid of a mucus trail. Rhabdocoelans possess a rostral organ capable of extension. The rostral organ is capable of extending one-third of the organism’s body length (Wrona & Koopowitz, 1998). Small prey can be ingested whole, while larger prey with exoskeletons need a different form of consumption. Once capture the prey are attached at the pharynx of the flatworm. The rostral organ is capable of penetrating the invertebrate’s exoskeleton. Immediately following penetration from the rostral organ mosquito larvae were immobilized (Wrona & Koopowitz, 1998).

Mosquito larvae are typically suspended within the water column. This separates the flatworms and prey by a distance of the water depth. However, adult rhabdocoelans were occasionally observed swimming within the water column and near the surface, and juveniles were observed to occur mainly in open water (De Roeck et al., 2005). This suspended swimming behavior would likely increase contact time of predator and prey, leading to an increase in predatory effectiveness.

It is known that these orders of turbellarians are predators and that they feed on mosquito larvae. To effectively control mosquito populations, the predators must be able to significantly reduce the number of mosquito larvae in theory. Triclads were observed to greatly reduce mosquito larvae density (Tranchida et al., 2014). This mosquito density decline was observed to occur throughout the mosquito breeding season. Although mosquito density was significantly reduced by triclad flatworm predation, the extent of this reduction diminished with increasing water depth (Tranchida et al., 2014). This restricts the effectiveness of predation to small, shallow habitats. Mosquito populations in wheel barrows, drums, and buckets would be less effected by predation because these containers are deeper water habitats. However, shallower habitats including tires, bird baths, and small lowland puddles would be ideal habitats to increase the effectiveness of predation and increasing the potential of biological control through flatworms.

Another factor of predation effectiveness is water velocity. Flow is detrimental to the effectiveness to reduce prey density. In a study testing prey density reduction from Tricladida with various water velocities, the greatest level of predation occurred at the slowest velocity (Hansen et al. 1991). If the greatest reduction in prey density occurred at the slowest water velocity one could infer that standing water would be the most effective habitat to observe effective predation. Although Hansen et al. (1991) did not have a control group with a water velocity of zero cm/s, all the other studies exhibited zero water velocity and contained significant reduction in mosquito larvae densities.

If turbellarians were to be effective biological predators they must be able to persist in the applied location. If one were to introduce these flatworms to stagnant water pools to control mosquito larvae they could be effective until cessation of the first generation. In order to achieve effective control the predators would ideally need to reproduce and survive seasons. Triclad cocoons were observed in study pots at the end of the study (Tranchida et al., 2014). The deposition of cocoons within the introduced water traps displays the ability of reproduction and survival past the first generation. If the predators can reproduce where introduced, and the cocoons could survive seasons or desiccation, introduction to small ephemeral water pools could prove effective at lowering mosquito populations.

Turbellarians could be possible candidates for biological control if introduced into the water holding containers. They can persist over seasons, and studies show they can significantly reduce densities of larval mosquitos in experimental outdoor settings. Flatworms would be most effective at controlling population densities of prey that are more closely associated with benthic substrates. Thus shallower water can increase predation on mobile, suspended prey.

Although it has been shown that they can be effective predators I believe it is not a feasible method to control mosquito populations for human benefit in a natural environment. This would require a land owner to disperse planarians throughout their property into all visible standing pools of water. It is rather unlikely to locate and distribute enough turbellarians into each pool of water. One more reasoning for my position opposing the realistic ability to control mosquito populations is dependent on the habitat of the adult mosquito. The adult mosquito is a terrestrial insect with the ability to fly. Thus if one were to deposit turbellarians into all the standing water pools throughout their property they are still likely to encounter mosquitos. As adult mosquitos are terrestrial flying insects they have the ability to travel. It is feasible for mosquitos to travel onto the landowner’s property without having any mosquito larvae habitat nearby.

Flatworms could potentially be effective biological control agents of indoor mosquito populations. If they were introduced into larval habitat within a greenhouse or similar setting, predation from flatworms could effectively control the mosquito population. The reasoning for the potential of effectiveness in this setting to be greater than in a natural environment lies with the control of movement of adults. If adult mosquitos are not freely allowed to immigrate into the environment larvae could be controlled effectively. If adults were to enter the greenhouse a few times a day, for example the opening of a door, the flatworms would be waiting for the prey, or mosquito larvae to hatch.

Predation on mosquito larvae by turbellarians did not completely suppress all present mosquito larvae within the habitats. Research demonstrates that flatworms are capable of significant reductions in mosquito larvae densities, but no studies showed complete population removal. If some mosquitos are to survive they are likely to reproduce elsewhere and still encounter humans.

References Cited

• De Roeck, E., T. Artios & L. Brendonck. 2005. Consumptive and non-comsumptive effects of turbellarian (Mesostoma sp.) predation on anostracans. Hydrobiologia 542: 103-111.
• Hansen, R., D. Hart & R. Merz. 1991. Flow mediates predator-prey interactions between triclad flatworms and larval black flies. Oikos 60: 187-196.
• Jennings, J. 1957. Studies on feeding, digestion, and food storage in free-living flatworms (Platyhelminthes: Turbellaria). Biological Bulletin 112: 63-80.
• Tranchida, M., S. Pelizza, M. Micieli & A. Marcia. 2014. Consequences of the introduction of the planarian Girardia anceps (Tricladida: Dugesiidae) in artificial containers with larvae of the mosquitoes Aedes aegypti and Culex pipens (Diptera: Culicidae) from Argentina. Biological Control 71: 49-55.
• Wrona, F. & H. Koopowitz. 1998. Behavior of the rhabdocoel flatworm Mesostoma ehrenbergii in prey capture and feeding. Hydrobiologia 383: 35-40.