Using insects to wage the war against our invasive plants

Why biological control?

Chironomid and fly damage on Lagarosiphon shoot tips at a field site in the native range. Extensive branching induced by chironomid and ‘silvering’ of leaves indicate where fly larvae have damaged the plant.

Generally, only a limited number of alien organisms that are imported into new environments become invasive. Those that do become invasive can have significant detrimental impacts on aquatic and terrestrial environments, affecting how they function and changing the distribution and population size of other species inhabiting the same space. One of the reasons why some plant species become invasive is that they persist in the new environment without the natural mechanisms of control that are otherwise in place in the region from which they originate.

Insects that feed on plants are a natural mechanism of control and can regulate plant populations by feeding on the photosynthetic parts, reducing plant vigor or feed on the reproductive parts reducing the seed production. Classical biological control is a scientific discipline that looks to re-unite the most effective mechanisms of natural control found in the native region to reduce the invasiveness of alien plant species in the introduced range. This usually involves the use of a highly specific insect, mite or pathogen that is considered a natural enemy of the target plant in the country of origin. Weed biological control agents are not genetically modified and are chosen on the basis of their specificity to the target species, minimizing non-target impacts.   

Established procedures

Each control programme is specific to the target species but modern classical biological control programmes usually include several key steps:

  • Surveys in the native range of the plant to collect and prioritize the most appropriate candidate biological control organism for the new environment
  • Importation into and the establishment of candidate agents in a quarantine facility in order to conduct laboratory assessments
  • Biology and host association studies to determine the interaction between the candidate agent and target plant and how this fits into the new environment
  • Host specificity testing to inform a risk assessment; a necessary pre-requisite before any biological control agent is considered for release
  • Mass rearing and the redistribution of the biological control agent to maximize the efficiency of the control programme

Targeting Lagarosiphon

Adult male shoot-tip mining midge (Polypedium sp.)

Although it’s a weed in many countries, Lagarosiphon has not previously been targeted using classical biological control. As a result a collaborative project between University College Dublin (UCD), Inland Fisheries Ireland (IFI) and Rhodes University (South Africa) began in 2008 to investigate the prospects and safety of this control method.

The key to Lagarosiphon’s successful invasion is its rapid growth rate and spread through vegetative fragmentation. Successful control will therefore depend on natural enemies that reduce the growth of shoots and minimize the viability of plant fragments.

Hydrellia fly adult sitting on a Lagarosiphon shoot tip damaged by the larval stages.

Focused surveys since 2008 in South Africa, part of the plant’s natural range, have resulted in the discovery of some promising candidate biological control agents. As part of the collaborative project several natural enemies have been imported into quarantine and are undergoing biology and host specificity studies. These include a leaf-feeding fly, Hydrellia lagarosiphon (Diptera: Ephydridae), and shoot-tip mining midge Polypedium sp. (Diptera: Chironomidae). Other natural enemies, like leaf-feeding moths were found to be damaging in the country of origin but still need to be assessed in future.

Leaf mining fly Hydrellia lagarosiphon

In July 2009 a leaf-mining fly Hydrellia lagarosiphon (Diptera: Ephydridae) was imported from South Africa into the quarantine facilities at UCD as part of a classical biological control programme targeting Lagarosiphon major (Hydrocharitaceae). As a previously unknown species the basic biology of the fly and its association with the plant was first assessed in the laboratory. The larval stages only feed on the leaves near the shoot tips reducing the photosynthetic potential of the plant. The fly completes its development in about 40 days, and adults emerge to reproduce and lay eggs on emergent plants and the water surface. The larval damage to shoots is significant even at low densities, and plant fragments (10-15cm in length) damaged by larvae are less likely to establish new plants, even at low levels of damage.

The impact of the fly is further enhanced when multiple fly generations are maintained on plants. Host specificity studies have been initiated and preliminary results indicate that L. major is the preferred plant but some Potamogeton species may sustain damage under confined conditions. Further experiments will need to be run to verify the significance of this potential non-target damage.

Shoot-tip mining midge Polypedium sp.

In June 2011 a population of a shoot-tip mining midge Polypedium sp. (Diptera: Chironomidae) was imported into the quarantine facilities at UCD. Although most non-biting midges feed on detritus, algae or are predaceous on other invertebrates this midge feeds on living plant material. The larval stages are plant feeding, starting on the stems and base of the leaves near the shoot tip and then later burrow into the meristem of the tip and into the main stem. The damage to the main and side shoots is severe stunting the growth of the plant. The immature stages of the life cycle are completed in about 40 days. The adults are terrestrial, live for less than 5 days and the female adult lays about 250 eggs.

The early indications are that the larvae do considerable damage to Lagarosiphon and do not necessarily promote fragmentation. Other encouraging factors are that the egg production is high and the generation time (egg to adult development time) is relatively short both contributing to a potential rapid population growth rate. Host specificity testing has been initiated on the chironomid but it is too early to provide reliable indication of its degree of specificity to the target plant.

Dr. J-R. Baars
Rosie Mangan
William Earle
BioControl Research Unit
School of Biology and Environmental Science
Science Centre (West)
University College Dublin
Belfield, Dublin 4
Ireland
Tel: +353 (0)1 7162346 wk
Fax: +353 (0)1 7161152
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