Insect population persistence: the do-or-die of dispersal

Posted by Hester Williams @HesterW123

Dispersal is an integral part of population dynamics. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for its own fitness, but also for population persistence and distribution.

Understanding the causes and consequences of dispersal is vital for population management and predicting population response to changes in the environment. This is particularly important in conservation and re-introduction efforts, biological control and management of alien species. Furthermore, the factors that determine the extent to which dispersers are selective and capable when searching for a new habitat is of interest.

A newly arrived population of a potentially invasive species is usually small, and dispersal could play an important role in its establishment success. Species with a high dispersal rate could end up spread out too thinly, resulting in the inability to find suitable mates, the loss of predator dilution or to defend against predators. These and/or other component Allee effects could scale up to a demographic Allee effect and ultimately lead to the demise of the population.

Reasons for dispersal

The reasons for dispersal are multiple and could include factors such as finding suitable host patches, finding potential mates, avoiding inbreeding with kin, avoiding intra- and interspecific competition, and escaping low or declining host patch quality.

Cues used during dispersal

The bee Chelostoma rapunculi makes use of a combination of visual and olfactory cues to find its host plant

When searching for a new habitat patch, insect dispersers make use of several cues, including visual (shape, size, colour) and/or olfactory cues (communication chemicals such as host-plant volatiles and pheromones). For example, the bee Chelostoma rapunculi makes use of a combination of visual and olfactory cues to find its host plant. Similarly, the Asian Longhorned Beetle, Anoplophora glabripennis uses both olfactory and visual cues to locate its host plant Acer negundo.

For certain insect species the chemical cues from colonised host plants are important in host location; in this case both cues released by conspecifics already colonizing the plant (pheromones) and plant cues induced by herbivore feeding (feeding-induced plant volatiles), or by oviposition, can influence the apparency of the host patch. This gives rise to a clumped distribution or aggregation of the insect species on selected host patches.

Using chemical cues from colonized plants can be both beneficial and detrimental. Particularly, it signals the availability and quality of food and the presence of conspecifics, thereby negating the Allee effect through dilution of predation risk, overcoming host defences and ensuring potential mates. Ultimately it reduces search costs, and other costs related to exercising vigilance during foraging and mate-finding. For example, the flea beetle, Phyllotreta cruciferae, makes use of a combination of male-produced pheromone and feeding-induced host volatiles to form aggregations under field conditions.

On the negative side, the volatiles from an herbivore-infested plant represent a food source with competitors and elevated risk of influx of predators and parasitoids. For example, feeding-induced volatile emissions from Nicotiana attenuata plants increased predation by a generalist predator, Geocoris pallens, on the eggs of the flea beetle Epitrix hirtipennis.

My study

My interest in the topic of dispersal is its role in the initial establishment and population growth of small, recently arrived populations of alien species – especially during eradication efforts when small populations are subjected to management actions such as host removal. During this process habitat is broken up into fragmented host patches, often with reduced numbers of individuals scattered over several patches surrounded by unsuitable matrix. Should these individuals roll the dice and aggregate?, and thereby form a population large enough to overcome Allee effects. If they remain scattered (no dispersion or inefficient dispersion), will they eventually die out?

Neolema ogloblini adult on stem of Tradescantia fluminensis.
Photo: Murray Dawson.

My studies use the leafbeetle Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis in New Zealand, as proxy for an invasive insect pest species. By studying the dispersal choices of recently released adults of N. ogloblini, I was able to determine that the beetle species utilizes cues from colonized host patches. The beetles responded to the presence of actively-feeding adults, but not to non-feeding adults, suggesting their response is motivated by feeding-induced volatiles and/or pheromones that are only released while feeding.

Experiments to determine how efficient the beetles are at finding patches of their host plant as influenced by the degree of isolation of a potential host patch and the matrix surrounding it, is to be completed this summer.

Results of my studies will ultimately give guidance on what eradication approaches are promising for particular invasive species.

Hester Williams is a PhD candidate in the School of Biological Sciences, University of Auckland and is stationed with the Landcare Research Biocontrol team in Lincoln, Canterbury. She is interested in invasion processes of both insect and plant species. Hester is supervised by Darren Ward (Landcare Research/University of Auckland) and Eckehard Brockerhoff (Scion), with Sandy Liebhold (USDA) as advisor. Her studies are supported by a joint Ministry for Primary Industries – University of Auckland scholarship. The project is an integral part of an MBIE program “A Toolkit for the Urban Battlefield” led by Scion.