If I ask you: ‘what is the simplest animal form you can think of?’ You are likely to think of a worm or a jellyfish. Very few – if any – would even think about insect larvae. We despise them, and see them as so simple that we even forget about their existence until we see a caterpillar crawling over our shoulders.
Larva is the earliest feeding life stage of many (many!) insects that undergo the process of metamorphosis – they are known as holometabolous insects. Holometabolous insects comprise the most diverse and fascinating insect groups ever to exist in this planet, including flies (Diptera, see below), beetles (Coleoptera, Fig 1A), ants, bees and wasps (Hymenoptera), and butterflies and moths (Lepidoptera, see Fig 1B). But their beauty, complexity, and strangeness (i.e., stalk-eyed flies) are rarely seen at the larval stage and therefore it is easy to overlook the importance of the larvae.
But don’t let simplicity fool you.
Larvae are at the at the core of future society development and ecosystem stability. This is because larvae play a wide range of ecological roles including prey, predator, hosts and parasitoids. Larvae are potential markers of ecosystem health and biodiversity. Larvae are also responsible for the majority of the economic burden of pests and also for the biological control of these pests through mass-rearing factories. Finally, larvae can serve as a sustainable source of food to – and can recycle organic waste produced by – animals and humans.
Ecological conditions during larval development can have important and long-lasting effects on individual fitness and population survival. Often though, we forget that, although small, larvae are part of their environment and are subject to multiple ecological factors that shape the quality of their surroundings. With this in mind, we designed an experiment to investigate how two major ecological factors – larval density and microbial growth in the substrate – shape larval development.
Larval density modulates intra-specific cooperation and competition that can alter food availability per capita, either accelerating or slowing down larval development. Microbes growing in the diet can associate with the larvae’s guts to increase nutrient absorption and/or serve as direct source of nutrients to the larvae. However, microbes can also modulate inter-specific competition for nutrients and space with the larvae (animal-microbe competition) to the point where, in some cases, microbes can secret chemicals that harm and kill larvae.
We decided to tackle part of this complexity using the larvae of the polyphagous fruit fly Bactrocera tryoni (Qflies) as model (Fig 2). In the lab, we varied (i) substrate nutrient composition, (ii) larval density, which modulates intra-specific interactions, and (iii) the amount of preservative in the diet to either favour or inhibit microbial growth, which modulates inter-specific interactions.
Some of the results were as expected: protein-rich diets were favourable for larval development (i.e., larva grew bigger and faster) while sugar-rich diets had the opposite effect, and high larval density was costlier to the larvae than low density. Remarkably though, diet-dependent effects were reversed when microbes were present, whereby sugar-rich diet with microbes became the most favourable diet for larval growth (measured as pupal weight). In fact, larvae in sugar-rich diets with microbes grew bigger than larvae in any other treatment, especially when larvae were in low density.
This demonstrates that both intra- and inter-specific ecological interactions – assessed through larval density and microbial growth in different diets – play a major role in determining the conditions where the larvae develop.
This is only the beginning of our understanding of how ecology affects development (‘developmental ecology’). Perhaps once we have a better idea of how ecology plays a role on insect development we will be able to predict and intervene on the increasing loss of insect biodiversity worldwide, as well as use larvae as food, waste recyclers, and vehicles to fight against agricultural pests and disease vectors. Please find out more about our research in our paper1 published in Ecology and Evolution.
 Nguyen, B., Ponton, F., Than, A., Taylor, P.W., Chapman, T., Morimoto, J. (2019) Interactions between ecological factors in the developmental environment modulate pupal and adult traits in a polyphagous fly. Ecology and Evolution.