This is all you'll ever get...: Hidden Herbivores

When thinking about plant-herbivore interactions, it is important to remember that not all damage caused to plants by herbivorous insects is immediately obvious. In fact, the majority of herbivory on plants (and the most damaging) is often caused by insects which cannot immediately be seen. Many types of insects live almost their entire lives inside a plant, emerging only as adults in order to find a mate. This can result in entire food chains (also involving predators and parasites) being enclosed within a single stem, leaf, or root, and can make them particularly interesting to study due to the closed nature of the system.

Ganesh collecting galled stems at the native Rappahannock River site

In Phragmites australis, some internally feeding insects induce growth abnormalities in the stem, known as galls. Symptoms of galls are easily recognisable in the field (once you know what to look for), characterised by shortening and swelling of the stem near its tip, and the death of the top leaf (see below). During our summer research trips we collected around 100 stems per site, from a variety of both native and invasive Phragmites sites throughout the United States. We kept these stems on ice and once back at LSU stored the stems in a freezer to preserve the insects inside them.

Galled stems being readied for dissection by removing leaf sheaths

Stems were prepared for dissection by removing leaf sheaths and were then dissected using a splitting tool by cutting the stem from the bottom to the top, examining its inner space for any kind of insect or arthropod species. All organisms found were grouped into a particular recognisable taxonomic unit (RTU - visually distinct species) for each site and stored in vials of 96% ethanol for preservation. We also recorded on a data sheet exactly what we found in each stem and any other relevant information.

The dissection station

So over the course of this past semester, Randee (one of the undergraduate students working our lab) and I dissected a total of 2025 stems. It was a lot of repetitive work and hours under the microscope. Each type of insect we found would be given a funny name for future identification such as "narrow white guy", "claw worm", "orange pupae", "emerging wasp", and "boring brown mite", and we also photographed each new type of insect we found. In total we collected around 240 arthropod samples.

Lipara sp. with inquiline

The organisms which actually cause the galls are fly larvae (immature stage) from a genus called Lipara, of which there are a number of species: Lipara pullitarsis, Lipara rufitarsis, Lipara similis, and Lipara lucens). Three of these are established in the United States. Lipara similis is one of these species, and is easy to identify thanks to the dark colouration at both ends of its body (see picture below). Lipara pullitarsis and Lipara rufitarsis are also established (see picture above), but distinguishing between these two species as larvae is very difficult. Larvae of other insect species can also be found living inside the galls with the Lipara. These are commonly known as inquilines, and while some have no negative effects on the Lipara, some can be predatory or parasitic.

Lipara similis

The next step in this project will be to finalise sorting and identification of all of the insects collected. The identification process will be difficult, as it's notoriously hard to identify immature insects. Once identifications are complete, the data can be used to answer a number of interesting ecological questions. Some of the questions we are interested in is how community composition of the galls differs between invasive and native Phragmites, and also how it may vary with latitude. Results will be forthcoming in the next couple of months, and I'm looking forward to seeing if all our hard work collecting and dissecting stems has paid off. For now, enjoy some of the pictures of the hidden herbivores of Phragmites.