by Joseph DeSisto
Today’s story comes from the tiny but remarkable twirler moths, the Gelechiidae. The moths themselves are small and typically brown, attracting little attention, while the caterpillars go by names like splitworm, bollworm, pinworm, tuberworm, and so on, usually referring to the plant tissues they consume.
Human interest in twirler moths, then, is largely focused on their relationships with the plants that both humans and twirlers relish — potatoes, tomatoes, grains, almonds, and many others.
Twirler caterpillars live sheltered lives — they typically either live inside twigs, fruit, leaves, or other plant tissues, or else use silk to roll leaves into shelters. This way the caterpillars can dine in relative peace, with fewer predators able to find and attack them.
Shelters can be effective against spiders and wasps, but they haven’t stopped humans from using all kinds of strategies, from pesticides to natural predator, to keep twirler caterpillars out of their crops. With good reason — many of these caterpillars cost farmers a huge portion of their annual yield.
Today we are going to focus on one of these moths, the Guatemalan potato moth, Tecia solanivoria (GPM). GPM caterpillars feed underground by boring into a potato and hollowing out a set of tunnels, in which they live until ready to emerge as adult moths. By eating out the inside of the potato, the caterpillars not only damage the crop themselves, they also allow fungi to enter the potato and begin the decomposition process, resulting in a rotten vegetable.
The caterpillars can feed on potatoes both before and long after harvest. As a result, they are easily spread when infested potatoes are shipped across long distances. Although GPM is native to Guatemala, it was first discovered in Costa Rica where it had been introduced in imported potatoes. Entomologists estimate that the introduction occurred in 1970, but the species was not formally described until three years later (Povolny 1973).
Why so long between introduction and description? Here I must editorialize — if Central America had had an active expert in gelechiid taxonomy, the species that was causing so much damage would surely have been discovered much earlier, perhaps even before the species ever left Guatemala! Instead the species was described by a Czech taxonomist, by which time Central American farmers were reporting 20-40% crop losses to the unnamed pest (Povolny 1973). The “outsourcing” of tropical biodiversity research to scientists in first-world countries can have dire consequences.
For reference, here’s a map showing Central America and northwestern South America. GPM originated in Guatemala (upper left) and has quickly spread southeast as far as Ecuador and Colombia. Click to enlarge. Map data: Google, Landsat.
Before 1970, the most significant potato-eating caterpillar in Latin America was the potato tuberworm (Phthorimaea operculella — try saying that out loud). Unlike GPM, the tuberworm is a leaf-miner, munching its way through the 2-dimensional world inside leaves. After its introduction to Costa Rica, however, GPM saw a meteoric rise, and quickly stole the show as the most economically injurious caterpillar for Latin American potato growers (Carrillo and Torrado-Leon 2013). In 1983 GPM was accidentally imported (via infested potatoes) into Venezuela from Costa Rica, and South America became the new front in the moth’s conquest. By 1996 the caterpillars had reached Ecuador, and in 1999 a population was even introduced to the Canary Islands off the coast of Northwest Africa.
Managing GPM is an economic necessity. Unfortunately, this can be difficult since once the caterpillar hatches and starts making its tunnels inside a potato, you’ve pretty much lost the potato. Pesticides are generally targeted toward the more vulnerable eggs and adult moths, but these stages are short-lived. Timing is crucial, and only pesticides applied at the right stage will reduce crop losses. Despite this, potato growers frequently err on the side of caution, applying pesticides intensively throughout the growing season (Carrillo and Torrado-Leon 2013). This is not only a waste of money, it can also lead to the moths becoming resistant to the pesticides, not to mention the environmental and human safety consequences of pesticide overuse.
The most effective methods of reducing GPM infestation are not chemical but cultural (Gallegos et al. 2002). Growers can help protect their plants by tilling the soil to break up clumps (havens for moths and eggs), planting their potatoes deeper in the soil, and removing “leftover” potatoes from the soil.
Here’s the twist: under certain conditions, it can actually benefit the farmer to let the caterpillars feed.
In 2010, Poveda and colleagues experimented with GPM abundance in potato fields in Colombia. Surprisingly, fields in which caterpillars were allowed to feed in moderation actually showed higher total yield than when caterpillars were completely excluded. It turns out that when caterpillars feed, the chemicals in their saliva are strewn about the potato, and the plant is able to recognize and react to these chemicals.
Each potato plant contains many tubers, the things we call potatoes. If less than 10% of these are infested with caterpillars, the plant compensates by diverting resources to the remaining tubers. The result: a greater number of extra-large potatoes, and an increase in overall yield. Plants with limited caterpillar infestations yielded 2.5 times as much useable potato mass as plants in which there were no caterpillars at all.
The lesson? Plant-insect relationships are complicated, and understanding the subtle details can matter. So can the taxonomy of small, “boring” moths. There are a lot of important things we wouldn’t know about GPM if it weren’t for the hard work and dedication of entomologists from Colombia to the Czech Republic. With invasive pests an increasing problem, we need to make sure to invest in both applied and basic research, and study even the insects that most people would rather ignore.
This post marks the start of a kind of experiment for me. Starting here, I will be posting a new article every Tuesday and Thursday. Here’s the catch: each post will focus on a different family of invertebrates, and I can’t cover the same family twice! The goal is to try and write about as many families as possible, starting with Gelechiidae.
Carrillo D. and E. Torrado-Leon. 2013. Tecia solanivora Povolny (Lepidoptera: Gelechiidae), an Invasive Pest of Potatoes Solanum tuberosum L. in the Northern Andes. In: J.E. Pena (Ed.), Potential Invasive Pests of Agricultural Crops (126-136). Boston, Massachusetts: CABI.
Gallegos P., J. Suquillo, F. Chamorro, P. Oyarzun, H. Andrade, F. Lopez, C. Sevillano, et al. 2002. Determinar la eficiencia del control quimico para la polilla de la papa Tecia solanivora, en condiciones del campo. In: Memorias del II Taller Internacional de Pollila Guatemalteca Tecia solanivora, Avances en Investigacion y Manejo Integrado de la Plaga, 4-5 June 2002, Quito, Ecuador pp. 7.
Hayden, J.E., S. Lee, S.C. Passoa, J. Young, J.F. Landry, V. Nazari, R. Mally, L.A. Somma, and K.M. Ahlmark. 2013. Digital Identification of Microlepidoptera on Solanaceae. USDA-APHIS-PPQ Identification Technology Program (ITP). Fort Collins, CO. 7 July 2015 <http://idtools.org/id/leps/micro/>
Povolny D. 1973. Scrobipalpopsis solanivora sp. n. — a new pest of potato (Solanum tuberosum) from Central America. Acta Universitatis Agriculturas, Facultas Agronomica 21(1): 133-146.