Junyan Chen, Husseneder, Claudia, Sun, Qian
Junyan Chen, Garima Setia, Qian Sun and Claudia Husseneder
Formosan subterranean termites, an insect pest familiar to most Louisianans, attack trees and crops and damage structural wood. The yearly cost for Formosan termite control and damage is $500 million in Louisiana alone.
Termites are social insects. They live in colonies characterized by a division of labor. A mature Formosan subterranean termite colony contains queens and kings that produce offspring, soldiers for colony defense and workers for building, foraging and brood care. Flying termites (alates) are produced between April and July (Figure A). They disperse to establish new colonies and become kings and queens. After swarming, alates shed their wings and find a partner by running in tandem. The pair finds a nest site underground, mates and produces their first brood. The new king and queen do not forage, but they raise the first workers using their own fat reserves until the workers are old enough to forage and take care of the brood.
The diet of Formosan subterranean termites consists of wood, which is low in nutrients and difficult to digest. The workers forage and feed the colony and have a complex community of gut symbionts, which are microorganisms that live in the termite gut for mutual benefit. These symbionts include five species of protozoa and hundreds of bacteria species. Protozoa and bacteria are both types of single-celled organisms. Termites will starve to death without these microorganisms, which are vital for extracting nutrients from wood.
The initial colony development is the most critical phase in the entire life cycle of the colony. The growth of the new colony is dependent on the resources that the colony founders invest in the first offspring. Because symbionts are vital for the termite colony, swarming alates have to pack essential protozoa and bacteria into their guts before they disperse to found a new colony. The pair feeds their first brood of workers with that “starter culture” of their own microorganisms.
Researchers from the LSU AgCenter entomology department set out to study how the abundance of protozoa and their bacteria symbionts changes in the early stages of colony development. In May 2021, alates were collected from swarms in Baton Rouge and New Orleans (Day 0). At Day 2, a total of 126 pairs were picked to raise new colonies in the lab. The first eggs were observed in 10-to-15-day-old colonies (Figure A). Larvae with short antennae and small white bodies hatched around Day 20. Larvae developed into eight to 15 workers per colony around Day 30 and one to three soldiers around Day 70. During each of these milestones of early colony development, reproductive pairs were sacrificed to assess numbers of the largest protozoa species (Pseudotrichonympha grassii) by counting them under a microscope. Their internal bacteria symbiont Candidatus Azobacteroides pseudotrichonymphae (CAP) was enumerated by sequencing a bacteria specific marker gene. Pseudotrichonympha grassii (Figure B) plays a major role in wood digestion, and CAP is responsible for capturing nitrogen from air. Nitrogen cannot be derived from a wood diet, but it is important for protein synthesis.
The P. grassii numbers in kings and queens increased 10- to 30-fold over the first 40 days of the new colony, followed by a rapid decrease when the first generation of workers emerged (Figure C). The number of CAP bacteria followed this trend as expected since these bacteria are internal symbionts of the protozoa. They represented less than 10% of bacteria in young pairs at Day 2; however, CAP bacteria proportion in pairs increased to around 40% in 40 days before declining toward day 70. During these early stages of colony development, CAP was the dominant bacteria species in all kings and queens.
The decline of protozoa and CAP bacteria in reproductives (the queens and kings) with the emergence of the worker caste signals the transition from internal to external colony nutrition. During early colony development, the founder pairs are the sole source of nutrition for the offspring. After workers emerge, they receive symbionts from the parents to cultivate in their own guts and they become the caste specialized for foraging and feeding the colony.
This study is expected to lay the groundwork for further investigation into the three-way relationship between termite hosts, the wood-digesting protozoa in the termite gut and their internal bacterial symbionts. This research might lead in the future to novel termite control methods by disrupting the balance of key microorganisms in the termite gut.
Junyan Chen is a doctoral candidate student, Garima Setia is a Master of Science graduate student, Qian “Karen” Sun is an assistant professor and Claudia Husseneder is a professor and adviser for Chen and Setia in the Department of Entomology.
(This article appears in the winter 2022 issue of Louisiana Agriculture.)
Figure A. Stages of early termite development: Winged termite alates of the Formosan subterranean termite were collected from the field (Day 0); Male and female alates break off their wings before forming a pair that runs in tandem to find a nest site (Day 2); First termite eggs were observed between Day 10-15 in lab colonies; Around Day 15-20, the white and semi-translucent larvae hatched; Around Day 30-40, workers were observed in the lab colonies recognizable by their developed mandibles (mouthpart).
Day 0 – Alate
Day 2 – Tandem running pair
Day 10-15 – Eggs laid
Day 15-20 – Larvae hatched
Day 30-40 – First workers
Figure B. Termite gut and protozoa as seen under a microscope. The whole gut was pulled out from the body of kings and queens and examined under the microscope at different magnifications.
Figure B1. The whole termite gut has been extracted, and the hind gut was pricked by a dissecting needle (viewed under 10X magnification).
Figure B2. Protozoa leak from the pricked hind gut (viewed under 100X magnification)
Figure B3. P. grassii protozoa under 400X magnification.
Figure C. Change in numbers of symbiotic microorganisms in the early stages of colony development. The number of P. grassii protozoa and the number of CAP bacteria rapidly increased during the first 15 days after the new colony was established; after 40 days, when the first workers emerged, their numbers started to decrease.
Figure C1. P. grassii protozoa number changes during the colony developing.
Figure C2. The number of Candidatus Azobacteroides pseudotrichonymphae bacteria changes during the colony developing.