Soldier Proportion Research Articles

Maternal determination of soldier proportion and paternal determination of soldier sex ratio in hybrid Reticulitermes (Isoptera: Rhinotermitidae) termite colonies.

Altruistic caste, including worker and soldier (derived from worker), plays a critical role in the ecological success of social insects. The proportion of soldiers, soldier sex ratios, and the number of workers vary significantly between species, and also within species, depending on colony developmental stage and environmental factors. However, it is unknown whether there are sex-linked effects from parents on controlling the caste fate or not. Here, we compared soldier sex ratios, soldier proportions, and population size among a four mating types of Reticulitermes amamianus (Ra) and R. speratus (Rs) (male × female, mRa × fRa, mRa × fRs, mRs × fRa, mRs × fRs) and demonstrate that the soldier sex ratio and worker population size of hybrid colonies skew to colonies of king's species, while the soldier proportion skew to queen's species. The survival rate of offspring resulting from interspecies hybridization was significantly higher for mRa × fRs than for mRs × fRa. The results of this study demonstrate the asymmetric influence of kings and queens on caste determination and colony growth, which can contribute to our better understanding of parental influence on the colony dynamics of social insects.

Influence of Soldiers on Exploratory Foraging Behavior in the Formosan Subterranean Termite, Coptotermes formosanus (Blattodea: Rhinotermitidae).

Termites are eusocial insects that live in organized colonies consisting of reproductives, workers, and soldiers. Soldiers are specialized for defense but are expensive to maintain, as they are incapable of husbandry and must be fed and groomed by workers. The soldiers of several species influence foraging behavior by acting as scouts that initiate foraging or by mediating worker behavioral plasticity during food exploration. These behaviors imply that soldiers may play a keystone role in termite colony function, apart from defense. Subterranean termite workers tunnel through soil in search of food while accompanied by varying proportions of soldiers, depending on the species and colony conditions. Previous studies have shown that soldiers accelerate worker exploratory tunneling behavior in two Reticulitermes species, the colonies of which contain fewer than 2% soldiers. This effect, however, is unknown in other subterranean species with different soldier proportions. In this study, we examined the influence of soldiers on exploratory foraging behavior in the Formosan subterranean termite, Coptotermes formosanus Shiraki, which is an economically devastating invasive species that maintains a relatively high soldier proportion (about 10%). When 100 foraging workers were grouped with 0, 2, 10, or 30 soldiers in two-dimensional foraging arenas, we found no significant effect of soldiers on the tunnel length, branch pattern, food source interception, or food collected within 96 h. These results suggest that C. formosanus colonies maintain food exploration efficiency regardless of soldier proportion variation.

A High Soldier Proportion Encouraged the Greater Antifungal Immunity in a Subterranean Termite.

Termites possess a mighty social immune system, serving as one of the key obstacles to controlling them biologically. However, the dynamic mechanism coordinating the social immunologic defense and caste distribution of the termites remains elusive. This study used the Coptotermes formosanus Shiraki and an entomopathogenic fungus as a host–pathogen system and experimentally manipulated a series of groups with different caste compositions of workers and soldiers. Then, the impact of demography on the behavior and innate immunity of termites was explored by analyzing the fungus susceptibility of the respective caste, efficiencies, and caste preferences of sanitary care, as well as the expression of the immune genes and phenoloxidase activity. Overall, to ensure the general health and survival of a group, the infected workers were found to sacrifice their survivorship for maintaining the soldier proportion of the group. If soldier proportion was limited within a threshold, both the survivorship of the workers and soldiers were not significantly affected by the infection. Correspondingly, the infected group with a higher proportion of soldiers stimulated the higher efficiency of a non-caste-biased sanitary care of the workers to the nestmate workers and soldiers. Moreover, the innate immunities of the infected workers were found to be more intensely upregulated in the group with higher soldier proportions. This suggested that the adjustable non-caste-biased sanitary care and innate immunity of the workers would contribute to the flexibility of the worker–soldier caste ratio in C. formosanus. This study, therefore, enhanced our understanding of the functional adaptation mechanism between pathogen-driven social immunity and the demography of the termites.

The production of soldiers and the maintenance of caste proportions delay the growth of termite incipient colonies

In a termite colony, the incipient phase is the most critical part of the life of the colony. The quality of the investment in the first offspring by the primary reproduc- tives may determine the rate of success of the colony to survive the first year and its growth rate in the following years. However, termite colonies possess a physiological constraint, forcing the group to maintain a relatively fixed caste proportion. During the development of the incipient colony, there is therefore a conflict for the group on the developmental pathways of larvae into workers or soldiers. On the one hand, the more workers produced, the more work forces would be available to provide for the primary re- productives, the brood and the nest maintenance (overall nurturing capacity). On the other hand, some larvae must develop into soldiers to maintain the caste proportion, reducing the potential number of workers. Using incipient colonies of Coptotermes gestroi (Wasmann), we investi- gated the cost of maintaining the soldier proportion over the growth of the colony within the first year. Our results showed that an incipient colony maintains a stable soldier proportion regardless of the stress imposed. The resources redirected into the replacement of soldiers not only reduced the total number of workers, but it also reduced the overall growth of the colony by delaying the development of the remaining eggs. Our observations suggest that in termite incipient colonies, because of physiological constraints, the maintenance of the soldier proportion overrides the devel- opment of the colony.

Studies of Precocene I, Precocene II and Biogenic Amines on Formosan Subterranean Termite (Isoptera: Rhinotermitidae) Presoldier/Soldier Formation

The regulation of the soldier caste in termites involves a self-regulatory mechanism of negative feedback once a specific proportion is achieved. In this process, juvenile hormone (JH) plays an important role. Two groups of compounds with known JH regulatory functions in insects were tested for their effects on Coptotermes formosanus Shiraki soldier caste formation. One group was the antijuvenile hormone agents precocene I and precocene II. The other group was biogenic amines including dopamine, octopamine, serotonin and tyramine. Filter paper bioassays in Petri dishes showed that precocene I at 100 μg/dish significantly delayed the formation of the first presoldier and the first soldier. Additionally, precocene I significantly reduced the soldier proportion at 40 d after treatment as compared with controls. Precocene II had no effect. We also found no biogenic amine effect on termite soldier formation. These results may reflect the importance of precocene I in caste control and a reduced importance in bi...

Physical contact between soldier and worker is essential in soldier self-regulation of Coptotermes formosanus (Isoptera, Rhinotermitidae)

In a termite society, the soldier proportion is regulated at a species-specific level with a seasonal fluctuation. A feedback mechanism in soldier proportion regulation is well recognized, while the proximate means by which the presence of soldiers inhibits the new soldier formation from workers remains mostly elusive. In the present study, some possible means for this inhibition were tested with the Formosan subterranean termite, Coptotermes formosanus Shiraki. The results showed that neither soldier-originated volatiles nor non-volatile chemicals without physical contact were responsible for the inhibition of new soldier formation. The essentiality of physical contact between workers and soldiers in soldier regulation was established. However, by physical contacts, as the soldier-replacement experiment showed, workers could not individually recognize or “count” soldiers to maintain a set soldier proportion. The possible feedback mechanisms for soldier regulation are discussed.

Effects of Temperature and Nutrition on Juvenile Hormone Titers of <I>Coptotermes formosanus</I> (Isoptera: Rhinotermitidae)

Colony organization via caste specialization in the Formosan subterranean termite, Coptotermes formosanus Shiraki, as in other species of termites, seems to be influenced by external factors such as nutrition and temperature. Here, we report on the influence of nutrition and temperature and the associated juvenile hormone levels on the regulation of the soldier caste. Two nutritional levels of food sources or five different temperatures were provided to C. formosanus workers in the laboratory. Juvenile hormone titers of these workers were monitored on 0, 12, 24, 36, 48, and 60 d after treatment. Termite survival and soldier-to-worker proportions also were recorded at each sample date. Workers fed a high-quality food source produced significantly more soldiers and this was associated with high worker juvenile hormone (JH) titers. JH titers in workers maintained at 20 and 36°C remained constant throughout the observational period with no presoldier or soldier formation. Under 24, 28, and 32°C temperature regimes, JH titers in workers climbed to a peak at day 48 and then sharply dropped at day 60 when soldier proportion increased to a specific value. At each sample date, JH titers in workers demonstrated a positive and statistically significant relationship to temperatures within the range of 24–32°C, but upon inclusion of temperatures below 24 and above 32°C, the correlation is no longer significant. The results show that favorable extrinsic conditions of temperature and nutrition allow for a higher soldier proportion threshold through the regulation of JH levels and caste differentiation in workers.

Formosan Subterranean Termite (Isoptera: Rhinotermitidae) Soldiers Regulate Juvenile Hormone Levels and Caste Differentiation in Workers

A caste structure is maintained in termite societies and juvenile hormone (JH) is generally regarded as the most important regulator in these termite colonies. Here, we demonstrate that the soldier caste regulates JH in workers of Coptotermes formosanus Shiraki. Worker termites (80–100 individuals) were placed in petri dishes with 0, 5, 10, or 20% soldiers. JH III titers of groups of these workers were monitored at 14, 28, 42, and 56 d. Any changes in soldier caste proportions also were noted at each sample date. On the first sample date, the JH levels in workers were similar among treatments with different initial soldier proportions, and no new soldiers were formed. Over the next three sample dates, the worker JH levels were higher for low initial soldier proportion treatments and vice versa. Concurrently, soldier formation increased with lower initial soldier proportions. JH titers in workers showed a positive and statistically significant relationship to soldier numbers until a certain soldier proportion was reached. These results provide evidence that soldier caste proportions regulate JH levels and thereby caste differentiation in workers. The means by which this regulatory mechanism may proceed is discussed.

Analysis of natural colonies of a social aphid Colophina arma: population dynamics, reproductive schedule, and survey for ecological correlates with soldier production

Field-collected 229 colonies of a pemphigine social aphid Colophina arma, which has a soldier caste in the first instar, were examined for its population dynamics, reproductive schedule, and ecological factors related tosoldier production. Colony size was small in July and became larger in August and on; this change probably reflects the flowering phenology of the host plant Clematis stans. Soldiers, which neither grow nor reproduce but are specialized for colony defense, were found throughout the seasons. Winged sexuparae, which migrate to Zelkova serrata and produce the sexual generation, appeared in late September. Midget first instar nymphs, which persist on C. stans for hibernation, were produced in October. Statistical analyses suggested that soldier proportion in colonies of C. arma might be related to the proportions of unwinged adults, colony size, and seasonal change. Sexuparae and midget nymphs were produced only in autumn, which reflects the overwintering roles of these morphs, and suggests that production of these morphs might be influenced by environmental factors related to the season, such as short photoperiod and low temperature.

Density-dependent induction and suppression of soldier differentiation in an aphid social system

We investigated the mechanism underlying the control of soldier production in colonies of a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the second instar. High aphid density was shown to induce soldier production in T. styraci. Analysis of natural colonies revealed that the soldier proportion tended to increase with aphid density but reached a plateau. Artificial diet experiments identified a similar plateau of soldier proportion under high-density conditions. In order to gain insights into the controlling mechanism of soldier production, the effect of soldiers on reproducing adult aphids was examined using the artificial diet system. It was experimentally demonstrated that soldier production was suppressed by coexisting soldiers, whereas coexisting non-soldiers facilitated soldier production. These results suggested that caste ratio in the colony of T. styraci is controlled by positive and negative feedbacks consisting of density-dependent induction and suppression of soldier differentiation.

Formosan Subterranean Termite (Isoptera: Rhinotermitidae) Wood Consumption and Worker Survival as Affected by Temperature and Soldier Proportion

The effect of temperature and soldier proportion on worker survival, soldier production, and wood consumption of the Formosan subterranean termite, Coptotermes formosanus Shiraki, was evaluated in the laboratory with termites from a single colony. Soldier production was influenced by temperature and the initial proportion of soldiers. On average, termites had a significantly higher survival rate when the initial soldier proportion was <20%. Soldier caste development was at its highest at 30°C, and when treatment started with no soldiers in the colony. Termite survival was significantly higher at 30°C than at 25 and 33°C after 12 d of testing. There was a significant interaction of temperature and soldier proportion on termite survival after 36 d. C. formosanus could support higher proportions of soldiers with an increase in temperature. There was no significant decrease in survival rates with increasing numbers of soldiers at 25 and 33°C. At 60 d, survival at 20 and 25°C declined significantly when soldier proportions exceeded 20%. At 30 and 33°C, termite survival did not decline significantly when soldier proportions increased from 20 to 30% (30°C) or 40% (33°C). Consumption rates increased significantly with rising temperature up to 30°C. Consumption rates tended to increase as soldier proportion increased. There was no significant interaction between temperature and soldier proportion on wood consumption rate.

Reproductive schedule and factors affecting soldier production in the eusocial bamboo aphid Pseudoregma bambucicola (Homoptera, Aphididae)

The reproductive characteristics of the soldier-producing aphid Pseudoregma bambucicola were studied in Kagoshima, Southern Japan, to know the factors affecting soldier production of eusocial aphids. The soldier proportion in aphid colonies was highest from October to November. In some large colonies, soldiers were observed in all seasons except in July when colony size was relatively small. Multiple regression analysis showed that the colony size was a principal factor affecting soldier proportion throughout a year. Other social or environmental factors such as aphid composition, host plant conditions and predator abundance were not always significant. Rearing experiments revealed that large colonies (≥1,000 individuals) produced soldiers in almost all seasons while small colonies ( 10 and >20, respectively. Because they have a small number of ovarioles (<15 on average), females should alter caste production within the same ovarioles according to changes in environmental conditions. Artificial removal or introduction of predators and reduction of colony size did not affect soldier production over two successive generations, revealing maternal effects on soldier production. Females cannot shift caste production quickly in response to changes in predator abundance and colony size. This is probably due to early developmental determination of castes within the mother's body.

Factors affecting the proportion of soldiers in eusocial bamboo aphid,Pseudoregma bambucicola, colonies

The proportion (percentage to all individuals) of soldier-type first-instar larvae in colonies of the eusocial aphid, Pseudoregma bambucicola, was measured at bamboo stands in Kagoshima Prefecture, Japan, in relation to colony size, percentages of adults and alatae, host conditions and syrphid natural enemies. A significantly positive correlation was found between the soldier proportion and colony size in all 3 months. Multiple regression analysis showed that the colony size and the proportion of alatae had highly significant partial regressions to the soldier proportion. Length of bamboo shoots on which colonies were observed showed a positive correlation to the soldier proportion, but this correlation was not supported by multiple regression analysis. The number of syrphid larvae living in colonies showed a negative, but insignificant, correlation to the proportion of soldiers.