Research Article |
Corresponding author: Atte Komonen ( atte.komonen@jyu.fi ) Academic editor: Michael Ohl
© 2022 Atte Komonen, Jyrki Torniainen.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Komonen A, Torniainen J (2022) All-day activity of Dolichovespula saxonica (Hymenoptera, Vespidae) colonies in Central Finland. Journal of Hymenoptera Research 89: 157-170. https://doi.org/10.3897/jhr.89.79306
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In social vespid wasps, colony activity varies at many temporal scales. We studied the peak season activity (number of individuals entering the nest per min) of colonies of the social vespine wasp Dolichovespula saxonica in its native range in boreal Finland. Six colonies were monitored non-stop for a full day, starting before sunrise and ending after sunset. Shorter monitoring was carried out before and/or after the full-day monitoring. All colonies were active before sunrise and after sunset, and the overall activity was positively linked with colony size. Activity showed irregular minute-to-minute cycles in all colonies. The broader within-day dynamics were idiosyncratic among the colonies: activity varied generally between 40–100% of the peak, there were usually a few peaks per day, and the timing of the peaks varied. Ambient temperature was not related to activity dynamics consistently. Our study provides high-resolution information about the all-day activity of D. saxonica and underscores high among-colony variability in the dynamics of vespine wasps.
Nest activity, social wasps, time series, traffic rate, Vespinae
In social vespid wasps, colony or nest activity varies at many temporal scales: within and between days, as well as over seasons. It is well established that the consistent seasonal change in the overall activity of successful colonies is largely related to change in colony size (
Colony activity can be divided in two interrelations: activity inside and outside of the nest. Studies on outside activity generally focus on flight activity, which is measured as traffic rate (i.e. the number of incoming and/or outgoing wasps). Colony-specific traffic rate and its daily variation are influenced by the developmental stage of the colony, ambient environmental conditions and time of day (
Most studies on colony activities of Vespinae have focused on Vespula (
In boreal and temperate climates, vespid colonies are annual, each founded by a single queen (
We studied the all-day activity of D. saxonica colonies in the native range of the species in boreal Finland at peak season. Flight activity was measured as traffic rate (number of ingoing individuals) with one-minute accuracy, which allowed to examine high-resolution variation in activity. We asked: 1) are the traffic rates and traffic rate dynamics similar or idiosyncratic among colonies; 2) what are the characteristics of the traffic-rate time series; and 3) does ambient temperature influence within-day dynamics? To understand among-colony variation in traffic rate, nest characteristics and parasitism were recorded.
The study was conducted in Jyväskylä, Central Finland, which belongs to the middle boreal zone. The studied wasp nests (n = 6) were inside wooden bird nest boxes (1.3–2.0 m above ground; Suppl. material
To allow re-monitoring, the nests were removed, dissected and their characteristics were recorded some weeks after the full-day monitoring (Suppl. material
In all nests, we and nine assistants did one non-stop, full-day monitoring close to the peak activity of the season (14 August 2020 in one nest and 14–23 July 2021 in five nests; hereinafter peak season). Peak season was determined based on a few shorter midday monitoring (Suppl. material
Traffic rates were expressed and analysed as individuals per minute, except in figures where longer time periods were needed for illustrative reasons. For example, within-day activity was illustrated using centered moving averages, calculated over seven 5-min periods (i.e. 35 min). Odd number of 5-min periods was used for illustrative reasons, and the 35-min moving average was used because it adequately smoothed the data and revealed underlying trends. The average full-day traffic rates were calculated between sunrise and sunset. Coefficient of Variation (CV) and autocorrelation analysis were used to describe variation in traffic rates. There were only 12 minutes of missing data, which were replaced with the data immediately before the gap. Linear regression was used to analyse the relationship between the mean traffic rate and colony size, and between the mean traffic rate and its variation (CV).
To identify patterns in the sequence of traffic rates over time, we used an autocorrelation analysis. First, sequence plots and the augmented Dickey-Fuller Test (ADF), as well as autocorrelation and partial autocorrelation plots (with a maximum number of lags = 30) were used to detect trends and seasonal effects in the time series. For subsequent autocorrelation analyses trends were removed by transforming the data using differencing (d = 1), i.e. making the time series stationary. Transformation was needed to analyse patterns in the fine-scale, minute-to-minute variation in traffic rates.
To analyse the relationship between temperature and within-day traffic rate dynamics (i.e. the broad variation in the mean traffic rate over day), we used the Expert Modeler option in SPSS, which automatically identifies and estimates the best-fitting ARIMA or exponential smoothing models; only non-seasonal models were considered. Because temperature was measured at 30-min intervals, we made it continuous by replacing missing values using linear interpolation. In the Expert Modeler, the final model only includes those independent variables, which have a significant relationship with the dependent series. Model fit was judged by the non-significance of the Ljung-Box statistic and visual judgement of the residual autocorrelation plots; in Sippula, the non-significance was reached after automatic removal of outliers. No transformations were used.
All analyses were conducted with IBM Statistics SPSS 26.0, except the augmented Dickey-Fuller Test which was conducted with RStudio Version 1.3.1093 (library ‘tseries’).
During the peak season, D. saxonica flight activity started about half an hour before sunrise and increased sharply, whereas the evening decline was more variable (from sharp to gradual) and ended 28–77 minutes after sunset (Figs
Traffic rates (number of individuals entering per minute) for the studied Dolichovespula saxonica colonies in Central Finland from sunrise to sunset, and colony size.
Colony | Med. | Mean | SD | CV% | Min | Max | Sum† | # of combs | # of cells |
---|---|---|---|---|---|---|---|---|---|
Pitkäruoho | 8.0 | 7.76 | 2.80 | 36 | 0 | 22 | 8789 | 7 | 1784‡ |
Sulkula | 4.0 | 3.91 | 2.16 | 55 | 0 | 12 | 4279 | 5 | 1237 |
Eerola | 4.0 | 4.41 | 2.34 | 53 | 0 | 16 | 4859 | 4 | 1011 |
Sippula | 2.0 | 2.18 | 1.63 | 75 | 0 | 13 | 2396 | 3 | 662 |
Haukanniemi | 1.0 | 1.07 | 1.08 | 101 | 0 | 6 | 1037 | 4 | 585 |
Siirtola | 1.0 | 1.28 | 1.10 | 87 | 0 | 5 | 1439 | 3 | 525‡ |
Sulkula re-measured | 1.0 | 1.44 | 1.29 | 90 | 0 | 9 | 1456 | ||
Eerola re-measured | 1.0 | 1.55 | 1.53 | 98 | 0 | 10 | 1585 |
The overall level of variation in the 35-minute traffic rate differed among nests (CV% min-max = 36–101; Table
The number and timing of the peaks and lows varied among nests, and the lows of the 35-min moving average were generally about 40% of the peak (Figs
Parasitism rate was low. No parasitoids or parasites were found in two nests. Eight cocoons of Sphecophaga vesparum (Curtis) (Hymenoptera: Ichneumonidae) were found in one nest, which is only 0.8% of the total number of cells. Larvae of Aphomia sociella (Linnaeus) (Lepidoptera: Pyralidae) were found in three nests (2, 25 and 94 individuals). (Suppl. material
The mean daily traffic rate as well as traffic rate dynamics changed over season. Those two nests, which were re-monitored for a full day later in the season, had then a lower traffic rate, different timing of peaks and showed a more gradual increase (over 2 to 3 hours) after sunrise; the evening decline was inconsistent among the two nests. Furthermore, the timing of the mid-day activity peaks varied over the season, but subtly (Suppl. material
Our results support the long daily activity of vespine wasps (
The sunrise to sunset traffic rates varied among nests and reflected colony sizes. Daily traffic rate is a function of worker numbers and flight activity. Worker numbers are largely related to seasonal phase of a colony, but other intrinsic and local environmental factors have a role since nearby, mature colonies vary in size (
At the peak season, D. saxonica activity dynamics were idiosyncratic among nests, i.e. the timing of the activity peaks, based on a 35-min moving average, varied among nests. The morning peak, which was generally not the daily peak, was followed by a variable period of lower traffic rate. The only extant study of the within-day dynamics of Dolichovespula (one nest of D. maculata) indicated early morning and late evening peaks (
Traffic rate varied also at finer resolution and showed irregular cycles of a few minutes. The only study on the trip or inter-trip times of Dolichovespula (
Flight activity of wasps is affected by weather, particularly extreme temperature, rain, wind and light conditions (
Parasitism can affect colony size and hence overall colony activity, but unlikely affect within-day dynamics. In the studied colonies, parasitism rate by S. vesparum and A. sociella was low, so the effect of parasitism on any of the measured parameters is negligible. Low parasitism rate corroborates previous observations (see
Our study provides high-resolution information about the all-day activity of D. saxonica colonies. Despite the previous suggestions that vespid, or at least Vespula, colonies would have roughly consistent within-day dynamics, the observed idiosyncratic within-day activity among colonies challenges this idea. Together with the recent studies on Vespula our results suggest that the within-day dynamics of vespines as a whole are explained mainly in terms of environmental conditions rather than by any innate pattern of changes in colony needs. Furthermore, all colonies showed irregular cycles over a few minutes. This suggests that the trip and inter-trip times are roughly similar in colonies with different environmental surroundings. Future studies should verify the observed patterns in other Dolichovespula wasps and, even though laborious, they should combine high-resolution monitoring of colony activity with individually tagged wasps and carefully measured in-nest conditions.
We thank the Kone foundation for a research grant to the wasp project, and Essi Järvinen, Kristiina Lindell, Lauri Viitanen, Satu Leino, Sophie Siimes, Jenna Palttala, Joona Lähdemäki, Tatu Koponen and Joona Hirvensalo for the help in the field.
Funding was provided by a private Kone foundation for a wasp project lead by Atte Komonen.
Tables S1, S2, Figures S1–S4
Data type: Tables and figures (docx. file)
Explanation note: Nest characteristics, seasonal activity, time series model summary, minute-to-minute dynamics and correlograms.