DOI: 10.5061/dryad.jm63xsjg1
title: Effect of temperature on the post-diapause development rate, survival, and body mass of the solitary wasp Isodontia elegans

description: We examined the relationship of post-diapause rearing temperature to
 developmental rate, survival, and adult body mass of the solitary wasp
 Isodontia elegans using prepupae from trap-nests. Isodontia elegans is a
 member of a genus often found in trap-nests in North America and Europe.
 Trap-nests are commonly used tools for studying cavity-nesting solitary
 wasps and bees. In temperate zones, progeny in nests usually overwinter as
 fully prepupae before pupating and emerging as adults. An important aspect
 of properly using trap-nests is determining temperatures that affect
 survival and health of developing offspring.  After overwintering
 &gt;600 cocoons containing prepupae after the summers of 2015 and
 2016, we placed cocoons on a laboratory thermal gradient where offspring
 experienced one of 19 constant temperatures from 643°C; emergence of
 adults was monitored for 100 days.  Our conservative estimate for
 the critical thermal minimum for development is 14°C, whereas that for the
 critical maximum is ~33°C. Prepupae transitioned to adults most rapidly at
 29–33°C, but developmental rate was lower for some progeny exposed to
 temperatures ≥30°C. Offspring successfully reached the adult stage in
 &lt;100 days at of temperatures of ~19–33°C. Adults from cocoons
 reared at lower temperatures weighed on average 6-10% more than expected
 based on their head widths, whereas those reared at higher temperatures
 weighed 4–10% less than expected. The difference may be due to greater
 rates of water loss and lipid metabolism during development at higher
 temperatures. Pre-overwintering cocoon mass was a significant predictor of
 relative adult body mass, indicating that adult health is partly related
 to their condition before overwintering. The trends we observed were
 similar to those for the bee Megachile rotundata, which we previously
 studied on the same gradient apparatus. However, data are needed on many
 other species of wasps and bees from a diversity of
 environments.  
description: Source of the wasps   <em>Isodontia
 elegans</em> cocoons were obtained from trap-nests placed at sites
 within several km of Hillsboro, Oregon (45°31’ N, 122°59’ W; Spendal et
 al., 2021). Cocoons were removed from nests in late September in 2015 and
 early October in 2016. Cocoons were stored at ambient temperature in a
 barn in Oregon, before transport to Montana State University (Bozeman, MT)
 in November where they were kept at 8°C, 80% relative humidity until the
 following April 20 of each year. Cocoons were placed in #00 gelatin
 capsules, each with its long axis parallel to the capsule; the ends of the
 capsules had been punctured with pinholes to provide ventilation. In 2016,
 we measured cocoon mass (MC) prior to overwintering with a Metler AT 250
 analytical balance accurate to 0.1 mg, and gave each gelatin capsule a
 unique numerical code.   Rearing wasps on Thermal
 Gradient On 20 April of 2016 and 2017, we removed
 gelatin capsules containing <em>I. elegans</em> cocoons from
 winter storage, at which time all healthy cells would have contained live
 prepupae. All cocoons were left at room temperature until the following
 day so that prepupae placed on the coldest and warmest ends of the
 gradient would not face a rapid temperature change. On 21 April, they were
 placed on the surface of a laboratory thermal gradient consisting of four
 parallel aluminum bars each measuring 15 cm wide × 100 cm long × 0.8 cm
 thick (Fig. 1). The ends of the bars sat upon two 2.5 cm wide, hollow,
 square cross-section aluminum rods oriented perpendicular to the long axis
 of the bars. Water from a constant temperature hot-water bath flowed in a
 loop through the two rods beneath one end of the bars, while another
 pumped a water/propylene glycol mixture from a constant temperature
 cold-water bath beneath the other ends of the bars. A 2.5-cm-high clear
 Plexiglas cover was fitted over each bar to stabilize temperature, but the
 covers fit loosely enough to allow sufficient air exchange. The entire
 gradient was enclosed in a larger wooden cover. Covers remained in place
 throughout the experiment, except for brief intervals when we removed
 gelatin capsules containing emerged <em>I. elegans</em> or
 parasitoid adults. Temperature
 (T<sub>B</sub>) was measured every 10 cm along the bars, from
 0 to 100 cm with thermocouples inserted from below into the center of each
 bar. In 2016, we measured T<sub>B</sub> at each position on
 the bars every two days during the experiment. Because
 T<sub>B</sub> had remained so stable during the 2016
 experiment, we measured it every four days in 2017. The mean difference
 between absolute values of consecutive measurements at each position on
 the bars was 0.24 ± 0.01°C in 2016 (N = 2200) and 0.32 ± 0.02°C in 2017 (N
 = 1100).  T<sub>B</sub> extremes from the cold to hot end of
 bars ranged from 3.8–44.6°C, with some variation between bars. In linear
 regressions of T<sub>B</sub> on the position along each bar
 (all dates combined), r<sup>2</sup> values ranged from 0.998
 to 0.999 among the four bars in 2016 and from 0.994 to 0.999 in 2017. The
 relationship of T<sub>B</sub> to position along the bars was
 consistent between years. The slope of regressions of
 T<sub>B</sub> on position varied from 0.390–0.402 in both
 years. Rearing temperatures (T<sub>R</sub>) experienced by
 developing wasps were estimated using a separate set of temperature
 measurements in which thermocouples were inserted into the center of a
 cocoon via a small hole in the side of a gelatin capsule. We set the
 capsule on the surface of the gradient (with the covers in place) and
 waited until the temperature had equilibrated to the nearest 0.1°C. We did
 this at all 19 positions while recording T<sub>B</sub> at each
 position when the cocoon temperature had equilibrated. By regressing those
 temperatures on T<sub>B</sub>, we created an equation for
 converting T<sub>B</sub> to T<sub>R</sub>. Average
 T<sub>R</sub> experienced by developing wasps within the
 gelatin capsules at different 5 cm positions on the gradient ranged from
 6.2–43.1°C in 2016 and from 6.1–43.2°C in 2017.  On
 each gradient bar, we placed 19 rows of eight cocoons each, the rows 5 cm
 apart and aligned perpendicular to the long axes of the bars with the
 sides of adjacent capsules touching (Fig. 1). Before placement on the
 gradient, gelatin capsules were drawn blindly from a container, so that
 cocoons were randomly distributed. When on the gradient, the long ends of
 adjacent gelatin capsules in each row were in contact with one another.
 Each bar had 152 cocoons experiencing 19 temperature treatments. Gelatin
 capsules were anchored to the bars with double-sided cellophane tape, the
 center of each cocoon sitting on a line marking a 5-cm interval. The
 gradient was monitored daily so that adult <em>I. elegans</em>
 or parasitoid wasps could be placed in a freezer within their gelatin
 capsules soon after they emerged. In 2016, parasitoids included
 <em>Melittobia</em> sp. (Eulophidae) and
 <em>Monodontomerus</em> sp. (Torymidae) whereas in 2017 only
 <em>Melittobia</em> appeared, and in much lower numbers.
 Parasitoid-infested cocoons were discarded from analyses.
 On 31 July each year, 100 days after cocoons were placed on the
 gradient, all cocoons from which adults had not emerged were dissected. We
 recorded the developmental stages of occupants of failed cocoons as 1)
 prepupa, 2) pupa, or 3) unemerged, fully developed adult. For prepupae, we
 recorded whether they were pale yellow, soft, and flexible (and thus
 potentially still living when they were placed in the freezer) vs.
 desiccated and hard (and thus clearly dead prior to freezing).  For each
 adult wasp that emerged, we measured its head width (HW) to the nearest
 ±0.05 mm using a stereomicroscope with an ocular micrometer and its body
 mass (MA) to the nearest ±1.0 mg to using a Smart Weigh® digital scale
 accurate to ±1.0 mg. Data analysis
 We characterized the relationship between
 T<sub>R</sub> and the time elapsed from the end of
 overwintering and emergence of adults in two ways.  First, we used
 quadratic regressions to model the relationship between days to emergence
 (D) and TR for each sex and each year. From this, we also estimated for
 each sex a value for T<sub>OPT</sub>, the temperature to the
 nearest 0.1°C at which wasps emerged most quickly (i.e., optimum
 temperature for rapid development). The results were also used to estimate
 temperatures at which male and female emergence dates were most
 similar.  Second, to examine the relationship of
 developmental rate (1/D) to T<sub>R</sub>, we used Briere-1
 models, which estimate an array of parameters when the relationship of
 developmental rate to temperature is asymmetrical because it declines
 relatively abruptly after T<sub>OPT</sub> is exceeded and
 likely mimics the asymmetrical performance curves of enzymes (Briere et
 al., 1999; Kontodimas et al., 2004; Quinn 2017). Unlike the quadratic
 model, the Briere-1 model allows estimation of not only
 T<sub>OPT</sub>, but means and standard errors of 1) the lower
 developmental threshold (T<sub>MIN</sub>, the temperature
 above which development proceeds after diapause) and 2) the upper
 developmental threshold (T<sub>MAX</sub>, the temperature
 above which development ceases). The Briere-1 model takes the form 1/D =
 aT<sub>R</sub>(T<sub>R</sub> –
 T<sub>MIN</sub>)(T<sub>MAX</sub> –
 T<sub>R</sub>)0.5, where a, T<sub>MIN</sub>, and
 T<sub>MAX</sub> are fitted coefficients. 
 We used Kruskal-Wallis tests to test the hypotheses that the
 number of prepupae, pupae + unemerged adults, and adults at 100 days
 varied with position on the gradient (as a categorical variable). Each of
 the four bars was considered a replicate because of variation in the
 regressions of TB on position among bars. When plotting the numbers of
 each stage at different positions, we converted position to the mean
 T<sub>R</sub> across all bars at that position during the 100
 days so that an easier comparison could be made to the developmental rate
 plots. We tested the hypothesis that the sex of emerging adults varied
 with T<sub>R</sub> using logistic regressions, coding males as
 0 and females as 1. Means are based on data from the four gradient bars,
 on each of which there were eight cocoons per position at the start of the
 experiment. The number of cocoons used in the analysis for each bar ranged
 from 0 to 8 for each position, because the number of unparasitized wasps
 reaching each stage varied. The numbers were lower in 2016 when 32
 positions (of 76 total) on all four bars had ≥2 wasps parasitized.
 However, just 4 of 76 positions experienced that level of parasitism in
 2017, providing us with a clearer view of the effect of temperature on
 survival. We tested the hypothesis that
 M<sub>A</sub>, which varied even among wasps with identical
 head widths, was related to T<sub>R</sub>. We began by
 regressing M<sub>A</sub> on HW in separate analyses for each
 year and sex using linear regressions. We then calculated the body mass
 residual (M<sub>RES</sub>) for each adult as the difference
 between the observed M<sub>A</sub> values and the predicted
 values from the regression. This gave us an estimate of how much lighter
 or heavier each wasp was than expected based on its HW. For the 2016 data,
 we regressed MRES on T<sub>R</sub> to test the hypothesis that
 M<sub>RES</sub> varied as a function of rearing temperature
 (i.e., were wasps reared at higher T<sub>R</sub> lighter or
 heavier than expected?). For the 2017 data, we considered both
 T<sub>R</sub> and M<sub>C</sub> as potential
 explanatory variables in multiple linear regressions. In addition, to
 estimate the magnitude of the effect of T<sub>R</sub> on
 relative M<sub>A</sub>, we divided the
 T<sub>R</sub> range over which adults emerged into thirds. We
 then compared M<sub>RES</sub> values between the coolest and
 warmest thirds of the T<sub>R</sub> range using Mann-Whitney
 Tests, due to data being not normally distributed. Finally, for wasps in
 the cooler and warmer T<sub>R</sub> groups, we calculated the
 average percent they were above or below the mean body mass
 (M<sub>A</sub>) for the group. All means are reported ±
 standard errors (SE) and all statistical analyses were done using
 SigmaStat® v. 11.0, with the exception of the Briere-1 models, which were
 produced using TableCurve v. 5.01®. Because temperature and mass were
 normally distributed, all analyses of those were done on untransformed
 data.