Host patch quality increases parasitoid locomotor activity despite risk of egg limitation

Abstract

Theory predicts that egg load and foraging activity of insects should be positively related. Species that emerge with a low initial egg load and mature additional eggs as they age may limit initial foraging activity to avoid egg limitation. Similarly, females, which deplete their egg supply through oviposition, should decrease activity immediately afterwards and then increase activity as new eggs are matured. The authors used activity monitors to measure daily locomotor activity separately from exposure to host cues with the stink bug egg parasitoid, Trissolcus japonicus. The authors measured activity and egg load as newly emerged wasps aged and tested whether egg depletion from oviposition would reduce activity. Parasitoid activity increased with age in parallel with egg maturation, with both egg load and activity reaching their maximum about 6–7 days after emergence. Newly emerged wasps began activity later in the day and had lower activity intensity compared with older individuals. When given host experience, females that had laid many offspring, and had thus depleted their egg loads, unexpectedly increased their activity for up to 3 days. The present study results challenge the generally accepted positive relationship between foraging activity and egg load, suggesting that this relationship may be secondary to host patch quality. The authors provide evidence that host patch quality can influence not only within‐patch but also between‐patch foraging activity. Plastic responses in activity patterns may allow parasitoids to better exploit rich host environments while minimising foraging costs and decreasing likelihood of egg limitation in low‐quality environments.

The effects of LED daylength extensions on the fecundity of the pest aphidMyzus persicaeand the daily activity patterns of its parasitoid,Aphidius matricariae

“…To determine how parasitoid locomotor activity levels, as a predictor of foraging activity and ultimately parasitism rates, vary with photoperiod, experiments were designed and conducted using TriKinetics LAM25H infrared activity monitors (TriKinetics Inc., Waltham, USA). This method has been used previously to assess locomotor activity as a proxy for foraging and general vigor in biocontrol agents (Paul et al, 2022; Rasmussen et al, 2018; Wong et al, 2021) as well as determining their daily activity patterns (Abe et al, 2014; Ogino et al, 2019). Each monitor can hold 32 glass tubes, and automatically and continuously counts how many times the occupant of each tube crosses a set of infrared beams at the middle as long as the monitor is operating.…”

Partial refuges from biological control due to intraspecific variation in protective host traits

“…Parasitoids were released into arenas between 7 and 9 h after the beginning of the photophase, and the egg mass in each arena filmed until 2-3 h after the start of photophase the following morning to record parasitoid foraging behavior (=17-20 h of filming per trial). The 7-9 h of filming during the first day occurred during the peak activity times of T. japonicus (Paul et al, 2022), was based on preliminary trials to measure host finding success and parasitism rate, and was designed to give parasitoids more than adequate time to find and parasitize all eggs that their egg loads would allow, even in the largest egg masses. Egg masses were then removed from arenas and incubated under the same abiotic conditions for at least 40 days to record either stink bug nymph or parasitoid emergence by counting the number of emerged individuals.…”

“…Parasitoids were maintained on newly laid (<48 h) eggs of H. halys in ventilated plastic containers (width: 9.2 cm; height: 11.7 cm) and reared at 23 ± 1 C, 25%-50% RH, and a 16:8 h L:D photoperiod. Females were 4-10 days old when used in experiments to allow time for ovary maturation (Wong et al, 2021;Paul et al, 2022). Female T. japonicus used in experiments emerged in the presence of males and thus were assumed mated.…”

“…To test the prediction that parasitoids would exhaust their egg loads on larger egg masses, we used the number of observed ovipositions per available host egg as a proxy for egg load depletion, reasoning that for a given host species-specific level of acceptability, parasitoids would oviposit fewer times relative to the number of eggs in the egg mass when they had depleted egg loads, with values close to 1 indicating that parasitoids had adequate egg loads to exploit every host egg and values less than 1 indicating that several hosts would remain unparasitized (values were sometimes more than 1 if parasitoids oviposited in individual host eggs multiple times). We expected that evidence for egg depletion would be particularly strong if the number of ovipositions per host egg declined mostly for egg masses sizes of more than ~40-60 eggs, which corresponds to the range of maximum egg loads of T. japonicus in two previous studies in our laboratory (Paul et al, 2022;Wong et al, 2021). Additional behavioral analyses of oviposition rate, postoviposition guarding behavior, and probability of ovipositor rejections over time are described in Appendix S1: Methods S1.…”

“…(Hemiptera: Pentatomidae), with both species originating in Asia (Abram et al, 2020;Conti et al, 2021;Hoebeke & Carter, 2003). Trissolcus japonicus is a solitary, moderately synovigenic idiobiont parasitoid that, like related Scelionidae parasitoids of stink bugs (Field et al, 1998), usually parasitizes every egg in host egg masses, and then guards them for up to several hours (Haye et al, 2021;Paul et al, 2022;Wong et al, 2021). This parasitoid is currently being intentionally released or redistributed in its adventive range in Europe and North America (Abram et al, 2020;Conti et al, 2021).…”