Eutrichosoma mirabile Ashmead, 1904

Baker, Austin J. & Heraty, John M., 2020, Larval morphology and life history of Eutrichosoma mirabile Ashmead and description of a new species of Eutrichosoma (Hymenoptera, Chalcidoidea), Journal of Hymenoptera Research 75, pp. 67-85 : 67

Eutrichosoma mirabile Ashmead

Fig. 1A-H

Eutrichosoma mirabile Ashmead 1904: 375. Lectotype designated by Gahan and Peck (1946: 315): USA: Montana: Helena (female). Deposited in USNM.

Eutrichosoma albipes Crawford 1908: 158-159. Synonymy by Bouček, 1975. Holotype: USA: Texas: Dallas (female). Deposited in USNM.

E. mirabile ; Bouček 1975: 132-133. Redescription and identification key.

Biology and life history.

Eggs and first-instar larvae were found inside the early (green) seedpods of Vachellia constricta and associated with the presence of weevil eggs and larvae ( Curculionidae ). Eutrichosoma mirabile eggs are laid among the host eggs inside the seedpods between the ovule and the inner wall of the pod. Hatching of the parasitoid seems to coincide with or precede hatching of the host because parasitoid eggs were never observed without host eggs. The majority of planidia found were parasitizing first- or second-instar weevils (~85%). Typically, only one planidium was found per host, positioned anterodorsally on the body just behind the head attached by the mandibles; always on the external surface and never penetrating the cuticle. The remaining unattached planidia were observed crawling around near clusters of host eggs. Eggs and planidia were the only life stages of the wasps observed in the seedpods. While there may be several eggs and early instars of weevil (up to ~10) per ovule within a seedpod, by the time the weevils are in their fourth instar, there is only one individual per ovule remaining. Considering the wasps are ectoparasitic koinobionts, they are likely detaching then reattaching and repositioning themselves on their hosts between host molts or transferring between individual host larvae. Given the similarities between E. mirabile and chrysolampines (discussed below), it is assumed that the E. mirabile planidia remain attached externally to the weevil when it leaves the seedpod to pupate in the soil, where the parasitoid likely finishes development. We were not able to keep the weevil larvae alive outside of the pods to allow the parasitoid to develop further. Parasitism rates shown in Suppl. material 1: Table S3.

Egg (Fig. 1A ). Egg body length approximately 0.2 mm, maximum width approximately 0.07 mm; ovoid; caudal stalk about half as long as the body of the egg. Eggs separate, not forming tight clusters.

Planidium (Fig. 1B-H ). Length approximately 0.13 mm, maximum width approximately 0.05 mm; fusiform in shape. Body and cranium white, darkened around mouth (Fig. 1C ). Cranium with one pair of short, papilliform antennae (ant), one pair of longer, thinner pleurostomal setae (plst), and one pair of minute, lateral cranial spines (cs); postlabium (psl) large, flat, circular, and surrounding prelabium (prl); labial palp (lp) present; pleurostomal bridge (psb) present and connected by thin integument (Fig. 1E ). Thirteen body segments beyond head; terga lightly sclerotized and ring-like, encircling the body; band of 1-2 irregular rows of tubercles (tbs) on anteroventral side of terga II-XII; prominent dorsolateral spines (spn) on terga I and III-XI; setae (set) present on terga I-VIII and XII, with three pairs on tergum II and two pairs on tergum III; short cerci present on XIII (cer); spiracles on terga II, IV-VI (spi) (Fig. 1B, D, F-G ).

Determining if a first-instar larva is a type I planidium (i.e. undergoes hypermetamorphosis sensu Pinto (2009)) requires examination of subsequent instars, which we did not find for Eutrichosoma . However, the mobility of the larvae observed in the seedpods and inferred from their koinobiont ectoparasitc behavior suggests that Eutrichosoma behavior is congruent with other PLC larvae, even if their morphology is not as derived as Eucharitidae , Perilampinae , or Philomidinae , which are all more heavily sclerotized and generally lack ventral fusion of the terga. Larvae of Eutrichosomatinae and Chrysolampinae appear to be somewhat intermediate between typically hymenopteriform first instars of other chalcidoid taxa and the highly derived planidial larvae in the PLC.

Material examined.

USA: Arizona: Cochise Co.: Canadian Lane, Portal , 1426m, 31°55'1"N, 109°07'37"W, 28.viii.2016, A. Baker & S. Heacox, AB16.024 [2 larvae slide mounted, UCRCENT00513221-2 ]; 4.viii.2018, A. Baker, S. Heacox, L. Kresslein, AB18.007 [larvae in alcohol, UCRCENT00513223 ] deposition UCRC.

Figures

Figure 1. Eutrichosoma mirabile immature stages. A Eggs of Eutrichosoma mirabile (small, stalked) laid on top of the eggs of their weevil host (large, unstalked) within a seed pod of Vachellia constricta; inset: Eutrichosoma mirabile egg B SEM ventrolateral habitus image of a planidium of Eutrichosoma mirabile C planidium attached to host weevil larva D setal map of a Eutrichosoma mirabile planidium, modified from an illustration by Darling & Miller (1991) E head capsule of planidium, dorsal and ventral views F planidium TIII-IV, ventral tubercles G head, anterolateral view, showing the labial structure H TXIII with cerci, dorsolateral view. Abbreviations: ant = antenna, cer = cerci, cs = cranial spine, lp = labial palp, man = mandible, plst = pleurostomal seta, prl = prelabium, psb = pleurostomal bridge, psl = postlabium, set = seta, spi = spiracle, spn = spine, tbs = tubercles, I-XIII = terga numbered from anterior to posterior.