What's my age again? Assessing the impact of stink bug egg mass age on host recognition by egg parasitoids Trissolcus basalis and Trissolcus japonicus (Hymenoptera: Scelionidae)

Abstract

Wasps, from the genus Trissolcus, are egg parasitoids that are commonly used in biological control programs targeting stink bugs. They navigate a complex environment, relying on a diverse array of biochemical and ecological cues to locate their hosts. Through this endeavour, these parasitoid wasps have to discriminate between young and old eggs as development is only achieved in the latter. In this study, we evaluated the ability of two parasitoid wasps, Trissolcus japonicus and Trissolcus basalis, on utilising short-range cues and, more specifically, volatile organic compounds emitted by stink bug egg masses to locate their hosts. We hypothesised that (1) stink bug eggs (i.e., Halyomorpha halys and Nezara viridula) emit short-range cues that are exploited by egg parasitoids (i.e., T. japonicus and T. basalis) to locate their hosts in addition to insect chemical footprints; (2) Trissolcus spp. Have the ability to differentiate young eggs from older ones to increase their fitness (3) based on changes in the chemical profiles of the egg masses according to their age. Our behavioural assays suggested that T. japonicus did not respond to stink bug footprints, whereas T. basalis was significantly oriented toward the footprints of gravid host females. Both parasitoids preferentially oriented towards young eggs rather than footprints. The parasitism rate of T. japonicus was not significantly different between young and old eggs unlike T. basalis which preferred parasitising on young eggs. We identified γ-butyrolactone and β-funebrene in the headspace of N. viridula eggs and we discussed the putative role of these secondary metabolites on T. basalis locating their host. Behavioural, performance and VOCs collection of this study contribute to a nuanced understanding of host-parasitoid dynamics along with implications for developing effective pest management strategies.

Keywords: biocontrol; chemotaxis; host recognition; short-range cues; trophic cascade; volatile organic compounds.

FIGURE 1

Schematic representation of the footprint-assay and performance assays: (a) choice test of Trissolcus females between footprint and control (filter paper only); (b) choice test of Trissolcus females between footprint and young eggs; (c) performance with old eggs only; dperformance with young eggs; (e) performance with young versus old eggs).

FIGURE 2

Schematic representation of an olfactometer assay at 40° angle, connected to a vacuum pump: (a) parasitoid entrance; (b) neutral zone; (c) choice zone with egg masses.

FIGURE 3

Choice of (a) T. japonicus and (b) T. basalis between the filter paper with the footprints of a gravid female of its host and the control filter paper in a glass Petri dish at 15 min, 30 min and 1 h (40 replicates for each species). Stars *** indicate statistical difference (p ≤ 0.001) and ° marginally significant difference (0.08 ≤ p < 0.05). n.s states for non-significant difference.

FIGURE 4

Choice of (a) T. japonicus and (b) T. basalis between young eggs from their host and the filter paper with the footprints of a gravid female of its host in a glass Petri dish at 15 min, 30 min and 1 h (40 replicates for each species). *** indicate a statistical difference (p ≤ 0.001).

FIGURE 5

Choice of (a) T. japonicus and (b) T. basalis between young and old eggs of their host in olfactometer tests at 15 min, 30 min and 1 h (40 replicates for each species). Stars * indicate statistical difference (p ≤ 0.05) and ° marginally significant difference (0.08 ≤ p < 0.05). n.s states for non-significant difference.

FIGURE 6

The rate of emergence of stink bugs nymphs, the rate of parasitism and the rate of unhatched eggs for (a) Halyomorpha halys eggs (parasitised by T. japonicus) and for (b) Nezara viridula eggs (parasitised by T. basalis) in the no choice arena trials (40 replicates for each species). Stars *** indicate statistical difference (p ≤ 0.001) and stars * indicate statistical difference (p ≤ 0.05) and marginally significant difference (0.08 ≤ p < 0.05). n.s. Indicates non-significant difference.

FIGURE 7

Choice of (a) T. japonicus and (b) T. basalis between young and old eggs of their host in the double-choice arena trials at 15 min, 30 min and 24 h (40 replicates for each species). n.s states for non-significant difference.

FIGURE 8

The rate of emergence of stink bugs nymphs, the rate of parasitism and the rate of unhatched eggs for (a) Halyomorpha halys eggs (parasitised by T. japonicus) and (b) Nezara viridula eggs (parasitised by T. basalis) in the double-choice arena trials (40 replicates for each species). Stars *** indicate statistical difference (p ≤ 0.001) and ° marginally significant differences. n.s. Indicates non-significant difference.

FIGURE 9

Peak area of each compound identified in old or young eggs of Halyomorpha halys (blue eggs) and Nezara viridula (yellow eggs). Absence of letters means no statistical difference.