Potential Host Manipulation by the Aphid Parasitoid Aphidius avenae to Enhance Cold Tolerance

Abstract

During parasitoid development, the immature parasitoid is confined to the host species. As a result, any potential to modify the physiology or behaviour of the host could play an important role in parasitoid fitness. The potential for host manipulation by the aphid parasitoid Aphidius avenae to increase cold thermotolerance was investigated using the aphid host species Metopolophium dirhodum and Sitobion avenae. Aphids were parasitized at L3/L4 instar stage (5 d old) and allowed to develop into pre-reproductive adults (10 d old) containing a 5 d old parasitoid larva. A control group was created of non-parasitized pre-reproductive adults (10 d old). The inherent physiological thermotolerance (LT50) and potential behavioural thermoregulation (behaviour in a declining temperature regime) of parasitized and non-parasitized aphids were investigated. Results revealed no effect of parasitism on the physiological thermotolerance of S. avenae and M. dirhodum. Significant differences in the behaviour of parasitized and non-parasitized aphids were observed, in addition to differences between host species, and such behaviours are discussed in view of the potential for host manipulation.

Fig 1. Lower lethal temperatures (LT₅₀) ± 95% fiducial limit (°C) of non-parasitized adults and adults parasitized by Aphidius avenae of the aphids Sitobion avenae (indicated by circle symbols) and Metopolophium dirhodum (indicated by triangle symbols) acclimated at 20°C.

Parasitized individuals are represented by filled symbols. (Notations are as follows: Sa = Sitobion avenae, Md = Metopolophium dirhodum, P = aphids parasitized by Aphidius avenae, NP = non-parasitized aphids) (Sa-NP n = 220; Sa-P n = 237; Md-NP n = 294; Md-P n = 201).

Fig 2. The relative proportion of behaviours exhibited by non-parasitized adults and adults parasitized by Aphidius avenae of the aphids Sitobion avenae and Metopolophium dirhodum when subjected to a declining temperature regime from 20°C to -10°C at 0.75°C min^-1.

Behaviours include: 1) the aphid actively walked from the wheat (light grey); 2) the aphid dropped from the wheat (dark grey); 3) the aphid remained attached to the wheat to temperatures of -10°C (black). Significant differences are indicated by letter superscripts. (Notations are as follows: Sa = Sitobion avenae, Md = Metopolophium dirhodum, P = aphids parasitized by Aphidius avenae, NP = non-parasitized aphids) (Md-NP n = 40; Md-P n = 33; Sa-NP n = 42; Sa-P n = 46).

Fig 3. Mean temperature of dropping behaviour (°C) ± 95% confidence intervals for non-parasitized adults and adults parasitized by Aphidius avenae of the aphids Sitobion avenae and Metopolophium dirhodum acclimated at 20°C.

These aphids represent the same test individuals as indicated by the dark grey bar in Fig 1. (Notations are as follows: Sa = Sitobion avenae, Md = Metopolophium dirhodum, P = aphids parasitized by Aphidius avenae, NP = non-parasitized aphids) (Sa-NP n = 13; Sa-P n = 11; Md-NP n = 19; Md-P n = 16).