Global climate change is expected to increase average surface temperatures by 1.5 to 4.5 °C
by the end of the 21st century. Major variation in climatic conditions is predicted to occur,
including more frequent droughts and heat waves as well as higher mid-summer
temperatures. Water-deficit stress may lead to increased nitrogen levels in plants, which in
turn may increase their suitability for insect herbivores. In addition, the higher nutritional
quality of the insect herbivore diet may increase the acceptability to parasitoids for
parasitism. Parasitoid larvae depend completely on the nutrient quality of the host, which in
turn could influence the fitness of the emerging adult. In this study, the influences of waterdeficit
stress and high day-night temperatures on a tritrophic interaction, potato, Solanum
tuberosum L. (Solanaceae), the potato aphid, Macrosiphum euphorbiae (Thomas)
(Hemiptera: Aphididae) and its aphid parasitoid, Aphidius ervi Haliday (Hymenoptera:
Braconidae, Aphidiinae) were determined. The specific objectives were (i) to determine the
water stress levels and nitrogen concentrations in potato plants and to evaluate the effects of
water-deficit stress and high temperatures on the potato-aphid interaction by determining
nymphal development times, survival rates, adult longevity and life table parameters of M.
euphorbiae; (ii) to select a suitable parasitoid of M. euphorbiae for experiments; (iii) to
evaluate the response of A. ervi to various densities of M. euphorbiae as part of determining a
suitable host parasitoid ratio for experiments; and (iv) to determine the effects of waterdeficit
stress and high day-night temperatures on amino acid concentrations in potato plants
and on the number of mummies formed, pupal survival, development time, longevity, total
offspring per female and sex ratio of A. ervi.
Levels of water-deficit stress in potato plants were measured indirectly by stomatal
conductance (gs). No differences were found in nitrogen concentrations between water-deficit
stress and well-watered plants. However, total amino acid concentrations were higher in
water-deficit stressed plants compared to well-watered plants. White (2009) proposed the
plant stress hypothesis, stating that the performance of senescent feeders and consequently
population growth is enhanced on stressed plants, especially under water-deficit stress, due to
an increase in phloem nitrogen. In this study, moderate water-deficit stress at ambient
temperature had little or no effect on the performance (development time and reproduction)
as well as on population growth parameters of M. euphorbiae, rejecting the plant stress
hypothesis. Fluctuating high day/night temperatures, on the other hand, had a negative effect on aphid growth parameters, suggesting that M. euphorbiae could decline in abundance
during average summer mid-day temperatures exceeding 27.3 °C.
In August 2010, Aphidius ervi were collected from a sample of the potato aphid, M.
euphorbiae, in the field on Malva parviflora L. (Malvaceae) in Pretoria, South Africa. This is
the first record of this parasitoid from South Africa. The heat-stress hypothesis proposed by
Roux et al. (2010) was tested on A. ervi. This hypothesis states that an increase in mortality
of aphid parasitoids, especially weaker individuals exposed to above-optimum temperatures,
will occur. Those individuals that survive heat stress will pay a cost in their fitness measured
as reproductive output. In effect, the survival and percentage parasitism of A. ervi were
reduced at high day-night temperatures and the hypothesis was accepted. This indicates that
field populations of A. ervi could decrease during temperatures rising to 30 °C and above.
Water-deficit stressed potato plants may provide an enhanced food source for M. euphorbiae
due to a higher total essential amino acids content. In the current study, pupal survival of A.
ervi, percentage parasitism and the number of female offspring were the highest on M.
euphorbiae reared on water-stressed plants at ambient temperature. This, in turn, indicates
that parasitoid larvae benefited from the higher amino acid concentrations ingested by their
host. However, high day-night temperatures negatively influenced the performance of A. ervi
and therefore countered the positive effects of high amino acid concentrations in water-deficit
stressed potato plants. This study provides some information on the effects of extreme
climatic events. However, short-term observations on an individual level are not sufficient to
predict long-term effects on insect population dynamics, which depend on complex
interactions between biotic and abiotic factors.