Trichogramma pretiosum Riley is a generalist egg parasitoid that is mass released in biological control programmes to control various insect pests on different plant species. The parasitism rates achieved by T. pretiosum vary across host species and across plant species, which suggests that the species of host and its host plant could both influence the host searching performance of this parasitoid. A comparative behavioural study across the eggs of two moth species, Helicoverpa armigera Hübner and Spodoptera litura Fabricius, was conducted because these two host eggs have different characteristics. Helicoverpa armigera eggs are laid singly, whereas S. litura eggs are laid in clusters and are covered with fine scales from the moth abdomen. I tested how these and other differences in host species affect the behaviour of T. pretiosum females towards their host species.
The ovipositional responses of T. pretiosum females, from different natal hosts and of different ages since emergence, to eggs of H. armigera and S. litura were studied in choice and no choice tests. Helicoverpa armigera eggs were consistently preferred over S. litura eggs, regardless of the natal host and adult age. When only S. litura eggs were available as hosts, they were parasitized at statistically similar rates to H. armigera eggs. The adult lifespan and lifetime fecundity of T. pretiosum were variable but were significantly affected by natal host species and/or host species to which they were exposed. The ovigeny index (OI) was significantly lower in the parasitoids exposed to H. armigera eggs than in those exposed to S. litura eggs, regardless of the natal host, indicating that H. armigera eggs sustain the adult parasitoids better, through host feeding, than S. litura eggs.
Trichogramma females, like other egg parasitoids, use chemical cues that originate not only from their host plants and host eggs, but also from the adult insects that deposit the hosts. These last include the scales left by the ovipositing moths. These host-associated cues are used by parasitoids over long and short distances to find their hosts. Olfactometer tests demonstrated that the odours from moth scales and host plants attracted T. pretiosum females significantly, and thus function as long range cues. By contrast, the odour from host eggs did not attract them in this way. The strength of attraction was similar across the scales from H. armigera and S. litura moths. The female parasitoids responded most strongly to odours from tomato and cabbage, whereas odour from corn was not such a strong attractant. A non-host plant tested did not attract the parasitoids at all. The odours of moth scales from both H. armigera and S. litura were more attractive to T. pretiosum females than was the odour of the host plants tested.
When parasitoids have followed these long range cues to the nearby vicinity of their hosts, they respond to short range cues to localise the hosts. Moth scales did not attract female parasitoids over a short distance, but they did arrest their movement so that they remained longer in the area with scales. Trichogramma pretiosum females took significantly more time to localise S. litura eggs than it took for them to locate H. armigera eggs. Time to locate hosts did not increase significantly when the eggs had been washed with hexane, which suggests that cues other than chemicals are associated with the eggs and play a more significant role in host finding over short distances.
On their emergence, parasitoids are confronted by several options, including mating, searching for hosts, and seeking food. Season influenced the daily pattern of T. pretiosum emergence, with cooler winter temperatures extending emergence over two days. In summer they emerged earlier in the day and all parasitoids in the tests emerged during the first day. Parasitoids left their emergence sites by walking before their wings expanded fully. Adults of T. pretiosum visit flowers, presumably for nectar.
Olfactometer tests showed that both males and females of T. pretiosum were attracted to the odor of alyssum flowers. The attraction to flower odour was significant for males when they had had no access to nutrients for 24h, but was less significant when they had just emerged. On the contrary, the attraction to food was more significant in newly emerged females. For 1 day starved females, cues associated with host plants (tomato leaves) and food (alyssum flowers) were equally important. Nutritional sources influenced the adult lifespan and reproductive performance of the parasitoids. Honey was the best source of nutrition for adults of T. pretiosum, resulting in the longest lifespan and the highest reproductive output, followed by alyssum flowers, tomato leaves and water.
The results are discussed in relation to (i) understanding the entire mechanism of host searching behaviour of egg parasitoids, (ii) interpreting the behavioural interaction between generalist parasitoids and their different host insects and plants, and (iii) the better use of egg parasitoids in biological control. Throughout the discussion suggestions are made for further research.