During a three year study in rapeseed-mustard crop, the response of two ladybird species, Coccinella septempunctata and Menochilus sexmaculatus was dependent upon density and time of appearance of their prey. Mustard aphid, Lipaphis erysimi appeared during the second (2000 and 2001) and last (2002) week of January. Aphid appearance was highly dependent on the phenological stage of the host plant. It appeared earlier in rapeseed, Brassica campestris var. BSH-1 than mustard, Brassica juncea var. RH-30. The ladybird beetle's eggs were observed in the last week of January, 4-5 weeks before the maximum aphid population. A correlation between aphid number and the abundance of ladybirds' life stages (egg, larva and beetle) on these host plants revealed that the egg and larval density was positively correlated to aphid numbers. Adult beetles, however, did not show significant positive correlation with aphid numbers during all the three years of study. Coccinella septempunctata was more abundant than Menochilus sexmaculatus in these crops. Menochilus sexmaculatus population disappeared earlier than Coccinella septempunctata which was abundant until end of the crop season.
Pea aphids (Acyrthosiphon pisum) have been reported to produce winged offspring in the presence of predatory ladybirds. These offspring may leave host plants by flight after they have developed into winged adults. The inter-generational nature of this response raises the question about the chances of survival of aphids developing in attacked colonies. We studied the behaviour of predatory ladybirds on host plants by releasing adult 7-spot ladybirds (Coccinella septempunctata) on bean plants hosting either no prey individuals or colonies of 10 or 30 pea aphids. Interactions between predator and prey were recorded until the ladybird left the plant. Ladybird patch residence time increased with the number of aphids present on a plant but beetles generally left a plant before all aphids were eaten. The time budget of the ladybirds revealed a high proportion of time not spent in feeding activities. Predation rate was about one aphid killed per 10min residence time in both treatments with aphid-infested plants. Aphids that survived an attack by the predator or that were alarmed when a conspecific was attacked often emigrated from the host plant, and their number was of the same magnitude as the number of aphids killed by the predator. On average, pea aphid numbers at the end of an experiment were reduced to less than a third of the initial value. The results of the experiment show that attack by single ladybirds does on average not cause immediate extinction of small aphid colonies. The short patch residence times of on average less than two hours show that a predator individual that induces winged-offspring production in an aphid colony will not any longer be present in the colony when the induced offspring mature. To understand the adaptiveness of predator-induced wing development in pea aphids the probabilities of subsequent attacks on an aphid colony need to be investigated.
The aphid Uroleucon ambrosiae (Thomas) is one of the principal pests found on greenhouse lettuce crops, and there is no efficient biological control agent of this pest in Brazil. This work evaluates the aphid U. ambrosiae as a host for the parasitoid Praon volucre (Haliday), aimed at using P. volucre as a potential biological control agent of U. ambrosiae on lettuce. As Macrosiphum euphorbiae (Thomas) is a common host of P. volucre in the field, the development of the parasitoid was compared on these two aphid species. Twenty nymphs of the 2nd instar were kept with P. volucre for one hour at 22 ± 1°C, 70 ± 10% RH and a 12 h photophase. The size of the aphid's tibiae at the moment of oviposition indicated that there was no significant size difference between U. ambrosiae (0.6 ± 0.02 mm) and M. euphorbiae (0.7 ± 0.03 mm). When mummies were formed, M. euphorbiae had significantly longer tibia (1.5 ± 0.03 mm) than U. ambrosiae (1.4 ± 0.02 mm). No significant differences were detected in the percentage emergence (74.9 ± 7.92 and 87.5% ± 3.31 for U. ambrosiae and M. euphorbiae, respectively), or proportion of female offspring (56.2 ± 7.62 and 44.2 ± 7.14%). The development time from oviposition to adult and longevity of females and males of P. volucre reared on the two host species were not different. High parasitism levels were recorded for both host aphid species, but the percentage parasitism of M. euphorbiae (54.4 ± 4.40) was higher than of U. ambrosiae (35.6 ± 5.30). Female parasitoids reared on M. euphorbiae had longer tibiae (0.78 ± 0.01 mm) than those reared on U. ambrosiae (0.72 ± 0.01 mm). Our results demonstrate that the alternative host species U. ambrosiae, compared to the natural host species M. euphorbiae, affects the female's size, but did not affect parasitoid development time, longevity, emergence or sex ratio. The parasitoid P. volucre seems to be a good candidate for using as a biological control agent of U. ambrosiae on lettuce in Brazil.
1) Alloxystinae are major secondary parasitoids of aphids, important in both their ecology and pest management. 2) Two radically differing views of alloxystine taxonomy exist in the literature, in one of which the group is very diverse, in the other it consists of a few variable species. 3) We sequenced a variable nuclear gene region (ITS2) for 28 specimens of a morphologically clearly defined group which, in one view belong to a single species and in the other to four species. We find that the four putative species each carry a different unique allele with no intraspecific variation. We show that the probability of the observed distribution of alleles under the assumption of a single interbreeding population is very small and we reject the view that all specimens belong to a single biological species. 4) We discuss the implications of our results for aphid - parasitoid community ecology and the biological control of aphids with parasitoids.
The tritrophic interactions between two different plant-host complexes, Avena sativa-Sitobion avenae, Triticum aestivum-S. avenae and the aphid parasitoid Aphidius rhopalosiphi were studied with respect to odour learning and recognition by the parasitoid. The orientation behaviour of females towards odours from either uninfested or aphid-infested oat or wheat plants was tested in a series of dual choice Y-tube olfactometer experiments. Female parasitoids had the opportunity to gain a single oviposition experience on either the oat-S. avenae or wheat-S. avenae complex before the experiment. In the first set of experiments, where A. rhopalosiphi was reared on the oat-S. avenae complex, eight odour-bait combinations were tested. The females did not discriminate between uninfested oat and wheat. After oat complex experience, females responded to odours from the oat complex, but not to odours from the wheat complex. Consequently, in a direct comparison the oat complex was preferred over the wheat complex. After wheat complex experience, the parasitoid's orientation responses gave a different picture. Both, the wheat complex and the oat complex, were then shown to be equally attractive. Hence, in direct comparison no preference was recorded between the oat and wheat complexes. In a second set of experiments, where A. rhopalosiphi was reared on the wheat- S. avenae complex, a possible influence of any pre-adult or emergence-related host plant experience could be excluded as the same results were obtained as before. At first glance the responses towards the different odour baits seem inconsistent. However, the results may be explained using a simple model with two key odour components.