Felix Wäckers

A comparison of nectar- and honeydew sugars with respect to their utilization by the hymenopteran parasitoid Cotesia glomerata

Fourteen naturally occurring sugars were individually tested with respect to their effect on Cotesia glomerata longevity. Parasitoids kept with solutions of either sucrose, glucose and fructose lived for >30 days. This constitutes a factor 15 increase in life span in comparison to control individuals kept with water only. Stachyose, mannose, melezitose, melibiose, maltose and erlose increased parasitoid longevity by a factor of 11.2-6.9. Solutions of galactose and trehalose had a marginal, but still significant effect. Lactose and raffinose did not raise parasitoid longevity, while rhamnose actually reduced parasitoid survival. In an additional experiment, the relationship between quantity of sugar consumption and longevity was established for all 14 sugars. To study the effect of an unsuitable sugar in sugar mixtures, a range of glucose:rhamnose mixtures was tested. Even at 20% of the sugar mixture rhamnose suppressed the nutritional benefit of the 80% glucose. The nutritional suitability of the sugars shows a positive correlation with the previously reported gustatory response towards the individual sugars. Patterns of sugar utilization are discussed with respect to hydrolytic enzymes and carbohydrate biochemical characteristics. Our findings for C. glomerata are compared to patterns of sugar utilization reported for other species. The comparison between C. glomerata and its host Pieris brassicae reveals that the parasitoid is capable of utilizing a range of sugars that are unsuitable to its herbivorous host. This specificity opens up opportunities to select food supplements for biological control programs that selectively target the antagonist, without concurrently enhancing herbivore fitness.

Interaction of vibrational and visual cues in parasitoid host location

Abstract. Female parasitoids are guided by multisensory information during host finding. Individual cues are used in an interactive or a hierarchical manner according to the relative importance on the spatial scale of their effect. Unlike most studies that concentrate on single cues, the present paper investigates the interaction of two physical cues. The interaction of mechanosensory and visual cues was studied in the pupal parasitoid Pimpla turionellae (Hymenoptera: Ichneumonidae). This species uses, amongst other senses, vibrational sounding (echolocation in a solid substrate) to find its mainly endophytic hosts. Location and frequency of ovipositor insertions were scored on cylindrical plant stem models with single or combined cues. Single-cue experiments show that parasitoids use both visual and mechanosensory cues and achieve a similar precision of host location with either cue. The combination of vision and vibrational sounding increased the precision of host location by a factor of approximately two to three. We conclude that the two senses interact, resulting in an additive accuracy. Neither the visual nor the mechanosensory cue was favored when offered adjacent to each other on the same stem model. On the investigated spatial scale, both physical cues are used and seem to be equally important for host location in this species.

Tyloids in Pimpla turionellae (L.) are release structures of male antennal glands involved in courtship behaviour (Hymenoptera: Ichneumonidae)

Abstract Morphological and behavioural studies were performed on male antennal tyloids of Pimpla turionellae (L.) (Hymenoptera: Ichneumonidae). S.E.M. and T.E.M. investigations revealed that tyloids, located on 8th and 9th male antennomeres, are release structures of integumentary glands rather than sensory organs. These are bicellular secretory units consisting of one secretory cell, with a well-developed receiving canal, and a canal cell. The latter forms the evacuating canal, which connects the receiving canal to the external pore on the tyloids. Observations of mating behaviour showed that during the precopulatory phase, males typically perform antennal strokes during which the secretion containing tyloids comes in direct contact with the female antennae. Stroking intensity seemed to be correlated with female receptiveness, being more prolonged when females are initially unreceptive. When encountering receptive females, male stroking is usually limited or even omitted entirely. Results from behavioural comparisons between unmanipulated pairs and pairs in which male tyloids had been coated with glue, clearly indicate that the secretion mediates mate acceptance in females.

Combining parasitoids and plant resistance for the control of the bruchid Acanthoscelides obtectus in stored beans

Acanthoscelides obtectus (Say) and Zabrotes subfasciatus (Boheman) are the main bruchid pests of stored beans in widespread regions of Latin America and Africa. Host-plant resistance based on the protein arcelin is effective in reducing damage caused by Z. subfasciatus, but beans containing arcelin remain susceptible to A. obtectus. The compatibility of combining arcelin resistance with biological control by Dinarmus basalis (Rondani) was investigated in climatic chambers. Three arcelin containing bean varieties with high and intermediate resistance towards Z. subfasciatus (RAZ 36, RAZ 94 and RAZ 104) and an arcelin-free standard (Calima, susceptible to both bruchids) were investigated. Immature development of A. obtectus in arcelin-containing beans was prolonged by 15% as compared to the standard, allowing D. basalis to have access to suitable host stages for a longer period of time. Over a 20-week storage period, the combined use of resistant host plants and biological control agents yielded best results with the host-plant varieties RAZ 94 and RAZ 104. In both varieties, parasitoids managed to keep bruchid damage below 1% as compared to 4.7% in the arcelin-free standard, and bruchids were eradicated in 80% of the replicates. Control levels in RAZ 36, the bean variety with the highest resistance to Z. subfasciatus, were not greater than the standard. Our results show that the combination of certain arcelin-enriched bean varieties with the parasitoid D. basalis is favourable for suppressing damage by A. obtectus.

Field Infestation of Phaseolus vulgaris by Acanthoscelides obtectus (Coleoptera: Bruchidae), Parasitoid Abundance, and Consequences for Storage Pest Control

Abstract Over a period of 3 yr we collected 19 samples (1 kg each) of recently harvested beans (Phaseolus vulgaris L.) from eight small-scale farms in Restrepo, Valle de Cauca, Colombia. Initial infestation by Acanthoscelides obtectus (Say) was low, but frequent. At harvest, 90% of the bean samples were infested by the weevil. The average level of infestation was 16 weevils per 1,000 beans, with a maximum of 55 weevils. Infested beans usually carried multiple larvae with a maximum of 13 larvae per bean. Emergence data indicate that oviposition by A. obtectus in the field is confined to a very short period before harvest. This relatively narrow time window can be exploited for proper timing of control measures. Only one species of parasitoid, Horismenus ashmeadii (Dalla Torre) (Hymenoptera: Eulophidae), was recorded, emerging from 21% of the samples. Samples with parasitoids had an average of five parasitoids per 1,000 beans, with a maximum of 12 parasitoids. This represented a parasitization level of 18%. During the 16 wk of storage, two weevil generations emerged, which caused visible damage in 0.5 and 34% of the beans (average of 14%). Although H. ashmeadii was successful in attacking the first generation of A. obtectus in the field, it failed to attack or develop under storage conditions. This indivates H. ashmeadii cannot serve as a postharvest control agent.

Efficiency of vibrational sounding in the parasitoid Pimpla turionellae is affected by female size

The pupal parasitoid Pimpla (=Coccygomimus) turionellae L. (Hymenoptera: Ichneumonidae) attacks a broad range of lepidopteran pupae. The variation in host size translates into substantial differences in size among adult parasitoids. Pimpla turionellae shows a strong host size-dependent sex allocation, which indicates that parasitoid body size influences sex-specific effects on fitness costs. We tested whether the body size of females affects their capacity to locate hosts by vibrational sounding. We investigated parasitoids from three size categories with respect to their ability to find artificial hosts (cigarette filters) in cylinders made of thin or thick paper (20 or 65 g/m2, respectively). Using laser Doppler vibrometry, we measured vibration signals produced by large and medium-sized females during vibrational sounding. Large females produced vibrations of a higher intensity than, but a similar frequency to, their smaller conspecifics. The parasitoids ability to locate a hidden host model by vibrational sounding also improved with size. This indicates that parasitoid size is an important factor in determining the females host-searching success. We discuss the results in relation to the parasitoids behavioural ecology.

Perception of Chromatic Cues During Host Location by the Pupal Parasitoid Pimpla turionellae (L.) (Hymenoptera: Ichneumonidae)

Abstract Chromatic and achromatic plant cues are expected to be particularly important for parasitoids of endophytic pupal hosts, because these stages do not feed and therefore avoid volatile emission caused by plant tissue damage. Endophytic feeding can cause discoloration or desiccation, leading to changes in color and/or brightness of infested plant parts that may be visually detected by parasitoids. The role of color cues in the host-finding behavior of parasitoids is poorly understood, and the visual system of most parasitoid species has not yet been investigated. We studied color discrimination ability and innate color preferences in the pupal parasitoid Pimpla turionellae (L.) (Hymenoptera: Ichneumonidae) during location of concealed hosts. Responses to combinations of yellow and blue bands of different reflectance intensities were investigated on cylindrical models of plant stems. The parasitoid’s reaction to these chromatic cues was evaluated by scoring the number of ovipositor insertions into the colored bands. Female parasitoids discriminated blue from yellow irrespective of total reflectance and inserted their ovipositors significantly more often into the blue area. True color vision is demonstrated for the examined species, and responses to chromatic cues are discussed in relation to their importance for host location in parasitoids. Results of this study and of our previous work suggest that P. turionellae uses contrasts (chromatic or achromatic) rather than specific color characteristics in visual host location.

Substrate vibrations elicit defensive behaviour in leafminer pupae.

Late instar larvae and pupae of the spotted tentiform leafminer Phyllonorycter malella (Ger.) (Lepidoptera: Gracillariidae) react with defensive behaviour when attacked by one of their parasitoids, the eulophid wasp Sympiesis sericeicornis Nees (Hymenoptera: Eulophidae). Vibrations produced during the insertion of the ovipositor into the mine are known to be important cues by which larvae detect the presence of their enemies. The aim of this study was to investigate which frequency components elicit defensive reactions in leafminer pupae using synthetic vibrations. Sine vibrations and bandlimited noise stimuli were offered to both free pupae and pupae concealed in their leafmines. Using Laser vibrometry we measured the vibrations experienced by pupae inside their mines and assessed the influence of the mine. Pupae were shown to react to substrate vibrations, and do so over a broad range of frequencies. Behavioural reactions to noise stimuli were stronger than to pure sine stimuli. Mine tissue attenuated vibration amplitudes of the input signal from 5.1 to 22.6dB. However, as response thresholds of concealed pupae were only twice as high as thresholds of free pupae (which is adequate to 3dB) pupae inside their mine were more sensitive than expected. This discrepancy is discussed both in terms of the conditions of pupae and in terms of mine structure. The results indicate that broadbandedness of vibrations produced by hunting parasitoids during ovipositor insertion into the mine may be a major criterion used by leafminers to perceive parasitoid presence and to escape ovipositor stings.

The role of chemical cues in host finding and acceptance by Callosobruchus chinensis

We studied the response of female Callosobruchus chinensis to chemical cues emitted by cowpea seeds at different stages of bruchid infestation (uninfested, egg carrying, L1‐, and L4‐infested). Olfactory attractiveness was determined in Y‐tube olfactometer assays by testing individual seed categories against either clean air or uninfested seeds. Oviposition preferences between uninfested and infested seeds were determined in petri‐dish choice‐experiments. The olfactometer assays revealed that weevils discriminate between seeds containing different stages of developing bruchids on the basis of olfactory cues. While odors from uninfested and egg‐carrying seeds acted as attractants, odors from L1‐ and L4‐infested seeds failed to induce a positive response by the bruchids. When given a choice between uninfested and infested seeds in the olfactometer, weevils preferred uninfested seeds over L1‐ and L4‐infested seeds, but failed to distinguish between uninfested and egg‐carrying seeds. In the oviposition experiment as well, bruchids showed a distinct preference for uninfested seeds when offered in combination with L1‐ and L4‐infested seeds. This experiment further showed a reduced acceptance of egg carrying seeds. Our results indicate that C. chinensis females use chemical information during both host searching and host acceptance. Volatiles from uninfested or egg carrying seeds act as attractants, while deterrence increases as development of bruchid immature stages progresses.

Perception of achromatic cues during host location of a pupal parasitoid

Visual cues employed by parasitic Hymenoptera havedifferent behavioral relevance during the location of thehost microhabitat from a distance and host location afterlanding. During the approach flight of bees and wasps,achromatic contrast is perceived first, followed by chro-matic (color) contrast (Giurfa et al., 1996). The distancefrom which a target, e.g., the host habitat, can be visuallydetected is determined by the size of the target area that pro-duces achromatic contrast with the background (Ne’eman& Kevan, 2001). Chromatic information is used over arelatively short range (Giurfa & Lehrer, 2001; Ne’eman K Pfannenstielet al., 1992; Smith et al., 1993; Potting et al., 1997). Despitethe importance of visual cues in host finding of hymen-opteran parasitoids, there are only three electrophysiologicalstudies addressing their spectral sensitivity (Peitsch et al.,1992; Mellor et al., 1997; Brown et al., 1998) and twobehavioral studies on color vision, i.e., the sensation ofchromatic contrast independent of intensity contrast(Wardle, 1990; Messing & Jang, 1992). While informationon chromatic vision is scarce, the role of achromatic cuesin parasitoid host location is largely unknown.For species that parasitize the pupae of endophytic hosts,i.e., hosts concealed within plant tissue, olfactory cues arenot readily available as the pupae do not feed, do not pro-duce feces, and pupation often occurs away from the larvalfeeding sites. Instead, these parasitoid species can use vibra-tional sounding and plant-derived visual cues for host loca-tion. Plant tissue damaged by host infestation can desiccateor discolor and serve as an achromatic host location cue forparasitoids such as