Host Choice in Tetramesa petiolata (Walker) (Hymenoptera: Eurytomidae)
This study was carried out to compare the host preference and oviposition behavior of T. petiolata in relation to different host plant species. Observations were made using stems of D. caespitosa contained in cages under laboratory conditions. Host plant preference was studied by introducing one adult T. petiolata female (4-5 days old) into a cylindrical cage containing four grass species including D. caespitosa, the natural host food plant. Female behavior was classified into five categories: grooming, resting, walking, searching, landing and oviposition. Walking and searching were distinguished by the attitude of the antennae. The mean time spent grooming and resting on each host food plant did not vary significantly between plants. In the oviposition choice experiments large number of larvae were reared from the host plant, but none were reared from other plants except for D. setacea, an unnatural host from which a few larvae were reared. This suggests that T. petiolata is at most potentially narrowly polyphagous and that females prefer one species much more than another. Since in the field, larvae of T. petiolata have never been recorded in the stems of D. setacea, the desposition of eggs in D. setacea can be regarded as a laboratory artifact and T. petiolata can be regarded as monophagous under natural conditions.
Insects of the genus Tetramesa Walker (Isosoma Walker; Harmolita Motsch.) (Hymenoptera: Eurytomidae) are small (2-3 mm in length), winged, generally black in color and feed on nectar as adult wasps. In the larval stage, they are phytophagous on cereals and grasses (Poaceae), the eggs being laid on the stems of their hosts (Claridge and Dawah, 1994). They are primarily a north temperate genus, comprising around 205 species world-wide (Burks, 1971; 1979). Sixty three species were recorded from America, north of Mexico (Peck, 1963) 61 species recorded from the former Soviet Union (Zerova, 1976, 1978) and 37 species from the British Isles (Claridge and Dawah, 1994; Fitton et al., 1978; Claridge, 1961). They are sporadic serious pests of cereals and grasses in various countries around the world, especially North America, where they are known as joint worms or straw worms (Philllips, 1927; Spears, 1978; Spears and Barr, 1985). Because of their general economic importance globally, they have been intensively studied in terms of their biology, taxonomy and ecology.
Tetramesa are extremely host-specific and previous studies have shown that they normally attack plant hosts of only one genus (Claridge, 1961; Phillips 1936), although Dawah (1987) found T. eximia (Giraud) to attack hosts of two closely related genera, Calamagrostis epigejos L. Roth. and Ammophila arenaria L. Link.
Taxonomic difficulties as mentioned by Henneicke et al. (1992), based on the extremely uniform morphology of adult wasps, has stimulated taxonomists to look for other discriminating criteria. These include host plant preference, mate choice and mode of larval life (Claridge and Dawah, 1994; Dawah, 1987).
Host habitat location and host location are problems shared by most species of insects involved in seeking food or oviposition sites (Vinson, 1981). Insect herbivores can be described as monophagous, narrowly polyphagous and broadly polyphagous, according to the number of plants species they exploit. Monophagous insects are those whose diet is confined to host plants of only one species. In this study, narrowly polyphagous refers to insects whose host plants are confined to a few (1-3) species, while broadly polyphagous refers to many species.
Thompson (1981) defines host specificity as the hierarchical ordering of plants species by ovipositing females. This means that in a simultaneous host choice trial, an ovipositing female will restrict her choice of host to the preferred one, or the top two or three. Singer (1982) offered host plants sequentially to ovipositing females and defined host specificity as the rate at which an insect becomes less discriminating as it searches. Singer's definition of monophagy means that a truly monophagous insect, enclosed with a plant which is not its specific host, will not oviposit. An individual which spends a long time in the search phase in which only a single host is acceptable will be classified as highly host specific. In Singers view, there is no truly monophagous insect. What is often overlooked is that by specializing on a type of tissue, phytophages specialize upon a period in the phenology of the plant (Trong et al., 1984).
According to Claridge (1961), Spears (1978) and Dawah (1987) Tetramesa species are very host specific but some (e.g., T. hyalipennis) are narrowly polyphagous i.e., feeding on small number of grass species belonging to a single genus or tribe. No previous studies have been done on host choice in T. petiolata. The aim of this present study was therefore to compare the oviposition behavior and preferences of T. petiolata in relation to different host plant species.
Materials and Methods
Observations were made using stems of D. caespitosa contained in cages under laboratory conditions. All females used in the experiments were reared in the laboratory and kept well supplied with food (a mixture of honey and water). All females used were 4-5 days old.
Host plant preferences were studied by introducing one adult T. petiolata female into a cylindrical cage containing four grass species including D. caespitosa, the natural host food plant. An open-ended tube containing the female was placed among the four plant stems and the female was allowed to climb out. The tube was then removed. The cages used in the experiment were kept well supplied with food in the form of 25% honey and water. The cages consisted of a cylinder of transparent plastic (height 800 mm, radius 250 mm). The cage was constructed using the base and lid of plastic cake container and cylinder of thin (0.16 mm) clear acetate sheet. Each cylinder included an 180 mm window, covered with nylon mesh to allow adequate ventilation. In addition, circular holes (120 mm dia.) were cut and fitted with sleeves of netting to enable access without allowing the insects to escape. In the experiment, a female was released into each cage and left for one hour. The time spent performing different activities was measured using a stop-watch.
All plants were at the same stage of development and stems were chosen in such
a way that each had two pairs of fully expanded leaves and a compact shoot and
which had similar leaf sizes and internode distances. The experiments were carried
out using a randomized arrangement of plants and a total of ten replicates were
run. Female behavior was classified into five categories: grooming, resting,
walking, searching, landing and oviposition. Walking and searching were distinguished
by the attitude of the antennae. In walking the antennae are held parallel with
one another or only slightly apart but parallel with the substrate, whereas
in searching the antennae are continually tapped against the host plant surface.
Recording of female behavior was started after a 5 min settling period, or when
the test female first landed on a plant, whichever comes first. Recording ended
after 4 h. Walking was not recorded because it often lasted for long periods.
The experiments were carried out in an environmental chamber maintained at 18°C
and with a 16 h light: 8 h dark photoperiod. Each female was left in the cage
for 24 h. The observations were carried out between 12.30 h and 16.30 h.
The following plants were used; Deschampsia caespitosa, Deschampsia setacea (Huds) Hack. Dectylis glomerata (L.) and Phleum pretense (L.). All stems were dissected after 18 days and examined for the presence of larvae.
The dates on which insects were introduced into cage were coinciding with the flight period of T. petiolata (late May- mid June). This was done to ensure that the grasses used in the experiment were at the correct stage of development for possible selection by T. petiolata. Observations on oviposition behavior were made using stems of D. caespitosa contained in cages under the same condition. The results were analyzed statistically using a one-way analysis of variance (ANOVA).
Results and Discussion
From the collected data, the mean total time spent performing each behavior was calculated, including grooming, resting and searching on each host plant, by females in 10 replicates (Table 1). The number of landings and attempted ovipositions made by each female on each host plant was counted (Table 2).
The mean time spent grooming did not vary significantly between plants (F = 1.63; df = 3; p>0.05); similarly the mean time spent resting on each host food plant did not vary significantly (F = 1.80; df = 3;36; p>0.05).
Phillips and Emery (1919) carried out breeding experiments on North American species of Tetramesa and showed that a high degree of host specificity exists amongst them. In the above oviposition choice experiments large number of larvae were reared from the host plant, but none were reared from other plants except for D. setacea, an unnatural host from which a few larvae were reared. This limited laboratory experiment suggests that T. petiolata is at most potentially narrowly polyphagous and that females prefer one species much more than another. Since in the field, larvae of T. petiolata have never been recorded in the stems of D. setacea, the deposition of eggs in D. setacea can be regarded as a laboratory artifact and T. petiolata can be regarded as monophagous under natural conditions.
The above results agree with those of Dawah (1987) who demonstrated the oviposition
preferences in T. hyalipennis and T. eximia. He indicated that
these species are narrowly polyphagous, but they are still highly host specific
because they attack two closely related species of Gramineae. Many phytophagous
insects show a high degree of host specificity, being either monophagous or
narrowly polyphagous (Hodkinson and Hughes, 1982). This certainly appears to
be true for Tetramesa sp. Two of the thirty-nine species of Tetramesa
studied by Spears (1978) were found to be monophagous. Other grass feeding
Tetramesa were found to be even more host specific (Claridge, 1961).
|| Mean time spent by T. petiolata grooming, searching
and resting on different plant species
||Mean number of landings and attempted ovipositions female
of T. petiolata when given a choice between four plant species and
number of larvae reared from each host plant.
Oviposition preference in female insects involve sensory detection of the host
plant or parts of the host plant. Dethier (1976) hypothesized that the process
of finding a plant upon which to feed or oviposit is governed at each step by
various arrays of sense organs of which chemoreceptors play an important role.
Hawke et al. (1973), Van and Veen (1981) and Austin and Browing (1981)
stated that sense organs on the tip of ovipositor are presumably used for assessing
the suitability of the host, but that the sense organs involved in host finding
are located on the antennae. The insects do not select host plants at random
but by means of several criteria, probably plant volatiles, color and form or
odors emanating from the hosts habitat. Many parasitic Hymenoptera use
olfactory cues to orient first towards a potential host habitat and second,
towards the host itself (Vinson, 1981; Weseloh, 1981). Pinto and Stouthamer
(1994) and Norlund (1994) noted that microhabitat preferences occur in many
parasitoids such as Trichogrammatidae and Ichneumonidae, where in these species
parasitization of a host egg depends on the crop plant it is laid on.
This study shows that adult females of T. petiolata are discriminating in their selection of plants in which to oviposit. Oviposition behavior in T. petiolata appears to be released only by highly specific stimuli. It may be suggested that the responses reported here are predictable because the insects become conditioned to feed and even oviposit on the same type of plant on which they themselves developed. Linn et al. (2003) and Thomas et al. (2003) hypothesized that preference for natal host plant odours and assortative mating due to host-specific sex pheromones may well drive host specialization. Al-Barrak et al. (2004) added that the morphologically similar host-adapted forms of Tetramesa may indeed be an example of sympatric speciation in action.
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