IOBC wprs Bulletin Vol 22(9), 1999
Investigations in the relationship between Cydia molesta (Busck) (Lepidoptera Tortricidae) and its main host plant
Davide Natale, Letizia Mattiacci,1 Edison Pasqualini and Silvia Dorn1Institute of Entomology "G. Grandi", University of Bologna, Italy
Abstract: Cydia molesta (Busck) (Lep. Tor.) is a polyphagous pest mainly damaging peach and pome fruit-trees. The aim of the research is to study the host-plant location behaviour of virgin and mated females in a Y-tube olfactometer. This work introduces the first investigation on semiochemically mediated host location behaviour and adult conditioning of female C. molesta. The results show that both naive and experienced virgin females respond to volatiles emitted by peach (fruit and shoots) and pear (fruit and shoots) but only at high light intensity (3500 lux). Results provide a first indication of the habitat location strategy of this important pest.
Key words: behaviour, olfactometer, host plant volatiles, Cydia molesta
Of north-west Chinese stock, the oriental fruit moth (OFM), Cydia molesta (Busck), has been recorded on wide range of plant species in the family Rosaceae, mostly in genera Prunus and Pyrus. The larvae cause damage to their host by tunnelling and feeding in the shoots and/or fruits (Tremblay 1990).
The mechanisms which determine C. molesta host-plant location are unknown. However it has been hypothesised that although the species is generally linked to the stone fruit orchards, there may be biotypes with different host plant preference, with pear or apple being the main host of economic importance in some areas (Rothschild and Vicker 1991). This could occur in those areas in which stone and pome fruit are closely associated between them, such as for example in the Emilia-Romagna region (Italy). Here C. molesta completes 4-5 generations, the first 3-4 of which on peach and the last on pear and apple crops (Ivancich Gambaro,1978). In the last few years, OFM represents the most important pest on pome fruit in our region, with heavy infestations starting from July (Marzocchi 1993). At present it is not yet known if the migration is due to the availability of a new food source, pear ripening begins at the end of peach harvest, or if the species shows a particular preference for pear among other pome fruits.
This paper introduces the first investigation on semiochemically mediated host location behaviour of female C. molesta by means of Y-tube. The ecological aspects of learning in a phytophagous insect like OFM, are investigated as well.
Material and methods
Fruits and shoots of Prunus persicum L., cv Spring Lady, were collected from 6- to 10-years-old trees reared in orchards. Fruits of Pyrus communis L., cv Conference, were collected from 6- to 10-years-old trees in field as well; a 2- year- old tree, cv Abate fetel, was used for the shoots.
Virgin females C. molesta were reared in a climate room at 28 ± 2 °C, 50-70 % RH and light regime 16L: 8D, within a plastic cage (30x30x30 cm), supplied with water and honey. Three- to 5-day-old females used in the bioassays. In one set of bioassay, females were given an experience with plant odours by exposing them for 10 min to an airflow carrying the host-plant volatiles. The fruits or shoots were placed between the cage and a fan 50 cm spaced. Females were experienced on the morning before the experimental day.
The bioassay set-ups
The behavioural response of individual C. molesta females toward shoots and fruits from both the host plants was observed in a Y-tube olfactometer as described by Sabelis & van de Baan (1983), with slight modifications. The major modification to that set up consists in the length of arms (23; 23; 23 cm) and the diameter, 1,5 cm. In our set up an airflow of 200 ml/m was pumped up into two glass containers (13 x 11 cm), which we used as odour chambers, and then in the arms of the Y-tube. The temperature was 23 ± 2 °C and relative humidity 50 ± 5 %. Light intensity was 3500 lux (artificial light). The odour sources introduced in the odour chambers were the following: peach fruit vs blank, peach shoot vs blank, pear fruit vs blank, pear tree vs blank. During the bioassay individual, naive or experienced, C. molesta females were released at the opening of the common arm of the Y-tube for a maximum time of 10 min (50 replicates/plant and insect treatment). We recorded a choice if females crossed one of the arms of the Y-tube within 10 min. We recorded a no-choice if females would stay in the common arm for more than 10 min. The bioassay were performed from 14:00 and 16:00 pm.
Bioassay data were elaborated by means of chi-square test goodness-of-fit (Sokal and Rohlf, 1981) to determine if the response differed from a 50 : 50 distribution of moths over the two odor sources.
C. molesta choice response
The comparison between the treatments shows that experienced females had a higher response to fruit and plant volatiles than the naive ones. Moreover for the experienced females the overall response to pear volatiles (38-34%) tended to be higher than peach volatiles (30 and 32%). Only pear-fruit elicits a significantly different response (p<0.005).
C. molesta responsiveness
Experienced female increased mean responsiveness in all cases. The difference between responding and non responding experienced female became significant in the cases of response to peach fruit (76% responding, p< 0.005) and to pear shoots (70% responding, p<0.005).
The host location behaviour of females C. molesta seems to be semiochemically meditated. Furthermore exposure to host volatiles prior to the bioassay increased females responsiveness during the Y-tube olfactometer tests. The responsiveness of experienced females was always over 50% and exceeded 70% for the treatments pear-fruit (76%) and peach-shoot (70%). This increased responsivenes could be due to sensitization as found for other phytophagous insects (Papaj and Prokopy 1989). In contrast to our expectations females responded better with high light intensity (3500 lux), while for example it is known that mating behaviour occurs at much lower intensities (200-700 lux) (Rothschild and Vicker 1991). It could be therefore hypothesised that the host location behaviour could occur with a different temporal pattern than mating. The result carried out up to now can be considered encouraging. The research will continue involving different cultivars and likely a greater number of replicates to gain statistically significant differences.
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