Kairomonal studies with reference to M. vitrata female moths
The experimental results showed a significant variation on orientation behaviour of
Maruca female moths towards flowers of different pulses (Table 1). At 10 MAR (Minutes After Release), the maximum orientation of one day old females recorded in lablab (44.0%). Pigeonpea and cowpea stand next to lablab with the per cent orientation of 20.0 and 18.0 respectively when compared to 4.0 per cent towards control arm. At 20 MAR, the highest orientation (44.0%) recorded towards flowers of lablab and found to be significant to other hosts. On 30 MAR, also same trend was recorded and the orientation level ranged from 2.0 to 40.0 per cent on blackgram and lablab respectively. On 10 MAR, the response of two day old
Maruca females was more (28.0 %) towards pigeonpea and control arm (without host) followed by 18.0, 16.0 and 10.0 per cent on lablab, cowpea and greengram respectively. In 20 and 30 MAR also, the maximum orientation of female moths recorded towards control arm (26.0%) followed by pigeonpea (24.0%) and greengram (22.0%). There was no female orientation recorded from 10 to 30 MAR on blackgram (Table 1). At 10 MAR, the highest orientation of three day old
Maruca females recorded towards control arm (34.0%). Lablab and pigeonpea recorded orientation of 24.0 and 22.0 per cent and stand next to control arm (Table 1). On blackgram flowers containing arm, there was no orientation of
Maruca females observed during the entire observation period.
Among the response showed by different age groups of female moths to flowers of pulses, the significant orientation was recorded only in one day old females. The mean data showed that maximum female moth orientation was towards flowers of lablab (42.7%) followed by 21.3 per cent on flowers of pigeonpea and cowpea as compared to 4.0 per cent in control (without any hosts) (Fig 1). Whereas, the maximum orientation of two (26.7%) and three day old females (34.0 %) was found towards control followed by 25.3 and 22.0 per cent respectively on pigeonpea for same the age groups. It clearly showed that the one day old females are having strong orientation towards its preferable host than other elder age groups. This kind of behaviour might be useful to find its suitable pair for subsequent mating and oviposition for further progeny development. The above phenomenon was in conformation with the laboratory studies by
Sambathkumar (2023) who recorded maximum fecundity on lablab followed by pigeonpea when compared to other pulse hosts. Generally, volatile chemical compounds of host plant parts are mainly influence on identification of ideal site for oviposition and development. Bo
Feng et al., (2017) recorded β-caryophyllene as strong oviposition attractant to
M. vitrata in cowpea
. Apart from locating preferable hosts by female moths, host plant volatiles also have tremendous influence on various sexual responses and courtship behaviour of many phtophagous insects.
Effect of kairomonal odour on larvae of M. vitrata
Based on the settling behaviour of
Maruca larvae on different pulses using plastic basin (30 cm dia. and 10 cm height), it was clear that the most preferred hosts were cowpea and lablab which attracted about 32.4 and 30.4 per cent of the larvae followed by pigeonpea (23.6%) and they were statistically on par with each other. Greengram (6.8%) and blackgram (3.6%) had minimum orientation and about 2.8 per cent of larvae did not show preference for any hosts (Table 2). The experiment conducted with bigger container revealed a significant attraction in lablab (33.6%) and cowpea (32.0%) than other hosts. Pigeonpea (14.0%), greengram (11.2%) and blackgram (6.0%) were preferred less by the larvae.
During many instances, the plants that are under biotic stress, release many Volatile Organic Compounds (VOC) and this cause remarkable response in host insects which are receiving these chemicals (
Adhab, 2021). Hence, by knowing the insect pest attracting specific plant volatiles, it can be recommended the planting of non-crop hosts that could yield considerable amount of the same volatile and thereby the main crop get protected. The current study showed that the maximum orientation of
M. vitrata larvae was towards flowers of cowpea and lablab followed by pigeonpea than blackgram and greengram (Table 2). Similar kind of studies on
M. vitrata has not done earlier. Maximum larval orientation towards lablab followed by pigeonpea is in agreement with reports of
Sambathkumar et al., (2017) in terms of desirable nutritional indices values on growth and development and to support for maximum generation production of
M. vitrata. Results on paired‘t’ test of the present study revealed that the significant difference only existing in orientation behaviour of larvae towards greengram and pigeonpea. The reason for this significant difference on two hosts might be due to unavailability of enough space for the larvae to locate its highly preferred host (with mixture of odour) in plastic basin. However, in the bigger container, chance for mixing host odour is very less and hence, larvae could able to orient towards its most preferable host.
In the present study, less orientation of
M. vitrata larvae and adults recorded towards blackgram and greengram. Even though these two hosts are seem to be the best choice for the establishment of
M. vitrata, the poor larval preference might be due to availability these two hosts in the pool of other pulses. Therefore, the VOCs from highly preferred host plants and the combination of odour from all host plants play a crucial role in larval preference of
M. vitrata for feeding when many legume hosts are available. In case of phytophagous lepidoperans, selection of suitable host plant for its larval feeding is bestowed with the choice of mother moths (
Singer, 1981) and the emerged larvae have ability to discriminate hosts based on quality and emanating volatiles from the host plant. Further based on visual and a wide array of semiochemical cues from host plants further, suitability will be decided by olfactory, gustatory and tactile cues
(Harris et al., 1999). Likewise, larvae would search thoroughly and settle on suitable food among complex matrix of food source.
Response of different age groups of males of M. vitrata to female moths
The results on response of one day old male moths of
M. vitrata to different age groups of females showed that, at 10 MAR, the orientation of male moths was maximum (34.0%) towards two days old females followed by 28.0 per cent towards one day old females when compared to 4.0 per cent towards four day old females. At 20 MAR, there was no significant variation in orientation of
Maruca males towards different age group of females and it ranged from 10.0 to 36.0 per cent on two and three day old females respectively. Same trend was registered at 30 MAR (Table 3). The response of two day old male moths of
M. vitrata showed significant variation with respect to its orientation towards females. At 10 MAR, maximum orientation was observed towards two day old females (40.0%) followed by one (24.0 %) and three day old females (16.0%). On control arm (without host), 16.0 per cent of males were settled. At 20 and 30 MAR, more number of male moths was oriented towards females of two day old (40.0%) and during the both times of observation, the least orientation (8.0%) of male moths recorded towards four day old female moths of
M. vitrata. The response of three day old male moths was almost similar from 10 to 30 MAR (Table 3). On 10 MAR, the per cent orientation ranged from 10.0 to 34.0 towards control arm and two day old females respectively. At 20 and 30 MAR also, the orientation was more towards two day old females followed by 18.0 per cent towards both one day old females and control arm (without host).
A pheromone is a substance secreted externally by an organism causing a specific reaction in a receiving organism of the same species led to interaction between organisms (
Nordlund, 1981). In recent past, there has been more use of pheromone mediated monitoring of insect pest population. Mean orientation response of different age groups of male moths towards females showed that maximum orientation of one day old males (35.3 %) towards two day old female moths followed by 28.0 and 13.0 per cent orientation towards one and three days old females respectively. Similarly, highest mean orientation response of two (40.0%) and three (31.3%) day old males also recorded towards two day old females (Fig 2). The present result might be due to the influence of releasing sex pheromones in two day old females to attract more number of male moths through series of differential electrophysiological stimulatory response in antenna of males. Age of insects is very important factor that significantly affect their mating success, mate preference and fitness outcome
(Lai et al., 2020). As the age of female moth increases there could be reduction in the rate and titre of sex pheromone release. The present results clearly revealed that the maximum male moth orientation is towards 2 days old female moths which coincided with more release of pheromone. Whereas on older (3 and 4 days age) female moths, the lesser male moth attraction has showed the decreased rate of pheromone emission. Similarly the male moths up to 2 days old attracted more towards young female moths (up to 2 days aged). This indicated that the maximum orientation between younger male and female moths and this response could be gradually reduced with increase in the age of both sexes. Earlier reports also indicating that, the mating delay between unsuitable pairs reduce the reproductive output of both sexes but female moths were more negatively affected than male moths (
Xia-Lin Zheng et al., 2020). Hence, the present study suggest that the ideal mating attempts can be done up to two days old male and female moths during pheromone related experiments.
The current findings are in conformity to the findings of
Downham (2003), who extracted maximum amount of attracting pheromones from two day old female moths of
M. vitrata. In contrast, the chance of pheromone release is very less in one day old female would cause less attraction. Similar kind of phenomena was observed in
Spodoptera litura Fabricius and
H. armigera (Hong-Lei-Wang et al., 2005). Results of present study may be further extrapolated with respect to frequency of adult moth mating rhythms. Since,
M. vitrata female moths of two days after emergence (DAE) released maximum amount of pheromones, the peak mating frequency might be expected during this time. This is closely in line with earlier findings on occurrence of highest mating frequency of
Mauca adults on 3 DAE (
Lu Peng-Fei, 2007). Many earlier EAG studies are also confirming these findings. Based on the present findings, it can be further studied the maximum attraction of different age groups of male moths of
M. vitrata to its females in order to find the ideal time and age for female moths to extract its natural attractants including the sex pheromones.
This phenomena also useful in designing specific kairomones based management techniques for
M. vitrata in future. Further studies are required to correctly identify chemistry of olfactory stimuli that are influencing the orientation behaviour of both larvae and adult moths in different hosts tested. If highly attractive chemical cues identified, this may be produced in large scale and utilized as alternate pest management tool for monitoring, mass trapping and mating disruption of
Maruca populations. Additionally, by applying these kairomones on non-host plants in nearby fields, both larvae and adult moths of
M. vitrata made to attract for feeding and oviposition, respectively on these plants to achieve incomplete growth and development of insects. Simultaneously, application of pesticides could be concentrated only on these non-host plants to reduce this globally distributed insect threat.
