Developing efficient pheromone traps-Spodoptera litura (F.)-Techniques to know!
In the rainy season, males started responding to pheromone traps soon after the sunset, with a small peak around 2000 hr (contributing only 10% of the total catch), and a small proportion continued to fly all through the night, with a peak around 0300 hr (19% of the catch). The same trend was noticed in the summer, with a peak activity around 0300 hr (24% of the total catch), but the initial activity was a little delayed. The observation with Rustrak flight detector fitted to a pheromone trap in summer also showed the same pattern, with a small peak at 2000 hr and the major activity at 0300 hr. The observation with night vision goggles also revealed the same trend, with 7% of the catch at 2000 hr and 41% of the catch around 0300 hr. it seems that under
normal conditions the, optimum; male activity would be around 0300 hr, irrespective of the season. Similar studies conducted at Tamil Nadu Agricultural University, Tamil Nadu, India by Dandapani (1985) with this species showed. highest catch between 2000 and 2200. hr, while Balasubramanian et al. (1985) reported peak catch at 2300-0000 hr and 0300-0400 hr. This could be due to the different chemical composition of the pheromone itself.
Calling behavior:
For successful mating, the behaviour of both the sexes can be put in to four stages which happens in sequence:
a. Pre-calling period,
b. calling period,
c. pre-copulation period and
d. copulation period
a.Pre-calling period:
In this insect the calling behavior is initiation by the female by releasing the pheromone. The activity was heist one day after emergence and decreased with age (Kawasaki,1986). The activity become higher with latter half of the dark perid. Ohbayashi et al.(1973) also eported that female calling, releasing of sex pheromone and copulation were observed throughout the night. Males become active after light off and showed peak about 1hr later followed by a less active period by the extrusion of clasper and hairpencil. Tamaki et al.(1973a,b) also reported that the responsiveness of males to the female extract in low concentration decreased 4hr after light off. This activeness is “searching while moving” type seems suitable to S.litura males which are highly mobile ( Oyama and Wakamura,1976). The mating behaviour and sex pheromone of Spodoptera litura F. were carried out. Peak of adult emergence appeared 1 to 2 hr after sunset or light-off. Mating also occurred most frequently 1 to 2 hr after sunset, and more than 80% of the pairs copulated before mid-night.While releasing the female (Yushima et al.,1973). They have also reported that the males and females when they are 4 day old after emergence from pupae could mate more than 97%. Most females started calling on days 1–4 after adult emergence. In three populations obtained in Saga prefecture in Kyushu mainland, Japan, two different patterns of pre-calling period (PCP) were observed: one was the above-mentioned pattern shown by most individuals, and the other was a pattern in which females sexually matured within several hours after emergence. Diel periodicity was shown in the time of calling activity, and its pattern varied among the populations. Pre-calling period was stable over successive generations in the laboratory (Murata et al.,2006).
b.Calling period:
Males of Spodoptera litura showed activity just after light off which was followed by a less active period. Then they became active and were highly active in the latter half of an 8 hr dark period. Females were less active than males and small activity peaks were observed sporadically throughout the dark period. Many females showed calling around 1 hr after light off and thereafter calling activity was observed continuously till light on. Males remained responsive to a sex pheromone from 1 hr after light off to light on.Two peaks in the male attraction pattern in the field resulted from high male and female sexual activity just after light off and from high rate of male locomotion later in the dark period (Kawasaki,1986). Mating behaviour represented as a typical calling position of female moths showed two peaks of activity during a 24 hr period. The major peak at 23 : 00 was 3 hr after initiation of the dark period and the minor one at 3 : 00 was just 1 hr before the end of the dark period. Response of the male moths to the sex pheromone, which was extracted from the virgin female moths, also showed two peaks coincided in time with those of the female's calling, and the mating period actually coincided with the time of maximal activity of the male response and the female calling (Ohbayashi Nobuo et al.1973).
Female calling:
Female calling starts just after lights off and higher calling continued for the next two days. However, calling continued throughout the dark period. The activity of walking and flying continued. This was confirmed through the use of “Actograph” (Kawasaki,1986).
Male calling:
Male start responding 1 hr after light off and continued till lights on. There was no change in this kind of activity with age progress (Kawasaki,1986).
Mating time, behavioural activity, and sex pheromone production and release of adult S. litura were examined under artificial light conditions (16L : 8D). Mating behaviour represented as a typical calling position of female moths showed two peaks of activity during a 24 hr period. The major peak at 23 : 00 was 3 hr after initiation of the dark period and the minor one at 3 : 00 was just 1 hr before the end of the dark period. Response of the male moths to the sex pheromone, which was extracted from the virgin female moths, also showed two peaks coincided in time with those of the female's calling, and the mating period actually coincided with the time of maximal activity of the male response and the female calling. The sex pheromone activity in the virgin female was high at the time of light-off and light-on, representing two peaks during a 24 hr period. On the other hand, release of the sex pheromone from the virgin female gradually increased from initiation of the dark period, and decreased toward the end of the scotophase. This inverse relationship indicates that the change in sex pheromone activity is related with the production and release of the sex pheromone in the virgin female (Ohbayashi Nobuo et al.1973). All adult activities took place during the scotophase. After emergence, calling, courtship and mating events increased with time and peaked in the second night. Mating peak were always following the calling and courtship peaks. Most paired insects performed the first mating within the 2nd day after emergence and peaked at midnight while remating peaked at dawn. Paired insects could mate up to 4 times, with an average of 1.9 ± 0.4 matings (Cong Li et al.,2012).
Studies also found that males of many taxa, including insects, displayed a heightened preference for novel mates combined with a decline in the propensity to re-mating with the same mate (Tan et al.,2013). However, two other studies in insects have found the opposite result: males significantly prefer previous to new mates for subsequent matings (Zeh et al.,1998; Xu, 2010).
c. Pre-copulation period:
There is no waiting period for copulation of females. However, the one day old females mate with two days old males and not withone day old males (Yushima et al.,1973).
d.Copulation period:
This insect is a nocturnal moth and all adult sexual activities (courtship, calling, mating and oviposition) take place during the scotophase (Li et al.,2012). Adult moths eclose at dusk, and no matings take place during the night of eclosion. Approximately 70% of mating occurs on the subsequent night after emergence, and those unmated will mate on the third night (Li et al.,2012, 2014). Paired insects can mate up to four times, with an average of 1.9 ± 0.4 matings (Li et al.,2012). Females begin to lay eggs on the subsequent night after first mating Li et al.,2012, 2014).
Trapping of males of Spodoptera litura in groundnut and castor cropping system.
In general, groundnut is grown mostly as monocrop in post rainy season, while it is grown as either intercropping with groundnut (4 to 6 rows) with castor or pigenpea one or two rows (in Saurashtra,Gujarat) or mono or mixed strip cropping in Saurashtra or in AP.. , Tamil Nadu, Karnataka. As we know the S.litura occurs in groundnut as much as in castor.
Which type of trap would be better for trapping of males using pheromone in a suitable dispenser?. Before going in far designing the trap for trapping the attracted males in a pheromone trap, it is better to know the development of trap in India.
Tamaki et al.(1973) identified the pheromone components first and they again visualized thetype of trap to be used. The authors (Yashima and Tamaki,1974) designed two types of traps to try the synthesized pheromone components in the field conditions. As per their description of the traps, Type I was a box trap (34.5 X 24 X 23.5cm) having a rectangular window (3 X 6 or 3 X 9cm) on each side. A net cage (24 X 15 X 10cm) was held on the top inside the box to enclose virgin females or synthetic pheromone sources. Type II trap was a cylindrical (20 X 8 cm) having 4 windows (6cmi9n diameter). The traps were filled with a dilute detergent solution (3% Gramin *, Sankyo Ltd,Tokyo) to prevent the escape of the males attracted. Traps were located at least 50m apart from adjacent traps and 1m above the ground level.
(Yashima and Tamaki,1974-this figure is drawn from the original article published)
The same authors reported their results as with 1 µg 50 males were trapped during 11-12th of September month, while, there was only 4 males trapped during 10-11th of the September month. The drastic reduction in the catch, they attributed, for the high wind velocity. These authors coined the name of the lure as “Litlure”. They also fixed the correct dose for Japan ecosystem as 39:1 ratio of the above two components of the pheromones, which could attract 318 males at 1 µg kept for 6 nights in 3 traps. Even 10:1 ratio of the two pheromone chemicals of S.litura could attract 305 males in the similar set up. They have also finalized the dose and time of retention of the chemicals as 1milligram in rubber septa as dispenser for 2 months period under field conditions. Yashuo Sato et al.(1978) developed a dry trap for field catch of the S.litura. The trap was a box (30 X 25 X 8 cm) made of polyvinyl chloride plates. The trap had two ducts (section:3cm X 3 cm) the opening of which were attracted to the perforated end walls. On the shut end of the duct, a valve made of perforated thin plates was hung to lead the incoming males and to prevent their escape.
Yashuo Sato et al.,1978- this figure is drawn from the original article published)
The authors have found that the weight of the valves had profound influence on the trapping of the males. Lower the weight of the valve (0.4g) more the % males trapped(80%), which means the valves as planned, retained the attracted males and not allowed for their escape. When they increased the weight of valves to 2 grams, their was no males could be trapped, probably, these highly weighed valves prevented the very entry of the attracted males. Interestingly, the width of splits in the valves do affected the trapping of males. In no split valves, there was no moths trapped, as no entry, while in3 mm split, there was 2 males and in 6 mm split, there was 51 males, while with 12 mm split, the males trapped were a little less (48.5 males). This showed that the split should not be more than 6 mm for optimal entry and preventing the males attracted. Obsoletely, as, hypothesized, too many males trapped and dead inside the trap may influence the further attraction of the males. There was no difference of the presence of the dead males inside the trap on further attraction.
It was Pawar et al.,(1984) who have constructed a trap from locally available materials, that was modeled upon a trap supplied by TDRI, London, was found to be more effective. The thus attracted was collected in a plastic bag through a funnel. This trap was designated as ICRISAT standard Trap.
The TDRI, London supplied a sleeve trap with a clearance space of 2.5 cm (entry space). But due to operation difficulties (fitting of lure after every 4 weeks-probably), they have changed the clearance space to 5 cm.
Probably that was the reason why there was not much encouragement in the trapping of males. When Dr.G.V.Ranga Rao took over the research on S.litura at ICRISAT, he adopted the trap (Ranga Rao et al.,1991a,b) that was used by Pawar et al.,(1984,1988).
Ranga Rao (1991a) published a excellent work on “development of standard pheromone trapping procedure for S.litura” They have screened four types of traps for this purpose. To put it in a simple way, 1.Small sleeve with either plastic or metal ring (10cm dia) which hold the sleeve with 5cm clearance spaceing.2. Big sleeve with in small sleeve with 20cm dia ring 3.Double funnel (15cm dia with vertical slit attached to to ring) having plastic funnel and another plastic funnel (20 cm dia)attached to plate with polythene bag 4. Single funnel trap., like the 3 type but with out small funnel. Traps were installed at 1m height, placed at 200 meter apart. They have observed the mobility of the moths for 14 nights. In the results, They have reported that there was no difference between single funnel or double funnel in trapping equally higher males. When they have used 16 traps/ha, there was no significant increase in trap (690/week) compared to 571 males with 8 traps and 445 males with 4 traps (CD:264.7). The most important observation they made was the escape of males after trapping. In small and big sleeves there was escape of trapped males (70 and 62), whereas, in in double funnel and single funnel there was no escape of trapped males. Finally they suggested that for mass trapping and mating disruption pheromone may not be useful, however, feasibility of forecasting through monitoring. For monitoring of S.litura they recommended 1m height in the initial and the data exhibits 4 m height also good at trapping the males (Ranga Rao et al. 1991b). Unfortunately, they have concluded that pheromone trap monitoring will not make the rightful impact on future pest control strategies. Also they have expressed a opinion that though double funnel trap was used, there was no significant moth catch.
What we should know in designing an efficient trap with pheromone dispenser?.
As suggested by Ranga Ran et al.(1991a) that ..”one needs to look for a better trap than this to ensure an efficient monitoring system”. This has to understanding the actual mobility (flying in and out). So, find out the facts about the mobility of this insect, a team was formed with author, three SRFs (trained in pheromone research in a ICAR scheme) visited the Main oilseeds research station at Tindivanam under TNAU, Coimbatore. One mg lure (rubber septa) was used in a two trap traps, one with 2 cm clearance space and another with 6 cm clearance space, keeping all other aspects, traps were prepared. Two traps were installed at 10 m distance and at each trap two persons were deployed for observation. The traps were installed at 12 night. Within a span of few minutes the males were started flying near the trap. More males were attracted to the trap. The entry of males by flying, landing and walking were observed. Males landed on the periphery and moved in to the trap, males entered in to the trap through flying, males moved out by flying, males moved out by walking were recorded. Entomologist know the insects and their activities. The following is the table which gives the males responded to the pheromone in two different traps with different clearance spacing. Our observation ended just 30 t5 minutes past 5 am.
|
Behaviour of male
|
2cm (Clearance)
|
% to the trapped
|
% to the attracted
|
6 cm (Clearance)
|
% to the trapped
|
% to the attracted
|
|
Males landed on the periphery and moved in to the trap
|
52
|
91.2
|
|
34
|
94.4
|
|
|
Males entered in to the trap through flying
|
5
|
8.8
|
|
2
|
5.6
|
|
|
Total males trapped
|
57
|
|
|
36
|
|
|
|
Males moved out by flying
|
1
|
|
1.7
|
9
|
|
15.8
|
|
Males moved out by walking
|
2
|
|
3.3
|
12
|
|
21.9
|
|
Over all escape of moths
|
3
|
|
5
|
21
|
|
36.8
|
|
Total moths attracted
|
60
|
|
|
57
|
|
|
Observed in one single night between 0100h to 0600h (Table is modified after the verification of original data on escape of males)
When we were observing the mobility of males, about 91 % males moved towards the lure and landed on the periphery to the trap in 2 cm clearance spacing, while it was 94% males landed on the trap with 6cm clearance spacing. If we see the males attracted it is almost similar in both the traps. The loss caused by escape of attracted moths are higher in the trap with 6cm clearance spacing i.e., 36.8%.The important point to be noted is that the escape from the attracted males are more in the 6 cm clearance trap. So the deciding factor is retaining the attracted males lies in the clearance spacing. If it is less the males attracted could not escape and it was only 5% in 2 cm clearance. The above given table shows the mobility of the males towards and away from the trap. Keeping this observation, we have modified the trap keeping the importance of clearance spacing in retaining the attracted males, a trap with 3.5 cm clearance spacing and 4.5 half cut funnel, and 55 cm sleeve length which facilitated the males moved in by walking and got trapped.
The above trap gives the behaviour of the males which are attracted to pheromone.
Based on the return movements of the males the below given trap is fabricated using the already available traps.
The clearance spacing was given 3.5 cm,the cut cylinder is of 4.5 height. This has worked well in trapping the males in groundnut and castor ecosystem (Nandagopal and Rathod,2007).
What the literature tells about the traps:
Wind velocity and moth catch:
Wind speed have negative relation with chemical mediation and moth catch. More the wind velocity less will be the moth catch. This is what Sutton (1953) equation for gas diffusion in a wind, the maximum distance downwind, for a threshold concentration of pheromone (minimum concentration required in order to obtain a male response), is expected to decrease with an increase in wind velocity (Nakamura,197). In the field the movement of males increased with an increase of wind velocity until a maximum was reached at a wind velocity of 1 m/sec.
Male adults of Spodoptera litura were released at fixed points and captured in a pheromone trap. By comparing the experimental results with those obtained from a simulation model for the male's attraction process, the active space of the sex pheromone of S. litura was estimated. The maximum range downwind of the active space for one virgin female was estimated as about 80m for wind velocity of 0.50m/sec. This distance decreased with an increase in wind velocity above 0.50m/sec, and decreased greatly with decrease in wind velocity below 0.50m/sec. The reason for the decreased range at low wind velocities was as a result of a deposition effect of the pheromone. Thus, in a windless greenhouse, the active space was limited to a small area close to the attractant. The active space increased proportionally with an increase in the emission rate of the pheromone (Nakamura and Kawasaki,1977) .
Apart from the requirement of temperatures warm enough for fight, we reasoned that wind speed may be the most important meteorological variable affecting trap capture through its influence on the ability of weevils to approach the trap, because they are not strong fliers (McKibben et al., 1988, 1991). Analysis of catches for 21 nights between 8.iii.75 and 3.vi.75 showed that optimal catches of males occurred at wind speeds of 1·0−2·5 m s−1. Wind speeds in excess of 3–4 m s−1 depressed activity (Marks,1977).
Do pesticides help pheromone dislocate the biological female to the males?
In an interesting experiment, Hongyi Wei et al.(2004) found that in a wind-tunnel bioassay, males reared from larvae treated with deltamethrin, malathion, or carbaryl were less likely to display behaviors resulting in the location of a sex pheromone source than males reared from larvae treated with endosulfan or a solvent control (acetone). Mating success was significantly decreased in pairs containing either males or females reared from larvae that survived treatment with deltamethrin. The ratio of the sex pheromone blend, Z9,E12-14:OAc to Z9,E11-14:OAc in females from the deltamethrin treatment group differed significantly from that of female moths in other treatments. (Hongyi Wei et al.,2004).
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Next: we will see about the trap development for Helicoverpa armigera
