Time Trends in Mortality for Conventional and New Insecticides Against
Leaf Worm, Spodoptera litura (Lepidoptera: Noctuidae)
Mushtaq Ahmad Saleem,
Ali H. Sayyed
To determine time trends in mortality for various insecticides, which are being used against cotton pests, the fourth instar larvae of Spodoptera litura was collected from Muzaffar Garh and tested for pyrethroids, organophosphate and new chemistry insecticides. The efficacy of the insecticides was examined by time-oriented mortality at LC50, through leaf-dip bioassays in the laboratory. In sodium channel agonists, endosulfan was the most efficient insecticide. The cholinesterase inhibitors tested, chlorpyrifos showed high efficiency while phoxim performed better in time-oriented mortality. Emamectin benzoate proved to be the most efficient insecticide in new chemistry insecticides tested. Spinosad and indoxacarb had almost similar LC50 and LT50 values. The least effective insecticide found was abamectin. The results are discussed in relation to Integrated Pest Management (IPM).
Insecticides have widely been used for more than 50 years. Although the goal of insecticides application is to kill all target pests, however survivors are common as target species develop resistance to particular compounds (Armes et al., 1997; Byrne and Toscano, 2001; Kranthi et al., 2002; Wei et al., 2004). In response to control failures, growers have habitually switched to newly registered insecticides, which have generally provided effective control. In the past decade, however, the availability of new insecticides has become increasingly difficult to predict and, coupled with product withdrawals, cancellations and regulation based on toxicological or environmental standards, it has become important to retain the efficacy of existing materials.
Several methods have been used to define time trends in insects after exposure to insecticides. For example, Stringer et al. (1964) defined the delayed toxicity of bait toxicants for Solenopsis invicta as <15% mortality at 24 h and >89% at the end of the experiment. Using Weibull function, Dell et al. (1983) define time trends to a changed physiological state (e.g., ecdysis) of the proportion of an insect population. Using this technique, Haverty and Dell (1984) estimated the time required to achieve 90% mortality for fast acting compound at fixed concentrations. Such values can be used to characterise slow-acting compounds (Remmen and Su, 2005).
The leaf worm, Spodoptera litura (Fab.) is a serious and polyphagous insect, its reproductive capacity and migration ability over long distances has made it an economically important pest of many agricultural crops. It has been reported to attack 112 cultivated species (Mallikarjuna et al., 2004). In the last few decades it has extended its host range to other crops such as cotton, mungbean, soya bean, cabbage and leafy vegetables and causes economic losses of crops from 26-100% (Dhir et al., 1992). Spodoptera litura is one of the first insect pests of agricultural importance in sub-continent to develop resistance to insecticides (Kranthi et al., 2002). It has developed resistance to variety of insecticides used (Armes et al., 1997; Byrne and Toscano, 2001; Kranthi et al., 2002; Sudhakaran, 2002).
Effective time trends of a pesticide, neem Azal-T/S in combination with nuclear polyhydrosis virus has been studied for Spodoptera litura (Kim et al., 2001), yet little is known about the role of conventional and new insecticides. The objective of this study was to examine time trends in mortality for various insecticides, which are being used against the pest of cotton and other economical crops in Pakistan and also to define the time required to reach >50% mortality.
MATERIALS AND METHODS
Insects: Full-grown larvae of S. litura were collected from arum crop from Muzaffar Garh in August, 2004 and were reared at 25+2°C and 60±10% relative humidity with light: darkness (14:10 h) on a semi-synthetic diet (Ahmad et al., 1995).
Insecticides tested: Commercial formulations of endosulfan (Thiodon® 35EC, Bayer Crop Science), cypermethrin (Arrivo® 10EC, FMC), deltamethrin (Decis Super® 10.5EC, Bayer Crop Science), esfenvalerate (Sumi Alpha®, Sumitomo), chlorpyrifos (Lorsban® 40EC, Dow Agro Sciences), triazophos (Hostathion® 40EC, Bayer Crop Science), phoxim (Volaton 40EC, Bayer Crop Science), methomyl (Lannate® 40WP, DuPont), thiodicarb (Larvin® 80DP, Bayer Crop Science), spinosad (Tracer®, Dow Agro Sciences), indoxacarb (Steward®15SC, DuPont), abamectin (Agrimec 1.8EC, Syngenta), emamectin benzoate (Proclaim® 1.9EC, Syngenta), lufenuron (Match® 5EC, Syngenta), diflubenzuron (Teflon 7.5WP, Helb Pesticides), methoxyfenozide (Runner® 24SC, Dow Agro Sciences) and fipronil (Regent® 36EC, Bayer Crop Science) were obtained from their respective manufacturers.
Bioassays for LC50: Bioassays were conducted on newly moulted
second instar larvae (L2) of S. litura using a standard leaf
disc bioassay (Anonymous, 1990). The population was tested at the first generation
(G1) following field collection. Test solution was made in distilled
water. Leaf discs of five centimetres diameter were cut from unsprayed cotton
leaves, which were washed and dried before using. These leaf discs were dipped
in the test solutions for ten seconds with gentle agitation and were placed
on tissue papers for drying with adaxial surface upward. On drying, these were
placed in 5 cm plastic Petri dishes having moist filter papers underneath to
avoid desiccation. Five L2 larvae were placed on each leaf disc.
Each treatment was replicated 8 times, including controls. Mortality was assessed
after 48 h of exposure. Larvae failed to show movement after a gentle touch
with a blunt lead pencil head were considered dead.
Time to effect (LT50): Time trends in mortality were determined by using serial dilution in leaf dip bioassay (Anonymous, 1990). For lethal time experiment, newly moulted L2 larvae of G1 were exposed to treated leaf discs at lethal concentrations (LC50 values). Forty larvae per insecticide were exposed in eight replications with five larvae per Petri dish. Mortality for dose-mortality relationship at serial dilutions of each insecticide was recorded after 48 h of exposure and for lethal time mortality relationship at 12 h interval after 72 h.
Data analysis: Mortality data was corrected by Abbotts formula and analysed by Probit analysis (Finney, 1971) using the software POLO-PC (Le Ora Software, 1987) for dose-and time-mortality regression lines. Significant difference was inferred by non-overlapping of 95% fiducial limits.
Sodium channel agonists: Endosulfan and three synthetic pyrethroids viz., cypermethrin, deltamethrin and esfenvalerate, which act on sodium channel, were tested. Endosulfan was significantly more toxic (p<0.01) than pyrethroids (Table 1). The most rapid mortality was with deltamethrin with LT50 19.8 h, which was significantly less compared with cypermethrin (30.2 h) and esfenvalerate (33.8 h) and endosulfan (42 h; Table 2). The correlation between LC50 and LT50 of endosulfan (Fig. 1) was better (R2 = 0.96) compared with deltamethrin (R2 = 0.87), esfenvalerate (R2 = 0.93) and cypermethrin (R2 = 0.94), respectively. It is possible that the variability in deltamethrin and cypermethrin, esfenvalerate or endosulfan activity was the result of differences in feeding status of S. litura, which had previously fed, would be able to withstand a longer period without feeding in the feeding inhibitor treatment. However other factors such as behavioural effects not tested in these studies might also have contributed in feeding.
Cholinesterase inhibitors: Phoxim is relatively an uncommon organophosphate
in Pakistan and therefore has significantly less (p<0.01) LC50
compared with chlorpyrifos and triazophos, which reflect its lower use in Pakistan
(Table 1). When these insecticides were used in bioassays
at LC50, the LT50s for all three organophospate compounds
Lethal concentration response at 48 hours interval of some
conventional and new chemistry insecticides against leafworm, Spodoptera
litura of arum crop collected from Pakistan
|1LC50 expressed in μg mL-1
Lethal time response at lethal concentration of some conventional
and new chemistry insecticides against leafworm, Spodoptera litura
of arum crop collected from Pakistan
|1LT50 expressed in h., 2Fiducial
limits at 90% probability level
However, the speed of kill of triazophos was 1.5-fold slower than phoxim and
chlorpyrifos (Table 2). The most rapid mortality >20% was
detected in the compounds tested after 24 h exposure with 0% mortality in controls.
Similarly, the correlation between LC50 and LT50 was identical in organophosphate compounds with regression coefficient for phoxim (R2 = 0.78), chlorpyrifos (R2 = 0.77) and triazophos (R2 = 0.83; Fig. 2).
New chemistry insecticides: These insecticides have a short history
of use in Pakistan compared with conventional insecticides. Emamectin is significantly
(p<0.01) more toxic compared with all other new compounds tested. Abamectin
was the least effective compound with significantly (p<0.01) higher LC50s
due to non-overlapping of 95% FL (Table 1).
||Relationship betweeen LC50 and LT50
values of an Organochlorine and Pyrethroids
||Relationship betweeen LC50 and LT50
values of Organochlorines
||Relationship betweeen LC50 and LT50
values of new chemistry insecticides
Emamectin benzoate (R2 = 0.80) and diflubenzuron (R2
= 0.82) provided lethal mortality at about 20 h after exposure at concentration
equal to LC50, which was significantly lower than other compounds
tested; spinosad (R2 = 0.93; 33.6 h), indoxacarb (R2 =
0.95; 38.2 h), abamectin (R2 = 0.97; 54.4 h), lufenuron (R2
= 0.96; 38.3 h), methoxyfenozide (R2 = 0.94; 28.5 h) and fipronil
(R2 = 0.99; 29.4 h; Table 2; Fig.
3). Although emamectin is derivative of abamectin (Mrozik, 1994), but epi-methyl
amino derivative is very effective against a broad spectrum of lepidopteran
pests, with good photostability and translaminar movement and lack of cross-resistance
with other commercially used pesticides (White et al., 1997).
The similarities in LT50 can be attributed to the mode of action
of the products. Synthetic pyrethroids, organophosphates and most of the new
products are nerve toxins (Thompson et al., 1999) and therefore would
be expected to have a relatively rapid action. The synthetic pyrethroids and
organophosphates are currently the most widely used products against S. litura
in Pakistan. Growers are consequently familiar with their rapid action in the
field. The presence of live larvae in a treated field one-week post treatment,
as could occur with stomach inseciticides, would in most situations, be regarded
as a control failure and result in a second application. An education programme
is therefore essential to inform users of these expected differences.
In term of Integrated Pest Management (IPM), the side effects that an insecticide causes in non-target populations are also important. The product such as chlorpyrifos, which is quite effective against S. litura, is also known to have broad-spectrum activity causing mortality in non-target beneficial populations (Corso, 1988) and therefore may not be suitable in terms of IPM. In contrast, although spinosad, indoxacarb and abamectin showed lower levels of mortality in the bioassays, this may not be reflected in differences in post-treatment field populations. However, these products show high levels of selectivity (Nowak et al., 2001) and preserve beneficial populations therefore these could lead to enhanced control surviving populations.
One of the basic aspects of resistance management is to devise approaches to
minimize reliance on insecticides so that the selection pressure can be alleviated.
Development of effective resistance management programme need to be based on
information on occurrence and degree of resistance and the local resistance
patterns in field populations of insect pests to different insecticides. Because
the history of pesticide application varies, resistance patterns also differ.
For such differences to be exploited they need to be properly documented. The
variation in resistance to the insecticides tested in this study was not large,
suggesting the possibility of common resistant mechanism to pyrethroids, organophosphates
and some new chemistry compounds. However, further studies are required to confirm
the hypothesis. Moreover, the low slopes obtained (Table 1)
from the probit assay data suggested that the population was heterogeneous.
Heterozygotes are the most common carriers of resistance, they are the most
important genotype from a resistance management perspective (Roush and Mckenzie,
1987). The widespread occurrence of heterozygosity in field populations may
contribute to rapid increases in resistance levels even as a result of just
a few insecticide applications. This phenomenon is exemplified by the transient
decline in pyrethroid resistance in several lepidopteran insects, for example
H. armigera strains following a withdrawal of pyrethroid use for five
years until 1987 in Turkey (Forrester et al., 1993). Reverted populations
were found to maintain a rather high frequency of resistance alleles, which
led to the re-establishment of high resistance after only a few selections.
The present study suggests that several of the products evaluated could have role in the management of S. litura in Pakistan. However, a field evaluation would be required to fully investigate the field efficacy and side effects to determine their field performance and IPM compatibility in Pakistani ecosystem.
The present work is part of a Ph.D program of the first author and sincere thanks are due to the Higher Education Commission, Government of Pakistan for granting the Ph.D scholarship and Director, Central Cotton Research Institute for providing laboratory facilities.
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