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Am. J. Trop. Med. Hyg., 73(2), 2005, pp. 343-349
Copyright © 2005 by The American Society of Tropical Medicine and Hygiene

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BEHAVIORAL RESPONSES TO DDT AND PYRETHROIDS BETWEEN ANOPHELES MINIMUS SPECIES A AND C, MALARIA VECTORS IN THAILAND

JINNAPA POTIKASIKORN, THEERAPHAP CHAREONVIRIYAPHAP*, MICHAEL J. BANGS, AND ATCHARIYA PRABARIPAI
Department of Entomology, Faculty of Agriculture, Kasetsart University, Bangkok, Thailand; United States Naval Medical Research Center, Silver Spring, Maryland; Division of Computer and Biostatistics, Faculty of Liberal Arts and Sciences, Kasetsart University, Nakhonpathom, Thailand


ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Behavioral responses of two field populations of Anopheles minimus complex species A and C for contact and non-contact actions of chemicals were compared during and after exposure to operational field concentrations of DDT (2 g/m2), deltamethrin (0.02 g/m2), and lambdacyhalothrin (0.03 g/m2) using an excito-repellency escape chamber. The two populations were collected from the Mae Sot District in Tak Province (species A) and the Tri Yok District in Kanchanaburi Province (species C) in western Thailand. Female mosquitoes of both populations rapidly escaped from chambers after direct contact with DDT, deltamethrin, and lambdacyhalothrin. The non-contact repellency response to DDT and the two synthetic pyrethroids was pronounced with An. minimus species A; however, non-contact repellency was relatively weak with An. minimus species C, but remained significantly greater than the paired controls (P < 0.05). We conclude that strong contact irritancy was present in both test populations, whereas non-contact repellency also played a significant role in the escape response of An. minimus species A.


INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Anopheles minimus s.l. Theobald is one of the most efficient malaria vectors throughout the eastern Asia.1,2 In Thailand, the An. minimus complex contains important vectors of malaria that are found exclusively in the forested hilly and clear forested foothill areas.3–6 Anopheles minimus s.l. was reported to be mostly endophilic and endophagic throughout its geographic range.7 After DDT was introduced to interrupt malarial transmission, An. minimus reportedly shifted to greater outdoor feeding and more zoophilic preferences for blood, particularly bovids.8,9 Even though DDT resulted in significant reductions of indoor-feeding mosquitoes, this control method did not completely interrupt transmission of malaria. This has been attributed partly to exophagic behavior of portions of the population and the persistence of a small number of vectors that enter and successfully feed indoors.10,11 Similar observations have also reported from Vietnam,12 raising questions on behavioral variations within the An. minimus taxon.

Based on morphologic and genetic variations, at least two closely related species of the An. minimus complex have been documented in Thailand and both have been incriminated as efficient vectors of malaria.4,6 Anopheles minimus species A is the predominant species and distributed throughout the country,6 whereas species C appears restricted along the western Thailand-Myanmar border, particularly in Kanchanaburi Province.4,13 Additionally, An. minimus species D has been reported in Thailand, but sufficient information is lacking to support the proposed sibling species status.13 Although An. minimus species A and C occur in sympatry in western Thailand, notable ecoethologic variation in feeding and resting behaviors, degree of anthropophily, and other bionomical aspects may influence vector capacities of these two sibling species.12,14

Anopheles minimus species A has shown a much greater (five-fold difference) endophilic behavior compared with species C.12 The An. minimus complex has also shown different response levels of response to intradomicilary use of insecticides.15–19 In Thailand, indoor house spray has been routinely conducted to interrupt human-vector contact and transmission.19 Understanding the behavioral responses of different species of mosquitoes, even closely related sibling species, to insecticides can facilitate vector control by selecting and implementing the most effective interventions possible and help to target the primary disease vectors.

Behavioral responses, namely insecticide avoidance, can be separated into two important and distinct categories: contact irritancy and non-contact repellency. Irritant responses result from physical contact with chemical-treated surfaces, whereas repellency is an avoidance response devoid of making actual contact with insecticides.20 Although behavioral responses have been recorded with various mosquito species and populations of Anopheles from Thailand using the excito-repellency test box,19,21–23 none have been recorded to compare the behavioral responses between species in the An. minimus complex (e.g., species A and C). Described herein are observations using the excito-repellency test system to quantitatively measure behavioral responses between wild-caught populations of An. minimus species A and C exposed to recommended field concentrations of DDT, deltamethrin, and lambdacyhalothrin.24


MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Mosquito collection. Anopheles minimus complex mosquitoes were identified based on morphologic keys.15,25 Species were differentiated by the presence or absence of the humeral pale spot on the costal wing vein. Anopheles minimus A has a wing costa without the humeral pale spot whereas An. minimus C has the humeral pale spot. A diagnostic enzyme, octanal dehydrogenase, indicated 95% concurrence with species A, which does not have the humeral pale spot. This spot is lacking in 73% of species C.6 Anopheles minimus A and C adult females were collected off human volunteer baits during the evening hours (6:00 PM to 6:00 AM). These volunteers (collectors) worked for the Ministry of Public Health. Behavioral tests were performed within 24 hours of capture. All mosquitoes were starved of blood and sugar 24 hours before the tests.22 Temperatures and relative humidity were recorded during the tests. Both populations were physiologically susceptible to DDT, deltamethrin, and lambdacyhalothrin (Chareonviriyaphap T and others, unpublished data).

Insecticide-treated papers. Analytical grade insecticide was impregnated on papers at operational field concentrations of 2 g/m2 of DDT, 0.02 g/m2 of deltamethrin, and 0.03 g/m2 of lambdacyhalothrin and prepared using diluent according to World Health Organization protocol.26

Behavioral tests. Tests were designed to compare two wild-caught populations in contact versus non-contact exposures using three different insecticides. Identical, specially designed test chambers (four per test trial) were used for all bioassays as previously described.27 The stainless steel outer chamber of excito-repellency testing device measures 34 cm x 32 cm x 32 cm (Figure 1Go), and faces the front panel with the single escape portal. The box is composed of a rear door cover, an inner Plexiglas glass panel with a rubber latex-sealed door, a Plexiglas holding frame, a screened inner chamber, an outer chamber, a front door, and an exit portal slot. Only female An. minimus specimens were used in excito-repellency tests. Mosquitoes were deprived of all nutrition and water for a minimum of 24 hours before exposure. Laboratory tests were performed during daylight hours only and each test was replicated four times. Observations were taken at one-minute intervals for 30 minutes. After each test was completed, the number of dead or knockdown specimens was recorded separately for each exposure chamber, external holding cage, and paired control chamber (without insecticide). Escaped specimens and those remaining inside the chamber, for both controls and treatments, were held separately in small holding containers with food and water and 24-hour mortalities were recorded.



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  		    FIGURE 1. Excito-repellency test chamber used to study insecticide behavioral responses.

 
Data analysis. A Kaplan-Meier survival analysis method was used to analyze and interpret the behavioral response data.20–22,27 Survival analysis was used to estimate the probability of escape time (ET) and compare differences in mosquito response among the two populations and three insecticides. Mosquitoes that escape were treated as deaths and those remaining in the test chamber were considered survivals.21 The ET50, ET75 and ET90 time in minutes for 50%, 75%, and 90% of the test population to escape, respectively, were estimated from data collected at one-minute intervals. Patterns of escape response were determined using the log-rank method.28 Stata statistical software (Stata Corp., College Station, TX) was used in the analysis.20


RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Two types of behavioral responses, contact irritancy and non-contact repellency, were observed with exposure to three insecticides and percent mortalities of escape and non-escape mosquitoes from control and treated chambers were recorded (Table 1Go). Patterns and rate of escape were stronger in An. minimus species A than An. minimus species C for all three compounds. In contact trials, percent escape of An. minimus A (92–96%) was significantly (P < 0.05) higher than for An. minimus C (50–90%), regardless of compound used. Similarly, percent escape by species A was also greater than that by species C for the two synthetic pyrethroids. In general, a relatively low number of mosquitoes escaped from the control chambers (12–23% for contact and 10–15% for non-contact). Mortality rates of escaped mosquitoes from both test populations were low (0–13.3%), whereas those that remained in the test chamber (non-escape mosquitoes) showed much higher mortality rates (43–100%). All non-escape specimens of species A exposed to deltamethrin and lambdacyhalothrin perished within 24 hours post-exposure (Table 1Go). High mortality rates (13.3%) of escaped mosquitoes from control chambers were observed with DDT. In non-contact trials, An. minimus species A demonstrated significantly strong escape responses to all three compounds compared with species C. After 30 minutes exposure, percent escape was approximately 96% for DDT, 92% for deltamethrin, and 87% for lambdacyhalothrin with An. minimus species A, while only 24% for DDT and deltamethrin and 18% for lambdacyhalothrin with species C. Percent mortalities of escaped specimens of both populations were very low, ranging from 1.1% to 4.5%. Mortality was not seen in non-escaped An. minimus species A after the 24-hour holding period.


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TABLE 1
Percentage escape response and mortality of Anopheles minimus A and C exposed to DDT, deltamethrin, and lambdacyhalothrin in contact and noncontact trials*
 
The escape patterns generated from insecticide-treated chambers are expressed in one-minute intervals for 50%, 75%, and 90% (ET50, ET75, and ET90) of the test population to escape from exposure chambers (Table 2Go). In contact trials, the ET50, ET75, and ET90 for An. minimus species A were 5, 12.5, and 24 minutes with DDT, 2.5, 6, and 16 minutes with deltamethrin, and 7, 23.5, and 30 minutes with lambdacyhalothrin, respectively. The ET50 for An. minimus C was 5, 8.5, and 6 minutes for DDT (0.02 g/m2). The ET75 and ET90 values for all three compounds for An. minimus species C could not be calculated (with one exception: lambdacyhalothrin ET75 = 12.5 minutes) because of insufficient numbers of mosquitoes escaping. Similarly, ET values for An. minimus species C in all non-contact trials could not be estimated. For non-contact trials, the ET50, ET75, and ET90 values were 5, 4.5, and 14 minutes for DDT and 5.6, 8, and 25 minutes for deltamethrin. The ET50 and ET75 estimates were 6.5 and 23.5 minutes, respectively, for lambdacyhalothrin.


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TABLE 2
Escape time (ET) in minutes for 50%, 75%, and 90% of Anopheles minimus A and C to escape from insecticide-treated chambers*
 
Comparison of escape responses between An. minimus species A and C in contact and non-contact trials using log rank analysis showed statistically significant differences in escape patterns between species in non-contact trials for all three compounds (P < 0.05). In contact trials, significant differences in escape responses between species were observed with DDT and deltamethrin (P < 0.05). Comparisons of escape responses between paired contact and control, contact and non-contact, and non-contact and control bioassays for each species for each compound were made. No significant differences between contact and non-contact escape for An. minimus species A were observed (P > 0.05). Escape probabilities in contact and non-contact trials were significantly higher than those in paired controls for all bioassays.

Figures 2Go–5GoGoGo show the proportions of mosquitoes remaining in the exposure and control chambers under different test conditions and chemical exposure. Strong repellency action was seen with An. minimus species A against all three compounds, whereas significantly less escape reaction was observed with An. minimus species C (Figure 5Go). In non-contact tests, An. minimus species A demonstrated a stronger escape rate with DDT than with either deltamethrin or lambdacyhalothrin (Figure 5Go). There were significant differences in escape responses seen in all contact trials compared with paired control and non-contact trials with An. minimus species C (P < 0.05). Escape patterns in all non-contact repellency trials for An. minimus species A were significantly greater than paired controls.



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  		    FIGURE 2. Escape probability of Anopheles minimus species A and C exposed to DDT and paired control chambers for contact and non-contact trials.

 


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  		    FIGURE 3. Escape probability of Anopheles minimus species A and C exposed to deltamethrin and paired control chambers for contact and non-contact trials.

 


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  		    FIGURE 4. Escape probability of Anopheles minimus species A and C exposed to lambdacyhalothrin and paired control chambers for contact and non-contact trials.

 


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  		    FIGURE 5. Escape probability of Anopheles minimus species A and C exposed to DDT, deltamenthrin (DEL), and lambdacyhalothrin (LAM) in non-contact trials.

 

DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In Thailand, An. minimus complex is comprised of at least two known species, species A and C, and both are important vectors of malaria in Thailand.3,4,6,15 After DDT was introduced for malaria control in 1949, An. minimus reportedly became predominately an outdoor-feeding species,5 although it appears that feeding behavior varies depending upon geographic distribution. Thus, insecticides may have little to do with any purported genetic selection or shift from an indoor to outdoor-feeding behavior.10 The failure to completely interrupt malarial transmission by An. minimus s.l. might be related to the behavioral diversity and innate response to insecticidal intervention.5,29

Studies have attempted to quantitatively describe and resolve the ecoethologic differences,9,10,12,14,30 genetic composition and diversity,4,6,12,30–33 and responses to intradomicilary use of DDT30 in this species complex. Experiments using recently colonized An. minimus species A exposed to deltamethrin clearly demonstrated the two primary avoidance responses: irritancy and repellency (excito-repellency).19 In our present study, we compared both behavioral responses in the two sibling species of An. minimus present in Thailand to three different residual insecticides used in public health with hopes that such information will facilitate targeting of specific malaria vectors and increase the effectiveness of vector control activities.

We observed unambiguous behavioral avoidance responses in An. minimus species A and C using an excito-repellency test system.27 All three insecticides produced rapid and striking irritancy in both sibling species. Moreover, very strong repellency responses to each compound were observed in An. minimus species A. Repellency reactions were similar to those of a recent laboratory colony of An. minimus species A from northern Thailand, which showed > 75% repellency to deltamethrin.19 Repellency responses were relatively weak in An. minimus species C, yet still significantly greater than the paired controls for all cases. Similarly, weak repellency of An. minimus species C from Pu Teuy Village (approximately 95% were confirmed as species C) to the three compounds was previously observed.19 Anopheles minimus complex from Pu Tuey village in Kanchanaburi Province was exposed to operationally standard concentrations of DDT (2 g/m2) and established medium lethal doses (LD50) of deltamethrin and lambdacyhalothrin that produced poor repellency activity.19 The relative inability to detect chemical signals or odors without physical contact with insecticide in An. minimus species C may be driven by evolutionary processes different from those in species A. Since 1990, Pu Teuy village has been considered a low-risk area for malaria, which has resulted in routine residual chemicals being applied more sparingly compared with more malaria-prone areas of the country such as Mae Sot District (Department of Communicable Disease Control, 2004, unpublished data). The differences in proportion of total houses sprayed with insecticides (i.e., insecticide exposure pressure) could be a factor affecting the avoidance behavior of these two closely related species.

One of the key components in preventing malaria transmission has relied mainly on methods that interrupt human-vector contact.34–36 Insecticides that have strong irritant and repellency attributes on vectors can perform this function without necessarily having to kill the mosquito to interrupt transmission. Repellency to insecticides in vectors has been recognized in several Anopheles mosquitoes.17,18,21,22,23,36–38 Compared with contact irritancy, this type of avoidance behavior could mitigate even more against selection of insecticide resistance in mosquito populations.

Anopheles minimus species A in Thailand, has been subjected to routine intradomicilary DDT spraying to interrupt malaria transmission for decades. DDT was applied either once or twice a year, especially in malaria-endemic areas of western Thailand. Although DDT was used for many years, no evidence of physiologic resistance has been detected in the An. minimus complex. We believe that innate behavioral avoidance of insecticide-sprayed surfaces by mosquitoes has, and continues to play, a significant role in delaying or preventing resistance from developing. Our findings confirm that strong behavioral avoidance of chemical residues is due to excito-repellent properties of these compounds and most likely contributes to interruption of feeding by mosquitoes and transmission of malaria.

Our findings indicate differences in behavioral responses between two species of the An. minimus complex in Thailand. We believe that these important observations can help explain some of the varying effectiveness of indoor residual spraying in various regions in Thailand. It is the understanding of behavioral avoidance and an appreciation for excito-repellency that indicate an important set of properties of residual insecticides and how they function to control disease transmission apart from contact toxicity alone.

Received January 12, 2005. Accepted for publication February 26, 2005.

Financial support: This work was supported by the Thailand Research Fund and the Kasetsart University Research and Development Institute, Thailand.

* Address correspondence to Dr. Theeraphap Chareonviriyaphap, Department of Entomology, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand. E-mail: faasthc{at}ku.ac.th Back

Authors’ addresses: Jinnapa Pothikasikorn and Theeraphap Chare-onviriyaphap, Department of Entomology, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand. Michael J. Bangs, Naval Medical Research Center, 503 Robert Grant Avenue, Room 3A40, Silver Spring, MD 20910-7500. Atchariya Parbaripai, Division of Computer and Biostatistics, Faculty of Liberal Arts and Science, Kasetsart University, Nakhonpathom 73140 Thailand.

Reprint requests: Theeraphap Chareonviriyaphap, Department of Entomology, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand, Telephone: 66-2-942-7131, Fax: 66-2-942-7130, E-mail: faasthc{at}ku.ac.th.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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