Fumento out of his depth on DDT

More Fumento? Have you been reading the web site of the Lake Wylie Pilot? Is this the same Fumento who promised:

"Now I am going to do the worst possible thing you can do to somebody who measures his life by "hits." I'm not going to write to you again,"

I guess that not writing to you does not cover writing about you.

Actually, not writing TO somebody is indeed not the same as not writing ABOUT them. Last I heard those words were not synonymous. As to Lambert's assertion that those were INDIAN mosquitoes and not SRI LANKAN ones, the expression "any port in a storm does come to mind." What? DDT checks their passports? It's the same type of mosquito; that's all that counts. From what I've seen, any time Lambert strays from John Lott he misses the target. As his blog continues to fade into the sunset he needs to learn the meaning of humility and become a single-issue blogger.

Tim: >since there are some DDT-resistance genes still on the population, the whole population would quickly become resistant once DDT is sprayed.

To play Devil's Advocate - yes, but that wouldn't stop it from preventing an immediate epidemic in the aftermath of the Tsunami - assuming of course you had a few hundred thousand tons of DDT and an army of trained spraying teams fully equipped and standing by.

A more general comment: WAS there an epidemic of malaria after the Tsunami? If not, Fumento's claims that indiscriminate DDT spraying was desperately needed rather fails under its own weight.

> Tracy: It's the same type of mosquito...

Except that Tim notes that the different populations have different levels of DDT resistance.

Tracy:

I just found it somewhat ironic that Fumento seemed to feel that hits were important enough to Tim that he would reduce them by never writing to him again and then he goes and publishes a column devoted to - yes you guessed it - Tim. If I had read that column the first thing I would do would be to say who is this Lambert guy and search him out.

Tim - have you noticed a huge increase in hits since the column came out. And if not does that mean that no one reads Fumento? Just ask'n.

Actually, Fumento [did write to me](http://scienceblogs.com/deltoid/2005/01/fumento9.php) after saying that he wouldn't write to me.

Fumento deliberately did not link to my post, presumably because he did not want to give me any traffic, so I can't count referrals and get an idea of his traffic. But I can use Blogpulse to [compare links to his blog and links to mine](http://www.blogpulse.com/trend?query1=http%3A%2F%2Ftimlambert.org&label…). Interest in his blog seems to be declining and is in any event much less than mine.

I think Ian made a good point. Have the areas affected by the tsunami shown a dramatic increase in the incidence of malaria?
If they haven't, the rest of Fumento's argument is redundant. Or is it still too early to tell?

I just reformatted my entire hard disc and am yet to download Adobe Acrobat so I can't currently read PDF documents.

This limits my searching a bit but this document from the WHO website reports on progress six months after the Tsunami.

http://www.who.int/hac/crises/international/asia_tsunami/6months/6month…

>Whenever and wherever a suspected increase in disease was noted, public health professionals from government, NGOs, UN and other agencies were rapidly deployed to the affected locations stem the outbreak. Outbreaks of measles, dysentery and hepatitis were thus detected and prevented from spreading further at a very early stage.

Malaria doesn't even rate a mention.

A google search for "Tsunami, Malaria, Sri Lanka" turns up a total of eight documents now of which refer to any major increase in the incidence of malaria.

This article is also of interest:

http://www.malariajournal.com/content/4/1/8

>Conclusion

>Although relocated people may be more exposed to mosquito bites, and their capacity to handle diseases affected, the environmental changes caused by the tsunami are unlikely to enhance breeding of the principal vector, and, given the present low parasite reservoir, the likelihood of a malaria outbreak is low.

As there is ongoing scientific debate debate and different aspects to DDT use in India still, it is import to keep the recent publications in mind.
One of this has this to say:

Tropical Medicine & International Health
Volume 10 Issue 2 Page 160 - February 2005
doi:10.1111/j.1365-3156.2004.01369.x

DDT indoor residual spray, still an effective tool to control Anopheles fluviatilis-transmitted Plasmodium falciparum malaria in India
K. Gunasekaran, S. S. Sahu, P. Jambulingam and P. K. Das
Summary

This study from two districts of Orissa State which are endemic for Plasmodium falciparum transmitted by Anopheles fluviatilis and A. culicifacies investigated the impact of dichlorodiphenyl trichloroethane (DDT) indoor residual spraying, in view of the ongoing discussion on phasing out DDT in India. Based on their high annual parasite incidence and logistical considerations, 26 villages in Malkangiri and 28 in Koraput district were selected for DDT spraying. For comparison, six and four unsprayed villages were chosen from the same districts. In each district, the prevalence of malaria infection and incidence of malaria fever, indoor resting density and parous rate of the vectors, and their susceptibility to DDT were monitored in six and three villages selected randomly from the sprayed and unsprayed groups respectively. Anopheles fluviatilis was susceptible to DDT while A. culicifacies was resistant. DDT residual spraying with 1 g/m2, was carried out in October-November 2001. Spraying 74-86% of human dwellings and 100% of cattle sheds brought down the indoor resting density of A. fluviatilis by 93-95%. This was associated with a significant reduction of incidence of malaria fever as well as prevalence of malaria infection from November to February in both districts. The spraying also seemed to impact on vector longevity, and a residual effect of DDT on the sprayed walls was observed up to 10-12 weeks despite re-plastering. Hence DDT spraying can still be an effective tool for controlling fluviatilis-transmitted malaria. Although this species is exophilic, its nocturnal resting behaviour facilitates its contact with the sprayed surfaces. DDT is still useful for residual spraying in India, particularly in areas where the vectors are endophilic and not resistant.

Introduction
In India, dichlorodiphenyl trichloroethane (DDT) and benzene hexachloride (HCH) were introduced into the public health programme for residual spraying in the early 1950s and malathion was introduced in 1960s. While HCH was banned from public health use in 1997, DDT and malathion are still being used. Although resistance to these insecticides in malaria vectors, particularly Anopheles culicifacies, has been one of the major technical constraints, their use has reduced and maintained malaria incidence at a level of 'only' 2-3 million cases a year (Sharma et al. 1996). In response to resistance development to DDT, synthetic pyrethroids were introduced during the last decade for indoor residual spraying (IRS), as well as for treating bed nets. However A. culicifacies is developing resistance in India to synthetic pyrethroids (Singh et al. 2002).

Primarily because of environmental concerns, a global treaty (the Stockholm Convention on Persistent Organic Pollutants, POPs) was signed which mentions the long-term aim of phasing out of DDT from public health use but which contains an amendment specifically allowing use of DDT for vector control provided that WHO guidelines are followed. There are only a few alternative insecticides to DDT. One of them, malathion, is not welcomed by communities because of its bad odour. Malathion and synthetic pyrethroids are approximately 2.5-fold more expensive than DDT per house treated per year (Raghavendra & Subbarao 2002). Therefore it is prudent to continue to use this insecticide judiciously wherever and as long as it is effective. The Government of India has decided to review the policy related to continuation of DDT spraying for the control of malaria and kala-azar based on results of field evaluations. This manuscript presents the results of a field evaluation by the Vector Control Research Centre (VCRC) from August 2001 to March 2002 to study the effectiveness of DDT IRS on incidence and prevalence of malaria in two districts of Orissa State, Malkangiri and Koraput, endemic for Plasmodium falciparum transmitted by A. fluviatilis and A. culicifacies (Parida et al. 1991).

SNIP

Coverage of DDT spray

Spray coverage of households and rooms in different villages of Malkangiri District ranged from 44.6% to 100% (overall: 74.2%) and 30.6% to 84.6% (overall: 48.6%) respectively. The corresponding values for Koraput District were 68.6-100% (overall: 86.2%) and 36.2-92% (overall: 56.6%). Coverage of cattle sheds was 100%. Some householders refused to allow spraying of their houses mainly because of a traditional custom of not allowing outsiders to enter their prayer rooms. Villagers do not object to spraying of their cattle sheds. Wall decolourization, bad smell, increase in bed bug nuisance, contamination of food grains stored above the false ceiling and social caste feelings against the spray-men were also some of the reasons attributed for the refusal.

Prevalence of malaria infection

Plasmodium falciparum was the predominant species, constituting 97.6% (n = 414) of the total malaria positives recorded in Malkangiri District and 99.2% (n = 619) in Koraput District. After spraying, there was 69% and 66.7% reduction of SPR, respectively, in the sprayed villages of Malkangiri and Koraput District (Table 1). The prevalence of malaria infection in the unsprayed villages increased significantly during the post-spray survey, conducted at the end of the transmission season, compared with the prevalence recorded prior to DDT spraying in both Malkangiri (OR = 2.85, CL = 2.04-4.46) and Koraput (OR = 1.45, CL = 1.04-2.04) Districts (Table 2). The increase was because of the accumulation of cases over the transmission period. In the sprayed villages, the reduction of malaria infection during the post-spray period was significant in both the districts (OR = 0.50, CL = 0.4-0.6; OR = 0.34, CL = 0.28-0.40 respectively) in comparison with the prevalence prior to DDT spraying. Further, comparison of 95% CL for the odds ratios indicated that the reduction of malaria infection in the sprayed villages was significant when compared with the unsprayed villages (95% CL do not overlap) in the two districts. It was clear from the comparison that accumulation of cases over the transmission period did not take place in the sprayed villages.

Incidence of malaria fever

In Malkangiri, incidence of malaria fever in the control villages increased in November and peaked during December and January. But, in the DDT sprayed villages, the MPI was reduced by from 84.1% to 91.7% over 4 months (mean: 85.6%) (Figure 1a). The reduction of MPI in the sprayed villages of Koraput District ranged from 62.9% to 83% (mean: 77%) (Figure 1b). The MH summary chi2 tests (Table 3) indicated that during the post-spray period there was a significant increase of malaria fever incidence in the unsprayed villages of Malkangiri (OR = 4.71, CL = 2.47-9.21) and Koraput (OR = 3.39, CL = 2.09-5.48) Districts. In the sprayed villages, after DDT spraying, there was an apparent reduction of the malaria fever incidence when compared with the incidence recorded before spraying, but the reduction was not significant (OR = 0.64, CL = 0.39-1.07 and OR = 0.69, CL = 0.42-1.12 respectively in the two districts). Comparison of the 95% CL for the odds ratios revealed that the reduction of malaria incidence observed in the sprayed villages was significant when compared with the unsprayed villages in the two districts (95% CL do not overlap).

Indoor resting collections and parous rate

Indoor resting mosquito collections (from both test and control villages prior to and after the spray) yielded a total of 3022 anophelines of 16 species in Malkangiri and 3059 Anopheles mosquitoes of 11 species in Koraput District. Anopheles culicifacies and A. fluviatilis constituted 27.7% and 26.7%, respectively, of the anophelines collected in the former district. In the latter district, A. fluviatilis was the predominant species (76.5%) followed by A. culicifacies (9.8%). In the two study sites, all 107 specimens of A. fluviatilis subjected to cytotaxonomic analysis were identified as species 'S'.

After spraying, the indoor resting collections of A. fluviatilis from the sprayed rooms were reduced by 93-97% for 3 months. In February, the reduction was 70.5%. Subsequently, there was a natural decline in the density of A. fluviatilis as was evident from a corresponding decline in the unsprayed villages (Figure 2a). In Koraput District, DDT spraying brought down the indoor resting collections by 99.7% in the first month and by 90.8% in the second month post-spray (Figure 2b). During the subsequent months, the reduction was 70.8-80.3%. Resting collections of A. fluviatilis in cattle sheds were reduced by 100% after the spray (Figure 3a) in Malkangiri District. In Koraput District, the impact was not discernable, as there was a natural decline in both sprayed and unsprayed villages (Figure 3b). The impact of the spray on A. culicifacies could not be discerned because of very low density during the study period in test and control villages in both the districts.

SNIP

Discussion

The DDT was successfully used in many Asian, African, Latin American and European countries between the 1940s and 1970s. In the highlands of Madagascar, where there was malaria resurgence in the 1980s following withdrawal of antimalaria activities, reintroduction of DDT spraying for a period of 5 years from 1993 to 1998 brought A. funestus-transmitted malaria back under control (Romi et al. 2002). In South Africa, spraying of DDT was highly successful from 1945 to 1995 in controlling the vectors of malaria, A. funestus and A. arabiensis, and there was no sign of DDT resistance in the vectors over this 50-year period. In 1995 the South African Government switched from DDT to pyrethroid spraying but over a period of 4 years this failed to control malaria and A. funestus reappeared, and was captured exiting alive from pyrethroid sprayed houses (Hargreaves et al. 2000). Moreover these mosquitoes were found resistant to pyrethroids but not to DDT. As a result the government, although it had the financial resources to continue with pyrethroid spraying, switched back to DDT spraying. This effected a 60% reduction in cases in 2001 and the problem of escape of A. funestus from sprayed houses also disappeared. In Swaziland, in southern Africa, no change from DDT to pyrethroids was ever made and malaria control remained very successful (Curtis 2002).

The present study assumes importance in the context of ongoing discussion on phasing out DDT. Introduction of DDT as a residual insecticide into the mosquito control programme had a major impact on malaria in India (Singh 1953; Rao 1958; Sharma 1984). This spectacular success prompted India to change the 'control programme' to an 'eradication programme', continuing DDT spray as the principal intervention method for the interruption of transmission. The programme continued to yield dramatic reduction (about 99.87%) of malaria incidence up to the late 1960s. In 1966 the programme experienced its first setback mainly due to the short supply of DDT (Rao 1958). While in the 1960s about 18 000 tonnes of DDT were sprayed annually, the momentum and funding for the eradication programme declined subsequently. In the 1990s, only 7500 tonnes of DDT were sprayed annually against malaria and leishmaniasis. This decline in spraying resulted in resurgence of malaria in some parts of the country (Sharma & Mehrotra 1986). In addition, other factors such as delayed supply of DDT, application of incorrect dosages and inadequate coverage aggravated the situation. Recurrent focal outbreaks and development of resistance in A. culicifacies, one of the major malaria vectors in India, also contributed to the declining effectiveness of DDT. While many factors were obviously responsible to such declining effectiveness, the problem of vector resistance and its association to the failure of DDT spray in India was emphasized by Sharma (2003). However, in fact, resistance was a problem only in areas where A. culicifacies is the principal vector. Furthermore, there is evidence that even where a standard WHO test only showed 11.5% mortality of A. culicifacies, spraying still had some effect on malaria (Sharma et al. 1982). Although this species was reported to be responsible for about 60% of the total transmission in the country (Raghavendra & Subbarao 2002), currently it would not be appropriate to decide which insecticide to use based on resistance development in A. culicifacies alone, as this species, in most of the areas of its prevalence, maintains only unstable malaria with epidemic potential only. Therefore, any such decision should simultaneously consider the areas experiencing stable malaria with persistent falciparum transmission by more efficient vectors such as A. fluviatilis, which is still susceptible to DDT.

The Malariogenic Stratification Committee constituted by Government of India in 1986 divided the country into seven strata. Three were refractory (not responding to control measures) with high receptivity and the remaining four were non-refractory with only epidemic potential of varying degrees (Sharma et al. 1996). If one categorizes the three refractory strata in terms of vector distribution, the first one has both A. culicifacies and A. fluviatilis as vectors in plain and hilly areas respectively, the second stratum comprises the north-eastern states of India where A. dirus, A. minimus, A. fluviatilis and A. philippinensis are the vectors and the third stratum consists of east central India. The terrain of this stratum includes mainly Jeypore and Singhbhum hill ranges. Anopheles fluviatilis is the major malaria vector in this area (Parida et al. 1991). In total, if the three refractory strata are considered, there are five vectors and among the five, apart from A. culicifacies, the four other species, viz. A. fluviatilis, A. minimus, A. philippinensis and A. dirus, are susceptible to DDT (Saha et al. 1999). Therefore, it is not appropriate to relate the ineffectiveness of DDT spraying in the country only to the development of resistance in A. culicifacies. It might be expected that DDT spraying would have been effective in areas where a susceptible species such as A. fluviatilis is the vector. But in fact east central India, including Orissa, where A. fluviatilis is the principal vector and susceptible to DDT (Sahu et al. 1990), has been declared as refractory to control measures. We conclude that the refractoriness is because of poor implementation of the spraying and general apathy among the people towards an endless programme of residual spraying.

Our study in the same region showed that DDT indoor residual spray with 74-86% household coverage brought down the abundance of A. fluviatilis resting indoors by 93-95% (by a joint effect of mosquito killing and induced exophily). This was associated with a significant reduction of incidence of malaria fever as well as prevalence of malaria infection from November to February during which period transmission of malaria would have otherwise peaked. Further, a residual effect of DDT on the sprayed walls was observed up to 10-12 weeks despite re-plastering. It is, therefore, clear that DDT spray could still be an effective tool to control fluviatilis-transmitted malaria in these areas if properly applied. Even when this species exhibits exophily (Das et al. 1990) its nocturnal resting behaviour facilitates its contact with the sprayed surfaces (Gunasekaran et al. 1994).

The major environmental concern of using DDT is the presence of DDT and/or DDE residues in soil, water and human blood. It is very likely that these residues arose from illegal diversion of DDT intended for indoor spraying to agricultural use (Curtis 2002). As emphasized above, the Stockholm Convention on POPs signed in 2001 and ratified recently has approved continued use of DDT only for vector control but not for agriculture. It is therefore necessary in India to institute strict regulation to prevent illegal diversion of DDT to agricultural use. There have been many other claims recorded about toxicity of DDT to humans but most of them neither showed convincing evidence (Sharma et al. 1996) nor withstood careful investigation (Curtis 2002). Therefore, DDT is still usable and should continue to be the insecticide of choice for residual spray in India for malaria control, particularly in areas where the vectors are endophilic and not resistant.

Tim,
Although you are citing the paper I quote again in entry 10 you seem not to have read it. It shows that even in a country like India where DDT resistance is an issue, DDT can definitely retain usefulness. The world is not so simple that mere presence of resistance makes DDT useless over whole regions.
It also gives a stunning summary of how South African events contradict your view.

So in South America, South Africa, and at least parts of India in 2004-5, DDT retains its value for managing malaria by IRS house spraying.

Also the WHO documents you posted acknowledge the WHO are under great pressure to discontinue DDT by NGOs, and my and other people's case that opposition to DDT by environmentalists is part of the story of re-emergence of malaria still holds good.

This is all a well documented part of the malaria story that you yourself keep on glossing over, so I'd go carefully accusing others of being out of their depth.

Do mosquitoes also develop resistance or immunity to insecticides besides DDT, e.g., deltamethrin, cyfluthrin, HCH, malathion?

If so, would an optimal strategy be to rotate among the various various pesticides?

Terry,

Because DDT is unusually chemically stable it takes longer to break down in the ecosystem than most other insecticides.

This explains its tendency to bioaccumulate in animal tissues (including in humans) and also means that it can affect a wide range of isnects and other arthropods (such as crustaceans)it wasn't directed at.

It also means that it is more likely that mosquitoes will be exposed to sublethal doses from DDT residue. This makes it easier for DDT-resistant strains to develop than for resistance to other insecticides.

You are correct that it makes sense to rotate insecticides. It also makes sense to restrict DDT use to where you get the most bang for your buck to slow the emergence of resistant strains.

One promising area of research in pest control is developing chemicals which disrupt the resistance mechanism and which therefore restore the effectiveness of chemicals to which resistance has developed.

"If they haven't, the rest of Fumento's argument is redundant. Or is it still too early to tell?"
You're missing the point. Environmentalists are bad and kill people. Whether malaria is resurgent in Sri Lanka is irrelevant.

"Tim, Although you are citing the paper I quote again in entry 10 you seem not to have read it. It shows that even in a country like India where DDT resistance is an issue, DDT can definitely retain usefulness. The world is not so simple that mere presence of resistance makes DDT useless over whole regions. It also gives a stunning summary of how South African events contradict your view."

You mean they prove that there is a ban on use of DDT for malaria prevention? I'm afraid I can't follow your reasoning.

"Do mosquitoes also develop resistance or immunity to insecticides besides DDT, e.g., deltamethrin, cyfluthrin, HCH, malathion?"

Yes.

"If so, would an optimal strategy be to rotate among the various various pesticides?"

It's in some ways better, particularly for medium term eradication, but of course you create subpopulations of resistant organisms to each which don't disappear, and they begin to overlap and you eventually end up with mosquitoes resistant to all of the above and then you are well and truly screwed. You can also select mutations for multiple insecticide resistance, including insecticides not discovered yet. The other approach is to use something until it is quite useless, then move on to the next; in the hopes that by the time you run out of options, something new will have come along. But leaning on insecticide in general for quasi-permanent insect control is a losing strategy, in the end.

z
Why not go and read the paper. It contradicts Tim in several ways. If you cant understand the paper, say where you don't. It covers several issues about which there are many entries on this site,

"Why not go and read the paper. It contradicts Tim in several ways. If you cant understand the paper, say where you don't."

Well, I don't understand how their spraying DDT in several provinces proves that there is a global ban on DDT spraying, which is killing people. I also don't understand why using a pesticide which kills only one species of malaria carrying mosquito "while A. culicifacies was resistant" is a better choice than one which kills both species.

Oh this is comedy gold:

As to Lambert's assertion that those were INDIAN mosquitoes and not SRI LANKAN ones, the expression "any port in a storm does come to mind." What? DDT checks their passports?

Well, in one sense it is true that there are no passport check for mosquitoes. However, very few malarial mosquitoes are capable of flying for the 50km it would take to cross the Palk Strait. Tracy may perhaps be confusing them with Boeing 767s.

z,

Well since they are propositions you've proposed, and not ones I've made, maybe they are true. Why you choose them in the light of what the paper has to offer I don't know.

On a another note, in an earlier thread I promised to search for left-wing nuttiness to give balance to over the right wing blather, and here's one outcome of it:

Group threatens Kari with court action over GMOs
October 24, 2005
Kenya Times
Maxwell Masava
Kenya Agricultural Research Institute (KARI) has, according to this story, been threatened with a court action if it fails to divulge information regarding genetically modified organisms.
The story says that yesterday, Africa Nature Stream — a non governmental organisation opposed to the introduction of genetically modified organisms in the country — gave KARI seven days to respond to questions regarding the GMOs and their impact on human life.
Speaking in Nairobi, the organization's chairman Masoa Muindi accused the government and Kari in particular of trying to introduce GMOs on behalf unnamed foreign companies without considering its impact on the environment and human health.
He accused Kenyan scientists of being money-minded and said the introduction of GM seeds in the country would destroy the economy. GM crops were likely to multiply and introduce diseases, besides degrading the environment, he said.
In a letter to KARI, which is also copied to the Office of the President, ministries, foreign missions and other government departments, the organization claimed that the new technology was a plot by Western countries to destroy African economies.

d, you are citing a paper about DDT in India which you allege contradict my views about South Africa. And yet you refuse to read my [post about South Africa](http://scienceblogs.com/deltoid/2005/09/ddt-use-in-south-africa.php) to find out what my views actually are and the research I cite there which was actually about South Africa.

Well Tim, since I've already posted a comment on this weblog acknowledging your South African posts this last remark of your is not true.

More to the point where did post your response to my numerous questions about South America?

"Mosquitoes "are almost certainly not going to become immune to DD T's most valuable attribute: its repellency," writes DDT expert Paul Driessen. Even in tiny quantities "DDT keeps up to 90% of the mosquitoes from even entering a home. It irritates those that do come in, so they don't bite; and it kills any that land on the walls, before they can infect another person."

Never did understand that assertion. Why, a priori, would irritancy not be subject to the same laws of genetic variability and natural selection, if it interferes with the mosquitoes' feeding? People vary in their sensitivity to irritation by poison ivy, for instance, with some people being completely resistant.

And, if the irritancy of tiny quantities of DDT keeps 90% of the mosquitoes from entering the home, how in the world do they get exposed to it and die?

>Why, a priori, would irritancy not be subject to the same laws of genetic variability and natural selection, if it interferes with the mosquitoes' feeding?

Presumably the selective pressure is much less because there are non-human food sources (not to mention people out of doors).

Resistance isn't cost-free. I don't know details for mosquitoes but i know that penicillin-resistant bacteria devote a significant percentage of their total energy consumption to manufacturing the penicillinase enzyme.

On a similar note, there's currently research under way on a virus which infects mosquitoes and kills them after 8-10 days. That's generally long enough for the mosquitoes to breed but not long enough for the malaria parasite to reach maturity.

So it should stop most cases of infection without creating selective pressure on the mosquitoes.

There was a recent study that found that DDT resistance can [be cost free](http://www.bath.ac.uk/news/articles/releases/ddtresistance080805.html). Mosquitoes can and do evolve resistance to the repellent effects of DDT by changing their behaviour. Instead of resting on the walls inside homes, they fly in, feed, and fly out again.

"The effectiveness of DDT was compromised by the insecticide's irritant effect, which led to a high proportion of bloodfed mosquitoes leaving huts and not resting indoors (Sharp BL, LeSueur D, Bekker P (1990) Effect of DDT on survival and blood feeding success of Anopheles arabiensis in northern Kwazulu, Republic of South Africa. Journal of the American Mosquito Control Association 6,197202)."
Goodman, C. A. , Mnzava, A. E. P. , Dlamini, S. S. , Sharp, B. L. , Mthembu, D. J. & Gumede, J. K. Comparison of the cost and cost-effectiveness of insecticide-treated bednets and residual house-spraying in KwaZulu-Natal, South Africa. Tropical Medicine & International Health 6 (4), 280-295. doi: 10.1046/j.1365-3156.2001.00700.x
http://www.blackwell-synergy.com/doi/full/10.1046/j.1365-3156.2001.0070…

Tim, if the bugs have to be in the air a longer time, that is a cost to their metabolic system. With the rising price of energy, it could bust their budget.