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What Can You Do to Prevent Rats, Mice and Mosquitoes from Invading Your Home?


What can you do to prevent rats and mice from invading your cupboard? How can you make your yard mosquito-proof? We speak to Chris Conlan, a vector ecologist for the County of San Diego, about the most common vector-borne diseases in our area and which animals carry them.

This is a rush transcript created by a contractor for KPBS to improve accessibility for the deaf and hard-of-hearing. Please refer to the media file as the formal record of this interview. Opinions expressed by guests during interviews reflect the guest’s individual views and do not necessarily represent those of KPBS staff, members or its sponsors.

MAUREEN CAVANAUGH (Host): I'm Maureen Cavanaugh. You're listening to These Days on KPBS. Well, summertime has finally arrived in San Diego. With the hot weather comes flip-flops, lazy afternoons and bugs. As you fight the long line of ants that have found their way into your kitchen, it may be comforting to know that pest control is not just an individual household concern. Somebody has got to make sure San Diego County is not overrun with flies and mice and rats and mosquitoes. That's the job of County Vector Control. Now, a vector is a carrier that transmits disease from one person to another and a lot of insects and rodents fit that bill. We thought it would be a good time to check in on County efforts to control the mosquito population and other dangerous pests and to take your calls about pest control at your house. My guest is Chris Conlan, vector ecologist with the County of San Diego. Chris, welcome to These Days.

CHRIS CONLAN (Vector Ecologist, County of San Diego): Yeah, thanks for having me.

CAVANAUGH: And I want to let you know, our audience, if you'd like to join the conversation, we invite you to do so. If you have questions or concerns about a pest control issue, give us a call. That's 1-888-895-5727, 1-888-895-KPBS. So, Chris, I want to start out with you by asking you, what exactly does County Vector Control do? What is your primary mission?

CONLAN: Well, we do a number of things but the lion's share of our resources are spent trying to control the mosquito populations around town. In addition to that, we also offer services to help the citizens of the county deal with rodent problems. We don't actually do the rodent control but we try to arm the citizens with the knowledge on how to do it themselves and we'll even get them started with a little kind of a rat control starter kit so that they can make that decision, whether or not it's something they want to do themselves or hire a professional pest control company to come out and do it. As well, we also monitor for other vector borne diseases like Hanta virus, which is carried by certain species of wild mice, plague, which is carried mainly by a ground squirrel population mainly at higher elevations though, not down in the lowlands, and we'll also test ticks for the presence of some vector borne diseases like Lyme as well as tularemia.

CAVANAUGH: And what's the need for Vector Control in the county? Why do we need a county agency for this?

CONLAN: Well, mosquito control is something that's not really performed by anybody else so that really does – is a responsibility that falls upon us. But as far as those other diseases go, they're kind of one of these things that they don't pop up commonly all the time but they do have the potential to do that. So by monitoring those diseases, it gives us a chance to fire an early warning shot and warn the public that, okay, look, we're detecting these things here, you really may want to be careful. It also alerts the local community and physicians that, you know, they maybe want to look for those diseases now because we're finding them in the wild.

CAVANAUGH: So it's part, actually, of Public Health.

CONLAN: Yes, it is. We are part of the Public Health. Umm-hmm.

CAVANAUGH: So let's start with what you mentioned, one of the most annoying and potentially dangerous summer bugs, mosquitoes. Now there's a vector if there ever was one, a mosquito.

CONLAN: Oh, yeah.

CAVANAUGH: What kind of diseases or illness do mosquitoes carry?

CONLAN: Well, obviously, the big one that everybody's hearing about, and for good reason, because it's the most common one we have now is West Nile Virus. We also have a couple of other mosquito borne diseases that have shown their presence in the county on occasion. One of those is called Western Equine encephalitis and the other one is called St. Louis encephalitis. Both are very similar to West Nile but, of course, West Nile is the one that's the most prevalent at the moment. It's an introduced disease and it's still kind of finding its equilibrium out there and, unfortunately, people are becoming sick from it in some cases.

CAVANAUGH: Right, now what is the danger of contracting West Nile Virus? Who is most commonly affected? We hear that it affects animals as well as humans, right?

CONLAN: Certain animals. Basically, anyone and any animal can get this stuff. It's just that certain people or certain species of birds, other animals, are more susceptible to it as far as getting very ill.


CONLAN: If people get it, four out of five people actually will get over it without any real issues with it. But that one out of five can have anything from a mild illness that might make them feel ill for a day or two to something as severe as a really nasty illness that'll put them in the hospital or even cause death.


CONLAN: And as far as risk factors go, usually what we see is that with increasing age comes increasing risk. It seems like if the immune system just isn't quite functioning like it used to, you're at slightly greater risk of having a harder time with the illness should you contract it. And the only way you can get it is from the bite of an infected mosquito.

CAVANAUGH: Any other diseases that mosquitoes around here carry?

CONLAN: Well, as I said, they have – we have Western Equine encephalitis…

CAVANAUGH: Right, right.

CONLAN: …which we've only found on a few occasions, as well as St. Louis encephalitis. There's also things like malaria, dengue fever, and these are diseases of really big concern as you move to more tropical areas of the world. But we're very lucky in San Diego, we don't have the two species of mosquitoes that vector that disease the most commonly. We do have one for malaria, but malaria is actually controlled just by the way that we live, by – You know, we have mosquitoes here that are very good at transmitting it but because we tend to be inside with our screens on at night and whatnot, it just pretty much keeps the disease at bay.

CAVANAUGH: I'm speaking with Chris Conlan. He is Vector Ecologist with the County of San Diego. We are talking about mosquitoes and eventually other dangerous pests that bloom in the summertime. And I wanted to ask you, Chris, what – I want to let everybody know that we are taking their calls at 1-888-895-5727. But before we get to the calls, I want to ask what measures the County takes to control mosquitoes? Do you drain standing water on County land? What do you do?

CONLAN: Well, it's a pretty comprehensive approach. We have technicians that go out and they'll respond to public complaints. They also monitor sites around the county that are known to be mosquito breeding places. And most of the time we control things through the use of pesticides. We have a couple of things in our arsenal that we use but the most common one is a bacterial-based pesticide that only affects the larval stages of mosquitoes, and that's pretty much it. And that can be applied by the ground, through just tossing it in the water or we also can drop it from a helicopter because we have a number of locations around the county that are known large mosquito breeding areas that are just inaccessible by foot and the only way to really treat those is through the use of a helicopter. And those are pretty much the primary ways that we do it. And then, of course, there's also checking backyard sources like, you know, pools from foreclosed homes and things like that that aren't being monitored anymore.

CAVANAUGH: That's an issue now, too, isn't it?

CONLAN: Yeah, and for those, usually mosquito fish is the preferred method for control on that. That's more of a biological control method. We can put fish in those pools and as long as someone doesn't come along and, you know, see the water looks mucky and put chlorine in it and kill the fish, the fish'll keep breeding and stay in that pool until the new owner shows up and decides to make it a goodlooking pool again.

CAVANAUGH: As I said, we are taking your calls at 1-888-895-5727. Right now, let's go to Ed in Point Loma. Good morning, Ed. Welcome to These Days.

ED (Caller, Point Loma): Hey, I wanted to ask about the mosquito fish actually. There's so few fresh water sources in San Diego and when you visit them, something like the San Diego River, they're almost always now covered in Gambusia and there are – I mean, and mosquito fish. And there are a number of native fish species that would gladly eat mosquito larvae and do a good job controlling things like the native sticklebacks and native San Diego killifish. I know they may cost a little bit more to breed but wouldn't there be some consideration to trying to use those in the program as opposed to an invasive species that competes against them and seems to out-compete them?

CONLAN: Yes. We're – And, actually, we are talking with Fish & Wildlife about that. And all these things require – there's a lot of, you know, red tape to go through with respect to wildlife regulations so we have been talking with Fish & Wildlife about that. There are organizations that are working on good methods to breed these other types of fish. The main issue is that because the mosquito fish have already been introduced long ago to most of our waterways, they're already there and for – We already have a rule within our department that we will not put mosquito fish in any, you know, waterways of the United States. They're primarily used for backyard sources like pools and fountains and things like that. So that's how we're trying to go that route.

CAVANAUGH: Thank you, Ed, for that question. You know, I have heard of, in other parts of the country, some communities use bats to control their mosquito populations. Do we do that here?

CONLAN: Well, you know, and again, this all started from an unusual study that was done many years ago but, unfortunately, as much as we like bats and we certainly encourage folks if they want more bats around their property, go right ahead, you know, build the bat houses, do what you'd like to do. But you can't really rely on them for mosquito control. They do eat some mosquitoes but, you know, a mosquito is very tiny and a bat has to expend a lot of energy to fly around at night and it's more likely to pick off a much larger insect and get more bang for its buck than to go after a whole bunch of little mosquitoes.

CAVANAUGH: Now what areas of the county are mosquitoes really prevalent?

CONLAN: It's actually rather spotty. If you live near any of our fresh water or even our salt water lagoons, you're more likely to see some. If you live anywhere near a body of water that is rather heavily vegetated with cattails or other thick vegetation in the water that will probably also produce them. And then you get all these transient sources that occur when residential areas have problems with, you know, as we mentioned, swimming pools that aren't being maintained, or also any little backyard sources. I mean, very often it's all those little tiny backyard sources that can add up to even more than what we deal with with a very large source somewhere else. So you've got to watch those, you know, like kids toys and wheelbarrows that fill up with water every time the sprinklers go off.

CAVANAUGH: That's what I was going to ask you, if somebody's seeing a lot of mosquitoes around their property, so that's the kind of thing to look for, just do a survey of your property?

CONLAN: Yep, do a survey of your property or give us a call. I mean, we'll send a technician out to give you a hand. In many cases it's easy. You look over the fence, you see your neighbor hasn't taken care of the pool. In other cases it's not so easy and we have to help you look around. It could be one of those, you know, underground drainage systems or a storm drain that's clogged up, which isn't readily seen from just walking around.

CAVANAUGH: Right. We are talking about vector control and, of course, vector is a carrier that transmits disease from one person to another and, as we've been saying, a lot of insects and rodents do fit that bill. My guest is Chris Conlan. He is Vector Ecologist with the County of San Diego. We've been talking a lot about mosquitoes but they are not the only vectors that you deal with. I wonder, are rats and mice more common this time of year? Or are they a sort of a seasonal problem?

CONLAN: They're around all year. What we usually see is that in the wintertime they tend to be a bit more of a problem. They're seeking warmth so they're more likely to come into houses. I mean, right now, it's – as you well notice, you walk outside, it's very warm so they're perfectly okay outside. But, again, during the winter we tend to see an increase in calls of people having issues with them in their house.

CAVANAUGH: Right. We're taking your calls at 1-888-895-5727. And on the line, Justin is in North Park. Good morning, Justin, and welcome to These Days.

JUSTIN (Caller, North Park): Hi. How are you?

CAVANAUGH: Just great. How are you?

JUSTIN: I'm very well, thank you. I wanted to mention, I'm actually a veterinary technician and you were talking about diseases that are carried by mosquitoes. One thing that I didn't hear was heartworm disease, which we're starting to see a lot of this year which in this area at least we've never had a problem with before. And I just thought it would be a good thing for our listeners to hear that heartworm disease is sprouting up this year and mosquitoes are the primary – or the only vector for it.

CAVANAUGH: Well, thank you, Justin, so much for that call. Chris, would you like to comment?

CONLAN: Yeah, actually we do have heartworm here. The species – there's only one species of mosquito that actually carries it here. It's called a tree hole mosquito. And it's not very common. It's kind of in little pockets. It tends to breed in holes created in like older oak trees and whatnot. You know, they have little rot spots in them and they'll fill up with water. So they tend to be a little more common in the foothills and as you get up into the mountains but there are areas in the lowlands that have produced them, and we find them in our traps from time to time and that is a concern. He's correct. We do have it and your dogs may be at risk, so it may be something that you want to check with your veterinarian on, about whether or not you want to have your dog medicated.

CAVANAUGH: Now you were saying this is not necessarily the time of year where rats and mice might, you know, burrow into your house to get warm but they could be outside in your backyard and so…

CONLAN: Oh, of course. And that's almost all year round. They're always around. You may not see them but they're almost always around.

CAVANAUGH: I wonder what diseases you're concerned about when it comes to rodents?

CONLAN: With rodents, probably our biggest concern, especially when you get close to more, you know, suburban areas would be Hantavirus. And that's only carried by a few species of wild mice, primarily deer mice and harvest mice, which usually don't invade homes. But in areas where you're maybe on the fringe of some wildlife habitat and things like that, there is a risk that they could come in. They're also known for going after maybe some outbuildings, like tool sheds and garages that are unattached and things like that. And if allowed to get in and set up shop, that could be a problem so we encourage people to take precautions. With rodents, we preach exclusion, you know, at best. Try your hardest to keep them from getting in. Plug all those openings. And that's something that one of our technicians can help you with if they come out to your house if you're having issues.

CAVANAUGH: And it really is amazing how small those openings can be for rats or mice to get in.

CONLAN: That's correct. A mouse can get into something the size of a dime and a rat only needs something the size of a quarter to get in. And once they're in, they'll start chewing that opening to make it bigger and all their buddies follow them.

CAVANAUGH: Let me ask you, when it comes to rats, I know, I've heard anecdotally, a lot of people in this community, there seems to be sort of like rat outbreaks in neighborhoods. Do you see this sort of thing happening?

CONLAN: It does happen. And, again, these can be related to a number of different things. There's some areas – some of the older neighborhoods with lots of lush vegetation that are well landscaped actually tend to be the bigger problems because you've got lots of cover and you have lots of food. And then other times you can have situations where maybe, in urban areas where people are leaving their dumpsters open, the rats have access to the trash, so they're amplifying the population that way. So there's a lot of things that go on and, generally, each case tends to be somewhat unique so it's a good thing, if you're not really sure where they're coming from, you know, give our office a call. We'll have a guy come out, take a look and they'll kind of teach you, you know, what probably the issue is in your particular area.

CAVANAUGH: And you were saying something about an actual kit that your office hands out?

CONLAN: Very often what we'll do, if people decide they want to do it themselves, we'll give them a starter kit, which basically amounts to a bait box and a couple of rattraps, some instructional videos, some pamphlets, a little piece of the proper – what we call hardware cloth, which is a type of galvanized screen that we recommend people put over any openings where the rats might be coming in. But the usual openings are going to be old rusted-out attic screens or large gaps underneath, say, garage doors or even other doors around the house.

CAVANAUGH: I am speaking with Chris Conlan, Vector Ecologist for the County of San Diego. We're taking your calls at 1-888-895-5727. Right now, Amy is on the line. She's calling from Bonita. Amy, welcome to These Days.

AMY (Caller, Bonita): Thank you. My question is about crows. I've lived in Bonita 25 of my 35 years in San Diego and the crows have descended in multitudes. Are crows vectors of West Nile Virus? And if the crow's a vector, what can we do to reduce the population?

CAVANAUGH: Thank you for that, Amy.

CONLAN: All right, let me address that. Yes, crows are. And crows have been increasing in population in our county. They're very smart birds and they're very successful and they do carry West Nile along with just about every other bird in this county. The reason the crows tend to get a bad rap is that of the birds out there, they're probably the most susceptible to West Nile Virus. When crows catch it, they almost always die. So they're the birds you're going to see lying around dead and which, for us, is a good tool. They're indicators. They let us know what's going on and we actually encourage the public to report freshly deceased crows for us to go out and pick up and test. But as far as them being the real problem, probably not. Because they die, it may be that they're actually being removed from mosquitoes feeding on them and picking up the virus faster than they can really become a real problem. So in most cases, it's probably the little songbirds and things like that that get sick for a few days and then recover, may actually be your bigger carriers of the virus than the crows even though the crows are dropping out of the sky.

CAVANAUGH: And can they – can these birds infect human beings or animals?

CONLAN: No, the only way that you're going to get the virus is through the bite of an infected mosquito. But these mosquitoes are picking up that virus by feeding on these infected birds.


CONLAN: So – And, again, once that crow dies, it's no longer of any interest for a mosquito.


CONLAN: And so it has to be alive and ill and that's where the virus has to be picked up.

CAVANAUGH: Let's take another call. Ben is in University Heights. Good morning, Ben, and welcome to These Days.

BEN (Caller, University Heights): Good morning.


BEN: Hi, I have a question. I have a question about ground squirrels.


BEN: I live by Black Canyon near Washington and it seems like ground squirrels are taking over the whole canyon and they're starting to come into my yard and eat my garden. And I wonder if there's any risk or any precautions I can take to get rid of them because they really have eaten half my garden and I have two kids that I'm worried about.

CONLAN: Sure, there's a lot of things you can do for – with ground squirrels. You can either buy the live traps and trap them out yourself. You can also use a baiting program. I mean, there are baits and bait stations available. We recommend that any time you engage in a baiting program be it for rats or squirrels or mice or whatever you're doing, that you use an approved and enclosed bait station and caution because you don't want to make – You want to make sure that other animals, pets and kids and whatnot aren't going to get to that bait and be able to take it. But doing those things usually will be able to get the population under control, at least on your immediate property. But as far as, you know, if you live near a canyon or whatnot, it's just going to be a constant battle. They'll come – You'll have to try to keep them at bay as best as you can.

CAVANAUGH: And without being alarmist, I did read that the instances of plague in San Diego County is usually carried by squirrels, is that right?

CONLAN: It is, but we haven't found any positive plague ground squirrels in the lowlands of this county for a long time. We're talking 50 years. Right – I mean, ever since I've been here and the records that show that I've looked at for the last 20 years, all the positive plague findings have been up in the higher elevations above 4,000 feet. So for those folks living down here, plague is probably not anything to worry about unless, of course, you're going camping up in the mountains, in which case you may want to follow some of the simple precautions like, you know, not feeding the squirrels or camping on top of rodent burrows where the – because it's the flea that's the vector there.

CAVANAUGH: I want to – I'm speaking with Chris Conlan. He is with the County Vector Control. Let's take another call. I do want to talk a little bit about ticks and fleas, though, before we leave this subject. Scott is in Carmel Valley. Good morning, Scott. Welcome to These Days.

SCOTT (Caller, Carmel Valley): Good morning. Thanks very much for taking my call. I've got a rat problem here in Carmel Valley and the rats seem to be using the fence, the wooden fence that divides my house from my neighbor's house as their highway. And they've been eating commercial Home Depot rat poison at a clip of almost two pounds – well, about a pound a day for the past, I want to say, almost three weeks. I started out with the little place packs that they eat through and have switched to bulk green pellets and a bait station and I just switched to another kind of poison. It seems like it's having no effect on them. I haven't found or smelled one dead rat yet.

CONLAN: Well, very often the rats go and die in secluded areas. But the other thing you have to remember with some of the poisons, you have to be very careful and read the label because some of those require the rat to feed on them multiple times over a period. And if you are not really religious about putting that stuff out there and keeping a constant supply of food, you may actually just be feeding the rats. So you got to be careful, watch the label. Basically, your situation sounds like a good candidate for giving our department a call and having one of our staff go out there because it may very well be there's something we could offer you in terms of advice that you could do to alter the habitat in your yard that might reduce the rodent population just by engaging in that activity.

CAVANAUGH: Thank you for that call. You know, Chris, one of the vectors, one of the pests that people don't think a lot about when they're out and about, walking in nature during the summertime, are ticks. Tell us about the main risks associated with ticks and how people pick them up.

CONLAN: Okay, well, ticks are rather commonly encountered in San Diego County. They tend to occur in more of a patchy nature. You can walk down a canyon and not see a one and then all of a sudden you'll walk down a 100 yard stretch and pick up ten of them. It's just the nature of the beast. The two diseases we worry about the most are Lyme Disease, which is carried by the Pacific black-legged tick, and then also tularemia, which is carried by a couple of other species of ticks we have, commonly referred to as dog ticks. And the main season for ticks here is the rainy season.

CAVANAUGH: Ah, okay.

CONLAN: So you're not going to get that many in the summer. They are around but they're far less common. So once the rains start in November or so, that's usually when ticks start to ramp up and you're more likely to encounter them. So we recommend that folks – you can use repellant to apply to your clothing and your legs and whatnot. Also, by staying in the center of the pathway, you're less likely to pick them up because they're kind of hanging out at the tips of the blades of grass and on some of the lower scrub near the edges of the trails and when you brush by, they just grab on and say thanks for the ride. And then they start wandering around and looking for a place to bite. But, luckily, ticks take a long time to make that decision where to bite so after you've done your little hike, especially if you've got a friend with you, you might want to just quickly double check each other and look and see if you've got any ticks on you and flick them off. And wearing light colored clothing makes it easier for them to spot, too.

CAVANAUGH: Let's take another call. Carla is in El Centro. Good morning, Carla.

CARLA (Caller, El Centro): Good morning. Thank you for taking my call.

CAVANAUGH: Yes, hi. How can we help you?

CARLA: Yes, my friend got West Nile Disease and he was in a coma for three weeks and he's still physically disabled and why can't we have a vaccine for people to take for West Nile Disease?

CONLAN: They are working on that but, unfortunately, vaccines for humans take a lot longer to get through, you know, the regulatory channels and whatnot. There's a lot more testing that has to be done. They do have a very good vaccine now for horses, so if you are a horse owner, because horses are very susceptible to West Nile, we strongly recommend that you consult with your veterinarian and get that horse vaccinated. But for people, unfortunately, there is no vaccine approved yet. It's still in the works, so for now you're just going to have to take those preventative measures by not breeding mosquitoes in your yard, wearing repellant. You may want to avoid activity during peak biting hours like dusk and dawn. And that's really all we've got going for you.

CAVANAUGH: I want to ask you, Chris, as you – you must do surveys about what is breeding and what – where the problem areas are. As you see the next couple of months developing, the summertime coming into full bloom, what are the main things that you want people to know? Are there any warnings that you're issuing? Anything of prime concern?

CONLAN: Well, that's – Actually, I'm glad you brought that up because prime season for West Nile Virus starts right now. It's usually July, August and September so these are the three hottest months, typically, for San Diego and the virus is most active during those months. And because we can't always predict where the mosquito populations are going to crop up, especially with respect to the foreclosures and whatnot, sometimes a pool can make it miserable for a two or three block square area. So just, if you are going to be out in dusk, you know, or even later on in the night, it's a good idea to go ahead and put on your mosquito repellant. And, again, we encourage people to go ahead and just check your yard out from time to time, make sure you didn't accidentally leave a bucket out there that's full of water and breeding mosquitoes.

CAVANAUGH: And do you foresee that this being a particularly virulent kind of a mosquito season?

CONLAN: So far, it actually hasn't been quite as bad as last year. Last year set records across the board. It got warm early and stayed that way and West Nile took off and it stayed strong right through the season. It's really just beginning to ramp up now. We've had a few positive birds discovered in the last two or three weeks, so it can be expected to start picking up speed here very quickly, so now's the time to be very vigilant especially for the general public. And call in those freshly deceased birds if you find them, especially – what we really want is crows, ravens, jays and birds of prey. If they're fresh, they're dead less than 24 hours and not all mangled, we'd be happy to pick them up and test them.

CAVANAUGH: I want to thank you so much. There's so much good information. Thank you, Chris Conlan.

CONLAN: Oh, you're welcome.

CAVANAUGH: He is Vector Ecologist with the County of San Diego. I want everybody to know that you can continue this discussion online. We encourage you to post your comments at

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