How to get rid of fleas in yards

Fleas in Outdoor Environments

Flea Life Cycle & Habitats

Adult fleas stay on their acquired host. Eggs are laid on the animal, but they aren’t sticky and fall off within a few hours. The eggs develop in the environment, eventually maturing into adults. 95-99% of flea populations consist of eggs, larvae, and pupae. Adults only make up 1-5% of infestations.

Infestations Begin Outdoors

Most infestations originate outdoors. Numerous small animals can carry fleas, including opossums, raccoons, foxes, coyotes, skunks, and feral dogs and cats. Flea eggs continually fall from infested wildlife, and infestations are passively spread around neighborhoods. Fleas eggs become adults within a few weeks. The adults jump onto dogs and cats venturing into the infested areas. Newly acquired fleas feed, mate, and begin laying eggs in 24-48 hours. When the pet re-enters the home, eggs start falling indoors.

Although less common, fleas can also hitchhike on people. They can jump onto the shoes and socks of a person’s who is walking through an infested part of a yard. Fleas are then unknowingly brought into the home to infest pets.

Re-Infestation Cycle

Feral animals can infest pets with fleas, and vice versa. There’s a continual re-infestation cycle wherever their territories overlap. Re-infestation pressure is usually higher than assumed, because most feral hosts are nocturnal and rarely seen. Outdoor flea populations can be a constant source of fleas within homes.

Insecticides for Outdoor Flea Control

Flea control is mostly focused on indoors and pets. As a result, few insecticides are recommended for outdoor control. Insecticide duration is a big issue outdoors. Contact insecticides kill existing adult fleas, but the effect is short-lived. The bulk of infestations are immature stages. New adults will continue emerging for a month or two. Thus, an effective insecticide needs to have long-lasting residual activity.

Adulticides

Adulticides target adult fleas and don’t provide lasting outdoor control. Permethrin and other pyrethroids are often used outdoors, but they perform poorly even when applied at high rates. Most adulticides significantly decline in activity by 7 day. They perform even worse against immature stages. After 24 hours, less than 50% of flea larvae get killed in soils treated with chlorpyrifos, bendiocarb, permethrin, diazinon, propetamphos, or carbaryl.

Insect Growth Regulators

The discovery of insect growth regulators (IGRs) led to a new paradigm in flea control. IGRs prevent eggs and larvae from developing into adults. And with their long-lasting activity, larval habitats can be treated to kink the life cycle. Juvenoid IGRs have both contact and oral activity. The most widely used compounds are methoprene, fenoxycarb, and pyriproxyfen.

Pyriproxyfen is the IGR of choice for outdoor use. Methoprene is unstable in UV radiation, and thus outdoor residual activity is relatively poor. Fenoxycarb showed promising results outdoors, but its manufacturer voluntarily suspended sales in February 1996.

Pyriproxyfen

Methoprene was the first juvenile hormone analog (JHA). It wasn’t suitable outdoors due to poor chemical stability. However, soon the phenoxy JHAs were discovered, including pyriproxyfen (trademarked as Nylar). These new compounds were photostable. This led to a new, effective way to control fleas outdoors.

Mode of Action

Pyriproxyfen mimics the biological activity juvenile hormone (JH), a natural hormone in insects. Strangely, these two compounds are structurally unrelated.

Juvenile Hormone

Insect larvae must shed their semi-hard exoskeleton to grow. Molting co-evolved with specific hormones, one of which is JH. When JH is present in high concentrations, larvae can’t molt or pupate into adults.

JH works with another hormone called ecdysone. When levels of JH lower, and levels of ecdysone rise, it triggers larval molts. When JH disappears, and levels of ecdysone rise, it triggers metamorphosis.

Juvenoids

Synthetic juvenoids (also called JHAs) work like JH, but they’re more chemically stable. Their stability allows them to compete for JH binding site receptors. Juvenoids bind to receptors and persist during periods when JH is usually absent. As a result, the larvae can’t mature.

JHAs are several times more active than JH. Pyriproxyfen is among the most potent. In addition, JH esterases can’t break down pyriproxyfen, so it persists in the insect for a long time. Unlike JH, pyriproxyfen is also toxic to eggs, and sterilizes adult females. Thus, it may be more than just a JH mimic.

How Pyriproxyfen Control Fleas

Eggs

Pyriproxyfen quickly penetrates flea eggs. Depending on the egg’s age, pyriproxyfen causes embryonic (no embryo forms) or post-embryonic (larvae die) effects. Freshly laid eggs won’t hatch. Eggs 1-2 days old may hatch, but larvae will quickly die.

Larvae

Larvae are highly susceptible to IGRs. Flea larvae can be exposed through contact, or by feeding on tainted adult feces. Pyriproxyfen prevents pupation. Instead, larvae transform into a larval-pupal or pupal-adult intermediates which can’t function or reproduce. Eventually they’ll die. Pyriproxyfen also damages the internal tissues of larvae, causing secondary lethal effects.

Flea larvae remain vulnerable until they’ve fully pupated. When larvae are exposed before spinning a cocoon, there’s a 98% reduction in adult emergence. They never become pupae. When cocoons less than two days old are exposed, there’s an 84% reduction in adult emergence. They’ll reach the pupal stage, but most never emerge as adults.

Pupae & Cocooned Adults

Flea pupae and cocooned adults are resistant to JHAs. They diffuse across the pupal case. This can cause control issues, as pre-emerged adults can stay quiescent in their cocoons for up to 5 months. This duration is called the “pupal window”.

Exposed pupae and cocooned adults will survive to emerge as adults. However, emergence occurs sooner (shortened pupal window). And emerged adults will die prematurely. 50% die within six days. Females are especially vulnerable, often dying in the first 48 hours of blood feeding. While alive, there’s no effect on egg production or offspring survival.

Adults

Pyriproxyfen is slightly toxic to adult fleas. Death isn’t immediate, but 96% die 8-10 days after contact. Females are more susceptible than males, since JH regulates more biological functions. JHAs typically aren’t effective against adults. Pyriproxyfen’s adulticidal effect is significant, because the main disadvantage of IGRs is their failure to kill adults.

Exposed female adults can’t lay viable eggs. They produce eggs prematurely. The eggs lack a yolk, appear darkened, and collapse when laid. However, they can lay normal eggs again after being free from exposure for 70 hours.

How Long Pyriproxyfen Lasts Outdoors

Pyriproxyfen has excellent outdoor residual activity. Duration depends upon the concentration, application rate, weather, and soil type. The most commonly cited study states that treated topsoil controls 80% of fleas for 3 weeks. In dry soil protected from rain, it can remain active for at least 63 days. Activity can decrease in wet soil because of leaching and degradation by soil organisms.

Pyriproxyfen performs better in sand versus clay. It binds to clay and organic matter, and then may not contact larvae. Sand, in contrast, is relatively inert and doesn’t affect activity. In sandy, partially shaded soil, pyriproxyfen provided significant control for 20 weeks. It lasted 44 weeks in high concentrations. Another study found high application rates provided 75% control for 11 months.

Pyriproxyfen also remains active on a numerous construction materials, such as wood, metal, and concrete.

Pyriproxyfen Concentrates

Tips for Spraying Yards

Before Spraying

Remove Debris

Flea larvae desiccate in open, sun-exposed areas. They only survive in humid, shaded areas. Outdoor flea hot-spots often occur where pets sleep, such as under a cool porch in the summer. Keeping these areas free of leaves and dense vegetation will make them less habitable for fleas. Mow, rake, or remove organic litter by hand.

Restrict Animal Access

If possible, block animals from reaching potential hot-spots. Then wildlife can’t drop flea eggs in these areas, and pets can’t wander in to acquire fleas. Common places to barricade include underneath homes, porches, decks, and verandas. Restricting access to these areas can be the most effective control method, since sprays can’t reach well. Even if spray is used, preventing access is a good idea. Residual activity can be substantially reduced if treated soil is displaced or buried from foot traffic and digging.

Where to Spray

Outdoor flea habitats are often poorly targeted. Viable habitats are limited, so spraying the entire yard is wasteful and irresponsible. It can cause undesirable environmental consequences. Treating yards may not even be needed in mild to moderate infestations. One survey only found fleas outdoors in 2 of 45 homes infested homes.

Total outdoor control is impossible if dogs or cats roam several acres of land. However, smaller areas of confinement can be treated, as well as confined sections of larger yards where pets spend the most time. It’s unnecessary to treat the whole yard or lawn. Before spraying, evaluate where pets frequently rest, and where there are existing fleas.

Climate Requirements

Flea eggs and larvae require habitats where temperature and humidity are stable. Suitable areas are limited, especially outdoors. Most eggs won’t survive. Infestations continue because fleas lay large numbers of eggs, and adults have a close association with their host and its resting sites.

Fleas develop in shady and wind-protected areas. Sunny locations can’t support fleas. Relative humidity (RH) must exceed 45% at all times. Survival isn’t possible under trees, or in maintained lawns, due to the low RH. Thus, spraying sunny areas of yards is unnecessary and ineffective.

Food Requirements

To survive, flea larvae must eat flea feces and eggs. Most eggs dropped from roaming animals won’t survive, because it’s unlikely they’ll fall into areas with larval food. Successful development occurs where both flea eggs and flea dirt fall from animals. These are usually places where the host grooms and sleeps. Itchy flea bites help facilitate this, as they cause hosts to scratch and dislodge eggs and feces.

Flooding

Larvae will drown in areas with heavy rainfall, sprinklers, or poor drainage. The water also dissolves and washes away larval food. Larvae stay in the upper few millimeters of soil. If soil moisture exceeds 7%, they’ll move to the surface. Most will drown when soil moisture exceeds 20%.

Foot Traffic

Flea larvae are unlikely to live in areas with substantial foot traffic.

Common Outdoor Flea Hot-Spots

Flea eggs fall anywhere invested animals visit, with most accumulating favored resting areas. Fleas successfully develop into adults in areas that are shady, cool, humid, and protected from wind. Outdoors, larvae are often found under dense mats of ground cover or shrubs. They avoid sunlight, moving under grass, branches, leaves, or soil.

Specific areas to target include dog houses, kennels, pet shelters, flower beds, mulch beds, crawl spaces, vegetation around buildings, areas next to the foundation, under structures (e.g. decks, patios, and mobile homes), and any other areas where pets rest in the shade.

Flea hot-spots are a major contribution to infestations, but may only make up a small portion of the area occupied by the pet. Outdoor hot-spots often occur shaded regions out of direct sunlight that retain moisture. Give special attention to where pets sleep. In the summer, hot-spots tend to be where animals go to escape the sun’s heat. Areas under homes and porches can be especially troublesome. Likewise, the soil in and around dog houses is frequently heavily infested.

The source of fleas outdoors is often evident. Upon walking near an infested area, fleas will quickly jump onto socks. Pets also quickly acquire fleas when they go into infested zones. A flea comb can reveal them easily.

How to Spray

Use a low pressure sprayer for an even distribution of insecticide. High pressures can disrupt the larval habitat and displace soil, resulting in an uneven chemical distribution.

Other Insects

Pyriproxyfen is a broad-spectrum insecticide. It affects a large range of insects, and can end up killing beneficial insects. Since it prevents larvae from becoming adults, it’s most useful against species where adults are active pests.

Be Patient

Larvae are most susceptible stage, but they aren’t killed immediately. Instead, they’re prevented from reaching adulthood. As a result, it can take a while before the effects of the treatment are noticed.