Cockroaches lay eggs that hatch into immature insects called nymphs. Although they are a fraction of the size of an adult cockroach, these baby cockroaches make up for their diminutive size by exceeding their elders in numbers and in appetite. If you discover these nasty nymphs outdoors, use the same strategies used indoors -- sanitation and physical and chemical controls -- to kill nymphs before they can mature and lay more eggs.
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Nymphs grow, but their shells, called exoskeletons, do not. The rapidly growing baby cockroaches go through several periods, called instars, of molting, or shedding their exoskeletons, covering a period of a month or more until they become fully grown adults. Nymphs are wingless and, like their elders, nocturnal. They also tend to outnumber their elders, since one adult female might lay between 10 and 50 eggs, depending on type of cockroach. You'll find early instar nymphs where they've hatched in leaf or garden litter or in cool, shaded places. These outdoor pests generally come indoors by accident -- carried indoors in deliveries, wandering into basements through cracks, or squeezing through rotted roofing, door frames or windowsills.
Outdoors, as indoors, the first step in roach abatement is to clean up and keep surfaces and enclosed spaces clean and dry. Outdoor roaches, such as the Pennsylvania or wood roach, Oriental and American Roach, also known as the Palmetto bug, lodge in cool, damp places where they have a ready source of starch -- rotting plant matter or soft wood. Rotate woodpiles and turn mulch frequently. Keep leaf litter and compost piles away from buildings. Eliminate litter containing cardboard, cans or tires where water collects. Keep potting benches tidy. Outdoor roaches like to congregate or nest in tree holes, especially the jagged openings in palms (Arecaceae), hardy in U.S. Department of Agriculture plant hardiness zones 7b through 10. Give them a power wash with soapy water and fill holes with quick patch cement. Outdoor roaches like old sewers and wood siding, too, so fix leaks in outdoor pipes and water lines promptly and replace "spongy" siding before insects can congregate under it.
Use only 1 inch of mulch where it contacts foundation walls to discourage baby roaches from setting up shop near buildings and caulk openings around windows and water lines -- including those lines from the air conditioning condensing unit. Remove standing water from gutters and standpipes. Traps, available at home and hardware stores bait nymphs and capture them on sticky paper. Neem sprays may not kill cockroaches, but neem oil-impregnated baits can inhibit growth of first-instar roaches, resulting in their deaths. By keeping outdoor landscaping clean and blocking access to cool, damp areas, you'll kill nymphs by slow starvation or at least encourage surviving nymphs to move their settlement on to the next neighbor's yard. For organic gardeners, a parasitic cockroach wasp may be available. The wasp lays its eggs on nymphs as they emerge, killing them from within.
The ultimate weapon in the war against roaches is chemical. Boric acid, recommended for indoor use is highly toxic to plants, so it should not be used in the garden. Place bait traps containing hydramethylnon or imidacloprid near wall borders, behind meter boxes or anywhere you find groups of roach droppings. Outdoor sprays containing imidacloprid labeled for roaches are most effective when used in cracks and crevices and as perimeter sprays. Read the label carefully. Some sprays must contact the insects to be effective, which means you must find their daytime digs to kill them. When handling pesticides, use them according to directions on the package and when the wind is calm. Wash well with soap and water after use. Store or dispose of spray cans and traps according to package directions.