How to Manage Pests
Pests in Gardens and Landscapes
Figure 2. Indian walking stick egg (actual size about 1/10 inch); the cap, or operculum, is the opening through which the first-stage instar will emerge.
Figure 3. If an adult female Indian walking stick is in danger of being harmed, it will splay its forelegs to reveal a bright red patch on its inner femora near the attachment point to the body. This red marking distinguishes the Indian walking stick from native California species.
Walking stick insects, order Phasmatodea, are mostly tropical insects that are considered an entomological curiosity because of their remarkable mimicry of twigs and leaves. Several species are popular in the pet trade and for grade school demonstrations and thus get moved extensively with some, such as the Indian walking stick insect, Carausius morosus, becoming established in many parts of the world. The Indian walking stick is native to southern India, but the precise time of its establishment in California is unknown; the first official finding occurred in San Diego County in 1991 and shortly thereafter in San Luis Obispo County. There has been an increase in homeowner reports of walking stick damage in the last 10 years along the Central and Southern coasts of the state.
The establishment of Indian walking sticks in California landscapes has occurred as a result of their escape from captivity or through discarding of eggs. Indian walking stick insects produce viable eggs without mating and broadcast them in their cages or containers; the eggs become co-mingled with their droppings (frass). The eggs are difficult to distinguish from frass and are easily tossed out in the process of cleaning the cage. Owners of walking stick pets must be responsible for understanding the biology of these organisms and caring for them properly, which includes bagging the cage debris and disposing of it in the trash.
The adult Indian walking stick is 2 3/4 to 4 inches long, wingless, and usually brownish with a long, thin body and legs (Figure 1). It can retract its legs seamlessly alongside its body, enhancing the insect’s sticklike appearance; the forelegs project forward adding to the overall length of the insect.
Eggs of Indian walking sticks look like seeds. They are ovoid, grayish-brown with a small, buff-colored cap (operculum) on one end through which the young insect emerges (Figure 2). The eggs are similar in size to the frass these insects produce, but the frass is rough in texture. There are five nymphal instars, each instar progressively larger with each molt. Nymphs are similarly colored to the eventual adult, and it is difficult to distinguish larger nymphs from adults.
A few walking stick species are native to California including the western shorthorned walking stick, Parabacillus hesperus, the gray walking stick, Pseudosermyle straminea, and species in the genus Timema. All of these species feed primarily on grasses and scrub brush—mostly woody shrubs—in dry wild areas and haven’t been reported as pests in landscapes. Adult females of the Indian walking stick can be readily distinguished from these species by the red markings at the base of their front legs (Figure 3).
Adult females can live for several months and produce several hundred eggs during their lifetime. Eggs hatch in 10 to 12 weeks at room temperature. The five nymphal instars take 4 to 6 weeks to complete before the insect reaches adulthood. No rigorous studies of temperature-dependent developmental durations or reproduction have been published for this species. Most knowledge has been gained from their use in the pet trade.
Indian walking sticks feed on a wide variety of plant species in California including but not limited to azalea, bramble, camellia, geranium, hawthorn, hibiscus, ivy, jasmine, oak, privet, pyracantha, rose, and some common garden vegetables.
Indian walking sticks reproduce mainly asexually (parthenogenetically), with females haphazardly dropping eggs while they feed or rest. Males are known but are rare; nothing is known regarding the conditions required to produce them. Females frequently feed and move through foliage at night but also can be observed feeding throughout the day. The conditions along the coast of California are mild enough for these tropical species to survive through the winter, although they do so mainly in the egg stage. Thus, most observations of activity and damage take place during the spring when nymphal stages emerge from over-wintered eggs and begin feeding. Multiple generations occur through the summer with nymphs and adults observed well into November or until the first frost.
Stick insects exhibit crypsis, a combination of color, shape, and behavior that makes them blend into the environment, thus avoiding detection by predators. When stick insects move through the foliage, they typically do so very slowly and with swaying movements that mimic the effects of wind on the plant. When disturbed, the insect retracts its legs and remains perfectly still, even when handled. Only when aggressively handled will the insects display yet another color phenomenon—aposematic, or warning, coloration. If the insect is in danger of being harmed it will splay its forelegs to reveal a bright red patch on its inner femora near the attachment point to the body (Figure 3). This color can startle predators, giving the insect a chance to escape. When a stick insect isn’t on a feeding site, such as walking on bare ground or poised on a man-made structure, they become easily evident, and due to their large size, can be alarming to some people.
Indian walking stick egg survival hasn’t been studied in California. The eggs in their native home are attractive to grain-feeding ants that mistake them for seeds and cart them back to their nests but never feed on them. Since the eggs are broadcast into the environment, the ants play a role in the egg’s survival by bringing them into the nest, where they remain undisturbed until hatching.
The worst damage typically is recorded in the springtime when nymphs are hatching from eggs and feeding on new flush growth of many plants species in landscape settings. They feed aggressively on small-leafed varieties of ivy and privet. Nymphs and adults feed heavily on the margins of mature leaves, creating a tattered appearance, or fully consume smaller, tender leaves (Figure 4). Populations develop unnoticed until significant feeding takes place. Damage reports in landscapes have been spotty and inconsistent from year to year on the Southern and Central coasts of California. The most frequent complaints arise from residents when they encounter walking sticks on windows, doors, or even inside homes on warm summer nights.
Indian walking sticks usually are mostly a nuisance in the landscape, although in some years and in some locations they can seriously defoliate plants and cause damage to valued specimens.
Don’t release Indian walking sticks or other exotic insects into landscapes or wild areas. If you are keeping these species as pets, when cleaning the cage put the debris into a bag and close the bag securely before throwing it in the trash. Do not toss loose debris outside, as it might contain eggs or the insects. If you no longer care to raise Indian walking sticks, place the entire contents of the cage including live insects and all debris into a plastic bag, tie the bag securely and place it in a freezer for at least 48 hours before throwing it in the trash.
No biological agents have been released for their control, and it is assumed that generalist predators feed upon them when encountered; no definitive studies have been conducted.
Removing and destroying the insects can reduce populations over time and reduce evidence of feeding; however, finding them is difficult due to their cryptic nature.
There is no research on pesticide efficacy against walking sticks. Pesticide applications aren’t recommended at this time, as they might kill beneficial insects occurring in the landscape. Care must be taken when using insecticides, especially when applications are made to edible plants.
Authors: D. H. Headrick, Horticulture and Crop Science, Calif. Polytechnic State Univ., San Luis Obispo, Calif. and C. A. Wilen, UC Statewide IPM Program, San Diego Co. Thanks to Judy Bell and Michael Bostwick of the San Diego Zoo for providing plant lists.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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