Pest population increasing


May 1, 2004

By: Hanif Gulmahamad

Exotic pests tend to increase to high populations when they are accidentally introduced into new geographical areas. This is largely because they are often introduced into new areas without natural enemies around to keep them in check.

These high pest populations frequently create problems – look no further than the multicolored Asian lady beetle (Harmonia axyridis ), which was intentionally introduced by the United States Department of Agriculture to control tree-feeding insect pests in the late 1970s. (Some sources claim the beetles were introduced in California as early as 1916.) While this introduction may have had some biological control value, this species has since become widely distributed – populations have increased to the point where PMPs nationwide are now fielding complaints from customers about “those biting ladybugs.”

Similarly, the western black widow spider, Latrodectus hesperus, has been on the increase on school campuses in California’s San Fernando Valley. There are three exotic structural pests that are directly responsible for the increase in black widow populations in these schools – the Oriental cockroach, Blatta orientalis; the Turkestan cockroach, Blatta lateralis; and the Indian house or decorated cricket, Gryllodes supplicans.

Figure 1. Oriental cockroach in a black widow spider web.

The Oriental cockroach has been well established and firmly entrenched in many areas of North America for such a long time that many PMPs do not realize that it is an introduced species. By comparison, the Turkestan cockroach and the decorated cricket are relatively recent introductions (Gulmahamad 1993a, 1993b, 1995, 2000).

The majority of LA school buildings are surrounded by asphalt and concrete, which become very hot when exposed to direct sunlight during summer. Because of a lack of vegetation and moisture, these hardscapes are hostile environments for most insects.

When cracks in these hardscapes are treated with a flushing agent, earwigs, peridomestic cockroaches, crickets and a few web spinners (Embiidina) tend to emerge. Most of the black widow spiders found at these schools are located at ground level or slightly above, and it appears that most of their prey species are crawling insects. This is based on a survey of prey species found in black widow spider webs at various school sites.

It is common to find Oriental cockroach (Figure 1), Turkestan cockroach (Figure 2) and the decorated cricket (Figure 3, next page) in black widow spider webs at these school sites. When populations of the cockroaches and cricket peak in the summer months, so do populations of the western black widow spider. There is a direct relationship between high populations of the peridomestic cockroaches and the crickets and high populations of the western black widow spider on school campuses.

Figure 2. Male Turkestan cockroaches in black widow spider web.

High populations of peridomestic cockroaches on school campuses create potential health and nuisance problems. When they lead to an outbreak of the western black widow spider in, on or around school buildings, the health and safety threat is clearly magnified.

Figure 3. Two Indian house crickets in a black widow spider web.

It’s raining roaches

Hidden cockroach infestations can be extreme. For example, I recently treated a hole in the root crown of a white mulberry tree, Morus alba, on a school campus with deltamethrin dust. Turkestan cockroaches continued to stream out of this hole for about an hour after treatment. Adult males, females and large nymphs headed straight for a storage building about 15 feet away from the tree. Large numbers of cockroaches climbed up the building and blackened the west wall of the structure.

Subsequently, these roaches succumbed to chemical intoxication and fell to the ground. I randomly selected four 1-sq.-ft. areas on the asphalt below the building and counted the cockroaches in each quadrant. I added the numbers together and then divided by four to get an average number of 34 cockroaches per square foot. With 111 sq. ft. of area covered by cockroaches, I estimated that approximately 3,774 large cockroaches exited this one tree hole. This number does not include the piles of small cockroach nymphs that exited the hole but quickly died on the root crown areas of the tree because they were more susceptible to the dust than the adults and large nymphs (Figure 4).

Figure 4. Dead Turkestan cockroach nymphs on tree root.

Because of the flushing action of the dust, large numbers of cockroaches exited the hole, climbed up the tree trunk and continued to ascend the large tree branches for some 30 feet or more above ground. Some of the male cockroaches took flight from the tree trunk and branches. Other roaches ran about on the tree limbs until they succumbed to the dust and fell down. On the way down, they hit the large mulberry leaves and created a sound similar to raindrops striking tree leaves.

Falling cockroaches also created a raindrop-type sound when they hit the asphalt below the tree. This “roach rain” continued for about one hour after the hole was treated.

Turkestan cockroaches in the tree hole were highly irritated by the deltamethrin dust – to the extent that if you were under the tree shortly after the treatment, they would run up your legs and into your pants. They would also fall into your shirt from above.

Those of you who have performed cockroach cleanout work for some time can relate stories of German cockroaches exhibiting this behavior after heavy infestations have been treated indoors. I have experienced that unpleasant situation many times before regarding German cockroaches. However, I’ll admit I have never before experienced cockroach rain involving a peridomestic cockroach outdoors falling from a tree! 

Earthquakes and subsequent pest problems

Taking action

While high populations of peridomestic cockroaches on school campuses can be rather disconcerting, future “roach showers” can be avoided by implementing sound, long-term integrated pest management strategies:

  • We permanently filled in all holes in the tree root crowns after the treatment.
  • We sealed earthquake-created cracks in the asphalt (see “Earthquakes and subsequent pest problems,” page 45).
  • We caulked cold joints, where concrete or asphalt hardscapes met building foundations.
  • We repaired asphalt that was pushed up as a result of enlarged tree roots.
  • We removed old black widow spider webs so that new webs – indicating fresh activity – could be addressed in the future.
  • We used sticky traps inside affected structures so that cockroach and cricket populations could be monitored.

The Los Angeles Unified School District does not routinely apply pesticides on a calendar basis for pest prevention purposes. Instead, pest populations are carefully monitored, and when appropriate and to the fullest extent possible, nonchemical intervention techniques are first employed before any decision is made to apply a low-risk or reduced-risk pesticide.

What clearly became evident from this integrated pest management program was that, by reducing the prey populations (cockroaches and crickets), we were able to reduce a predator pest population (black widow spider) to a tolerable level. It then became less labor-intensive to periodically check for new black widow spider webs, quickly remove the webs and crush the new inhabitants.

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