Experimental Yellowjacket Control Program 

Summary

1997 - 1999

John Albright, District Biologist

 

Summary:

 

In order to satisfy its responsibility to protect its constituents from pests of public health importance the Shasta Mosquito and Vector Control District proposed and experimental program to test the feasibility of yellowjacket control in public parks.  Combinations of trapping and baiting were tried and compared beginning in the spring of 1998 and continuing through 1999.  Statistics from the two years showed significant differences due to climatic changes from 1997 through 1999.  Although a wealth of data was collected in the two years so far, it is difficult to draw firm conclusions about the efficacy of treatments applied.  Although subjective observations indicate that yellowjacket baiting with poison was effective at some locations and not others, statistics indicate that all control efforts may have been ineffective.  This project needs to continue for more seasons to eliminate more of the variables such as differences in climate from year to year as factors influencing control.

 

Disclaimer:

 

Any mention of the brand name of products used in this project is intended solely to describe our activities, methods and materials used as specifically as possible.  Shasta Mosquito and Vector Control District does not endorse any specific products for use in pest control activities.  Any person doing pest control work should research the materials and methods best suited to their particular needs.  Always follow the label directions provided by the manufacturer when using any type of pest control product.

 

Yellowjacket Definition and Biology:

 

It is very important when talking about yellowjacket control that the reader understand exactly what a yellowjacket is.  It is quite common for people to refer to paper wasps as Ayellowjackets@.  Paper wasps are not yellowjackets.  Paper wasps are insects that make paper nests, somewhat resembling inverted umbrellas, under eaves and in similar secluded locations.  These wasps are generally rather large (compared to honeybees, for instance) with an elongated body, long legs and a tightly constricted division (thread-waist) between their thorax and abdomen.  They belong to the sub-family Polistinae and generally have small colonies of perhaps three to a dozen individuals. 

 

True yellowjackets are in the same sub-family as hornets (Vespinae).  They are generally about the same size or slightly smaller than honeybees and less slender than paper wasps.  Therefore they are referred to by a number of common names and misnomers which often relate to bees in some way, such as meat bee, ground bee, or ground hornet to name a few.  Unlike paper wasps, yellowjackets commonly have colonies of several thousand individuals that live in multi-tiered paper nests surrounded by a round paper outer covering, which makes the nest overall resemble a paper volleyball.

 

Unlike bees, yellowjackets commonly feed on other insects (alive or dead) and scavenge on carcasses of all types of animals.  Their scientific name, Vespula spp., comes from the Latin word AVespa@ which was the job title of an undertaker for disposing of the bodies of indigents in ancient Rome.  As the name implies, Vespula spp. serve an important function in cleaning up corpses from the environment.  However, their inability to distinguish between barbecued chicken and road-kill makes them an important pest species in areas of high public contact.  They are also attracted to sweets, which makes picnic areas all the more attractive to them. 

Yellowjackets have a one-year life cycle that begins with solitary fertilized queens, which over-winter in protected areas almost anywhere that they can squeeze their bodies into.  In normal years these queens begin to emerge from hibernation in April as the weather begins to warm.  They begin to look for sheltered areas to establish new yellowjacket colonies.  Typical nesting sites are usually close to the ground and include hollow stumps and logs, abandoned animal burrows, woodpiles, crawl spaces and wall voids.  Occasionally nests occur higher up in hollow trees, under eaves or in attics, but lower sites are much more common.  The queen builds a small starter nest from which the first workers will hatch.  Until her first worker offspring have matured she must gather food and maintain the nest by herself.  As soon as workers emerge, they take over housekeeping and food gathering chores.  From then on the queen stays protected within the nest where she is responsible for laying the thousands of eggs that it will take to keep the colony populated through the summer and fall.

 

In the Shasta Mosquito and Vector Control District area the most common yellowjacket species, Vespula pensylvanica reaches its peak population between mid-August and mid-September.  Up until this time all of the eggs laid by the queen will develop into worker yellowjackets.  In the fall she will begin to lay the eggs that will produce the queens and males that will mate to produce the following year=s yellowjacket population.  The new queens mate with the males and find cozy places to over-winter until the following April when the cycle will begin again.

 

Project Background:

 

Shasta Mosquito and Vector Control District (District) is entrusted with the responsibility of protecting the public in a 387 square mile area of Shasta County, California from health threats posed by insects and other animals found in the environment.  Methods of protecting the public depend on the degree of health risk that is associated with different pests species found in this area.  For many pests such as flies, fleas and cockroaches the District limits its activities to providing public information in the form of brochures and answers to questions from the public given by our biologist and other personnel.  Other pests such as mosquitoes require a regular annual program of control, which involves monitoring pest populations and active treatment of pest sources.  There is no clear consensus on what an appropriate level of activity should be for a vector control district to adequately protect the public from yellowjackets.  In California, mosquito and vector control districts are run locally and make these sorts of policy decisions based upon local conditions.  Some districts such as the El Dorado County Vector Control District feature yellowjacket control as arguably the most important function within their district.  Other districts have limited control programs, and many districts only provide information.  Until recently Shasta Mosquito and Vector Control District has fallen into that latter category.

 

In the summer of 1997 the volume of calls to the Shasta Mosquito and Vector Control District asking for help in controlling yellowjackets was alarming.  A serious yellowjacket outbreak was noticed throughout northern California that year resulting in serious stinging incidents and damage to recreational values in some public areas such as parks.  Shasta Mosquito and Vector Control District Manager, Bill Hazeleur felt that the District needed to reevaluate our role in yellowjacket control and make some effort to test the feasibility of yellowjacket control in areas of high public contact.  There was some concern initially that the program may put the District in competition with private pest control companies which do yellowjacket control in residential situations.  Therefore, yellowjacket control on private property was not the initial emphasis of the project.

 

The City of Redding Parks and Recreation Department (City) gave their permission for the District to use any city park areas for conducting the experiments.  In particular, the City asked that we see if some relief could be brought from the severe yellowjacket infestation that was plaguing a new park that was under construction in the Enterprise area of Redding.  The park, officially named Enterprise Community Park, is more commonly referred to as Kid=s Kingdom because of its large and popular playground area of that name.  The area was newly carved out of a 92 acre chunk of mixed riparian, oak and brush habitat along Churn Creek.  The yellowjacket problem at the park was driving the public out and forcing City employees to empty garbage receptacles after dark to avoid stings.

 

Two other parks in Redding were chosen for their ease of access, high level of public activity and easy access.  Lake Redding Park is a large, well-established park in the downtown Redding area along the Sacramento River with picnic areas, playgrounds, ballparks, museums and a hiking trail.  South City Park is in the downtown Redding area.  It has several baseball fields a tennis court and playground area.  The area around South City Park is much more urbanized than the other two parks and there are no significant waterways close by.  One residential site (Mad 1)  in the Enterprise area was chosen to which District personnel have easy access.

 

Since the decision to initiate the project was made after the peak of yellowjacket season in 1997, the project was not begun until the spring of 1998.

 

Materials and Methods:

 

Trapping of yellowjackets was done using Rescue7 yellowjacket traps baited with heptyl butyrate  (a yellowjacket attractant) and strips of turkey ham.  Yellowjackets that enter these traps pass through a funnel and are trapped within a plastic cylinder where they eventually die of exposure.  Traps were changed weekly, and the trapped yellowjackets were identified and counted.

 

Yellowjacket baiting was done using various combinations of different types of cat food and mackerel mixed with Knoxout 2FM7 microencapsulated Diazinon product mixed according to supplemental label directions.  The bait was placed in bait stations made from large pill bottles drilled to allow access by yellowjackets, but to exclude larger animals.   Attractiveness of the bait stations was further enhanced by suspending a cotton ball soaked with heptyl butyrate above the bait within the bait stations.  Bait stations were hung out of reach in shady trees to protect them from curious children and the intense summer heat common in Northern California.  The bait was replaced every two to three days as it became dry or rancid.  The theory behind such baiting is that foraging workers will take the bait back to the nest where it will be ingested by other workers, brood, and the queen.  This will hopefully kill  the whole nest.

 

Different methods were tried at the different sites to try to obtain information on the best and most efficient methods of control:

 

At the Kid=s Kingdom and Mad 1 sites trapping was begun in April in an attempt to rid the environment of newly emerged queens.  Since each queen will produce thousands of worker yellowjackets later in the season, the elimination of queens may cause a significant reduction in overall yellowjacket numbers later in the season.  Trapping continued until changes in the weather prompted a dramatic reduction in the yellowjacket numbers in the fall.  Traps were collected weekly and the trapped yellowjackets were counted in order to assess population trends throughout the yellowjacket season.  Baiting was done later in the season if yellowjacket foraging became significant enough to cause a pest problem.

 

At Lake Redding Park traps were set after the traps at Kid=s Kingdom and Mad 1 were no longer catching any yellowjacket queens.  Traps were collected weekly and the trapped yellowjackets were counted in order to assess population trends.  Trapping continued until the yellowjacket numbers dropped dramatically in the fall.  Baiting was performed here later in the season when yellowjacket pest risk warranted such activity.

 

South City Park was utilized as our untreated control site.  No early season trapping was done to catch queens and no baiting was done later in the season.  Traps were set after the last queens were caught at Kid=s Kingdom and Mad 1 and serviced weekly to monitor the yellowjacket population at South City Park until numbers dropped in the fall.

 

Yellowjacket counts were entered into a Microsoft Access7 database for storage and retrieval.  Microsoft Excel7 was used to summarize data and generate graphs.

 

Results:

 

Although a very few Vespula Sulphurea and Vespula Atropilosa were trapped their numbers were negligible when compared to Vespula pensylvanica numbers.  Therefore all future references to total yellowjacket numbers may be construed to mean Vespula pensylvanica numbers.  The period of queen emergence was from mid-April to July 10 in 1998 and from mid-April to June 11 in 1999.  The number of queens caught in 1998 was 101 compared with 58 in 1999 (Figure 1).  

Figure 1(Click to view graph)

Average number of total yellowjackets per trap in 1998 was 1306 compared to 4380 in 1999.  Peak yellowjacket counts occurred around Labor Day both years (Figure 2)

 

Figure 2(Click to view Graph)

 

Yellowjacket foraging in 1998 was not enough of a problem at any of the sites to warrant any significant amount of yellowjacket baiting.  Baiting was done from mid-August through September in 1999.  However, the population of yellowjackets at all of the sites tended to rise and fall at the same time regardless of what sort of treatment was done (or not done) at each site.

 

Discussion and Conclusions:

 

Wet weather continued through June in 1998 delaying the establishment of yellowjacket nests by the over-wintering queens.  In 1999 the warm spring led to earlier establishment of yellowjacket nests as indicated by an earlier rise in the number of workers trapped in 1999 and more total yellowjackets trapped compared to 1998 ( Figure 2).  This early colony establishment is a more significant factor in overall yellowjacket numbers and pest potential than the number of queens in the environment early in the season.  If the number of queens had been the more significant factor, the overall yellowjacket numbers should have been higher in 1998 when the total number of queens was nearly twice as high (101 total queens) than in 1999 (58 total queens).  However, since most of the 1998 queens emerged too late to develop large colonies (Figure 1) the total number of yellowjackets caught was higher in 1999 (4380 total yellowjackets per trap) than in 1998 (1306 total yellowjackets per trap).

 

Since no significant yellowjacket numbers occurred in 1998 and very limited baiting was done, no conclusions can be drawn from 1998 data about the efficacy of the treatments applied at the different sites.  Yellowjacket numbers were higher in 1999 and all of the proposed types of treatments were done (queen trapping and baiting, baiting alone, and no treatment).  However, the yellowjacket population increased and decreased through the season at the same time at all of the sites regardless of which treatment was done.  This may mean that none of our treatments did any good at all.

 

There is one important subjective observation that indicates that the baiting program created a more significant yellowjacket reduction than the statistics would indicate.  The benchmark that we used to determine that it was time to begin baiting was that City workers in the park were being bothered enormously by yellowjackets foraging at the garbage cans.  Within about two weeks of the onset of our baiting program this foraging behavior had virtually ended and did not recur for the remainder of the season.  It may be that the baiting was effective at killing nests within the park, but did not effect nests further away.  Since the yellowjacket traps were located on the periphery of the park they would continue to pick up yellowjackets drifting in from distant nests.  Perhaps the yellowjackets drifted in from distant nests in sufficient numbers to alter the statistics enough to cancel a reduction in the number of yellowjackets trapped from within the park area.  One observation to support this idea is that a nest was found and destroyed within the park, inside the area surrounded by traps.  Destroying this nest did not have a noticeable effect on the number of yellowjackets that were trapped the following week.

 

On the other hand, the most intensive baiting was done at the Mad 1 location because numbers there remained consistently higher there than anywhere else throughout the season.  Foraging remained significant there and trap numbers remained high despite the baiting efforts.  Bait acceptance at this location was also higher, with the bait stations often being completely emptied by foraging yellowjackets.  This would strongly suggest that a nest or two was close by and such intensive feeding on the bait should have killed these nests.  This did not happen.  One possible explanation is that this location may have been at the intersection of the foraging perimeters of many distant nests, none of which got a large enough share of the bait to be adequately controlled.

 

Yet another possibility could explain all of these observations.  The Knoxout 2FM7 that was used to prepare the bait was about five years old and may have lost some of its efficacy.  If this is true, the yellowjackets may have fed on the bait but continued to thrive.  The reduction in foraging activity at Kid=s Kingdom may have been due to the fact that yellowjackets were feeding at the bait stations instead of the garbage cans.  Since yellowjackets flying into the park from distant nests would have had to fly past both bait stations and traps to forage in the park, they may have been diverted from foraging within the park by the dual attraction of a food source and the heptyl butyrate attractant at the bait stations and yellowjacket traps surrounding the park.

 

Finally, based on this data, the length of time that the weather is favorable for nest development seems to be a much more important factor in overall yellowjacket population than the number of queens that successfully over-winter.  This calls into question the efficacy of trapping queens in the early season to reduce yellowjacket numbers.  However, a sample size of only two seasons of data is hardly sufficient to make any broad generalized conclusions about our observations.  Therefore we need to continue trapping and monitoring yellowjacket numbers for more seasons in order to add credibility and assure the effectiveness of any long-range control program we undertake.