ABSTRACT: Vectolex© sand granules were
applied to three Beverly, Mass. high salt marsh plots at 10 pounds/acre,
and three at 20 pounds/acre. Reduction of Aedes cantator larvae
in application plots was excellent, with mortalities being significantly
greater than in control plots. Larval control did not appear to
persist into the next flooding although rain-flood patterns made
the analysis less than ideal. A subsequent application in Newbury
was less effective. There was evidence of a statistically significant
reduction in copepods four days after the application although
considerable numbers remained.
INTRODUCTION
Bacillus sphaericus along with Bacillus subtilis var. israelensis (which as you know, is generally just called B.t.i.) and the juvenile growth hormone analogue, methoprene, represent the major agents of biological control of mosquitoes. The last two have been widely studied and adopted as standard tools in the mosquito control armamentarium. In fact B.t.i. as VectoBac© is extensively used on salt marshes by Northeast Massachusetts Mosquito Control and Wetlands Management District.
While B.t.i. works because the encysted stage has a high toxin content, B. sphaericus invades the intestines of the mosquito larva and is able to penetrate through them into the hemocoel and reproduce and then overwhelm the larva with toxin. Since B. sphaericus uses active bacteria rather than cysts there has been some suggestion that the bacteria may survive and yield longer control periods than B.t.i.
B. sphaericus has been around for quite some time. Our first NMCA Jobbins Scholar Stephen Wraight, in 1978, was working on B. sphaericus use in black-fly control, where it has found its greatest application. It has only slowly been adopted for mosquito control for a variety of reasons, among them being the availability of many less expensive alternatives, including in this particular category, B.t.i.
Last year, at the New Jersey meetings, three of us heard a presentation by Chort and coworkers on the successful use of B. sphaericus against several mosquito genera in different habitats.
Among these were salt marsh Aedes for which the product is not normally recommended. Abbott Laboratories were making available one-pound cans of their B. sphaericus product, Vectolex©,
impregnated on corn cob granules. We resolved to try this material.
METHODS
During the early summer of 1997 several of us spent time examining a number of sites for testing. We settled on a site in Beverly, Massachusetts off Rt. 127 to the east and very slightly north of the intersection of Hale street with West Street. The marsh, on privately owned land, is sandwiched between the highway and a railroad embankment.
We established nine rectangular plots of 400ft2 area marked at comers with lath and labelled. Three were treated with a dose of 10 lbs. Vectolex/acre (1.47 oz.), three with 20 lbs. Vectolex/acre (2.94 oz.), and three were left as untreated controls. A very dry summer, partly spent on site searches, left us unable to make our first applications until July 23. Applications were made with pre-weighed materials dispensed by a salt-shaker like device.
Mosquito larvae were censused by taking a minimum of five and a maximum of ten dips per plot. Mosquitoes were censused on July 23, 25, 27, August 6, 8, 11, 22. Pre-treatment numbers of larvae were substantial.
Zooplankton was also censused by the same collecting technique
except that ten dippers-full were poured through a plankton net
and the material was preserved in formaldehyde for later census.
Zooplankton was collected only on July 23 and July 27, but on
August 11, dippers were examined by eye and all plots showed substantial
zooplankton present.
MOSQUITO RESULTS
Figure 1 integrates all of the mosquito data. Plots began with similar numbers of larvae, if anything somewhat fewer in the controls. By July 25 at least 99% of the larvae, Aedes cantator, were dead. This was still so on July 27. Larvae in the control plots continued to do well and emergence was noted. Then the site dried out.
The site was slightly flooded by rains, and on August 1, 6 and 8 there were apparently healthy Aedes sollicitans larvae in the test plots. There was so little water that dipping quantitatively was not possible and the plots dried out again.
Then on August 21 a spring tide extensively reflooded the site. At this time fish became dispersed throughout the site and no mosquito larvae were found. During the earlier part of the study the fish had been sequestered into a section of the marsh which we were not using in the study.
We also did a small study at Newbury, with less clear results.
This may be because the Vectolex, a different batch, had been
in storage for more than a year. We hope to re-examine this question
next year.
Figure 1. Census of mosquito larvae for whole study.
ZOOPLANKTON RESULTS
Copepod numbers were quantified only on the day of application,
just prior to application, and two days later. As Figure 2 shows,
controls changed little, but there were statistically significant
reductions of copepod numbers in both B. sphaericus treatments.
These effects were largely upon the harpacticoid copepods which
considerably outnumbered the cyclopoids. The errors for these
data are large, still there appears to be a reduction (Table 1).
We plan to check this effect again, and also to correlate it with
direct laboratory applications. The latter are difficult because
we could not, in practicality, fractionate the material supplied
for field use into so small a dose as would be needed in the lab.
Figure 2. Copepod numbers on one pre-treatment date and two post-treatment
dates.
TABLE 1. Mean numbers of copepods per 10 dips in test and control plots, SE in ( ).
CONTROL 10 LBS./ACRE 20 LBS./ACRE
JULY 23 291 (82) 1040 (530) 1651 (707)
JULY 27 233 (141) 402 (95) 558 (269)
CONCLUSION
We achieved excellent control with Vectolex© at both doses within two days. After this, the situation was never ideal for evaluation. Rain was not adequate to follow the next hatch of larvae through their whole cycle. As far as we could observe, the material was not effective at this time, although it would have been better if we could have followed the larvae through the cycle. Also, since this larval brood was largely A. sollicitans, a difference in susceptibility could have been involved. Again the next tidal flooding brought fish which precluded a last check of the Vectolex© against larvae. Thus, at this time, we can only conclude excellent effectiveness against the first brood with no residual effectiveness against the next. It was our intention to test the susceptibility of Ae. sollicitans, but our site received some upland drainage and had little tidal flooding for some time. For whatever complex of reasons, we found that we had a hatch largely of Ae. cantator on the first brood.
Copepod numbers, particularly harpacticoids, were reduced significantly
four days after treatment, although large numbers remained, actually
exceeding those in the control plots.
FUTURE OBJECTIVES
As of now, we continue to be interested in a number of things:
1. Vectolex© as a possible back-up for B.t.i if resistance develops.
2. Vectolex© evaluation for fresh-water use.
3. Vectolex© shelf life is something we would like to know more about because of our lesser results at Newbury with year- old material.
4. Vectolex© effectiveness on Aedes sollicitans.
5. Vectolex© residual potential in the salt marsh.
6. Vectolex© effectiveness on late instar larvae.
7. Larger salt marsh treatment plots which would contain more diverse larval habitats within each test plot.
8. In-lab examination of the zooplankton toxicity question.
Chort, Dana, Claudia O'Malley, Michael Romanowski and Roderick Schmidt. 1997.
Four Weddings and a Funeral: Operational Trials of Vectolex CG©
in New Jersey. Proceedings of the N.J. Mosquito Control Association.