7967

• UCREFRP Catalog Number: 7967
• Author: Charbonneau, C. S., R. D. Drobney and C. F. Rabeni
• Title: Effects Of
• USFW Year: 1994
• Copyright Material: YES

Effects of Bli on invertebrate~

275

100
90
>- 80
~ 70
1560
~ 50
~ 40
o
Cii 30
a.. 20
10
o

o Control
B Vectobac-G (6 ppm)

11 19 27
Temperature (oC)

Fig. 3. Toxicity of Vectobac-Gf, (6 ppm) to Chironomlis riparius at
three temperatures in water-only exposure (N = 3, 10 larvae per repli-
cate). G tests indicate toxicity of Vectobac-G is dependent on tem-
perature (P < 0.(05).

it significantly reduced (18-88070) populations of Chironomini
and Tanytarsini [16]. Reductions of Chironomini (3CHi7%)
were also reported in a golf course pond after an application
of 3 kg/ha (0.5 ppm) of Bti wettable powder [16]. A Bli flow-
able formulation was applied to a California duck club pond
and resulted in a 76070 reduction in chironomid numbers [20].
A flowable formulation of Bti was also tested on field exper-
imental plots in California at 0.25 and I kg/ha. Chironomid
populations increased rapidly after an initial decrease, indi-
cating a short-term effect [I8]. Garcia et al. [17] tested strains
of Bti formulated by different companies (Sandoz WDC-l
and Goldberg ONR6O). The Goldberg formulation was tested
in a freshwater stream and resulted in 100% mortality of the
chironomids in <6 d. The Sandoz formulation was tested in
a freshwater reservoir, where there was 15 to 100% mortal-
ity of chironomids in 24 h, and in sewage ponds, where there
was 58 to 92% mortality.
Even though we concluded little or no impact to inverte-
brates in our field tests, toxicity tests conducted with field-
collected larvae determined that these species were susceptible

100
100 100 90
90 ~ 90 .~ 80
~ 80 a; 80 IZI Control ~ 70
~ 70 fJ Control t: 70 . Vectobac-G 0 60
0 0
60 II Vectobac-G ~ 60 ~ 50
~ 50 50
E E 40
E 40 Q) 40 Q)
Q) 0 0 30
0 30 0; 30 0;
Q; 20 a.. 20 a.. 20
a..
10 10 10
0 0 0
9.1 40.6 6.7 40.6
Depth (em) Depth (cm)
A B

to Yectobac-G under controlled laboratory conditions, espe-
cially at five times the operational level. Conflicting results
among laboratory tests with laboratory-cultured and field-
collected chironomid larvae and conflicting results among the
field enclosure tests and literature indicate that toxicity of
Yectobac-G can vary. Other researchers have reported dif-
ferences between laboratory toxicity tests and field studies
[59-60]. Estimates of the LC50 of Bti to mosquito larvae
were higher in the field than in the laboratory [60]. This dif-
ference could be from settling of the heavy insoluble crys-
tals [30] or microbial degradation of Bti crystals under field
conditions [59]. We believe that the variation between our
laboratory and field results was caused by a number of in-
teracting environmental factors that were present in the field
and may have reduced efficacy of Vectobac-G to benthic
organisms.
Of the conducted tests, temperature, water depth, mac-
rophyte surface area coverage, and instar differences affected
the efficacy of Yectobac-G to benthic chironomids. Temper-
ature was positively associated with mortality. Feeding rates,
both in mosquitos [31-32,61-62) and black flies 123) may ac-
count for the influence of temperature. Bti is a stomach poi-
son and must be ingested to produce a toxic effect. At higher
temperatures, filter feeders (including mosquitoes, black
flies, and chironomids) may increase their filtering rate and
as a consequence may ingest more of the toxin, resulting in
higher mortality.
Vectobac-G was less toxic to chironomid larvae in deeper
water. This decrease has been attributed to the settling and
rapid denaturing of the Bti crystal as it sinks [27].
Results indicating that macrophyte coverage above the
sediments reduces efficacy of Vectobac-G to chironomid lar-
vae have been previously reported, and the reduction was at-
tributed to the adhesion of settling Bti material (pellets and
crystals) to the leaves of submerged vegetation or the removal
of Bti by feeding snails and other organisms (28). The pre-
ceding factors result in decreased amounts of Bti material
available to benthic invertebrates for consumption, thus re-
ducing mortality. Algal mats also reduce the effect of Bti on
mosquito larvae [26], because the pathogen Bti remains on

rJ Control
IjFVIR
EI 5xRAR

14.3 37.1
Depth (cm)
C

Fig. 4. Efficacy of Vectobac-GI! to midges at three water depths. Water depth test I (A) and water depth tesl 2 (B) were conducted with
Chironomus riparius in water-only exposures (N = 3, 10 larvae per replicate). Field depth test (C) with Micropseclra I Tanytarsus) spp. was
conducted in the Minnow Pond. Minnesota Valley National Wildlife Refuge, Bloomington, Minnesota (N = 3. 10 larvae per replicate). Es-
caped larvae in water depth tesl 2 were not considered in the analysis. Vectobac-G was applied at the surface area recommended application
rate (RAR) of the lest vessels.