Fourth International Bemisia Workshop International Whitefly ...

Journal of Insect Science | www.insectscience.org ISSN: 1536-2442
initial 36 pairs started in the first experiment, 11 pairs
remained intact for the 24 day duration. Eight of the 11
pairs produced total brood female proportions ranging
from 0.59 to 0.77, while the other three pairs produced
sex ratios near parity. Five of the eleven pairs
produced total adult progeny that exceeded 500, and all
but one pair had total adult progeny in excess of 400.
The highest number of progeny produced was 608;
mean progeny for all 11 pairs was 439. Of the
remaining pairs of the original 36, surviving females
that lost mates soon began laying unfertilized eggs
based on records of subsequent adult emergence. The
decline of female progeny was quite rapid and usually
complete within three days following loss of the male.
These observations were substantiated in the second
experiment in treatments that involved isolation of
females following a period of coupling with a male. In
contrast, females initially isolated from males
produced only male progeny until being paired with a
mate, then began producing female progeny within one
day of being paired; only male progeny were produced
by females that remained unpaired for the duration of
the experiment. These results demonstrate that males
must be available for repeated copulations throughout
the lifetime of females for egg fertilization to occur.
Rapid shifts in sex ratio in both greenhouse colonies
and field infestations may occur according to the
availability of males for mating.
Relationship of Bemisia tabaci Adult
Mortalities and Population Control to
Imidacloprid Concentrations in
Cantaloupes
S. J. Castle 1 and J. C. Palumbo 2
1 USDA, ARS Arid-Land Agricultural Research Center, Maricopa,
AZ, USA.
Correspondence: scastle@wcrl.ars.usda.gov
2 Department of Entomology, University of Arizona, Yuma
Agricultural Center, Yuma, AZ, USA
Decision-making in chemical pest management
typically depends upon information concerning how
well a candidate pesticide performs against a particular
pest species in a particular crop environment. The
principal source of such information has traditionally
been accumulated field efficacy data produced under
variable trial circumstances, often outside the
experimenter’s control. Factors such as the type and
age of the experimental crop, the degree of pest
pressure from resident and immigrant populations, the
level of natural mortality due to biotic and abiotic
sources, etc., all potentially influence the outcome of
each experimental trial. Thus, efficacy profiles for
insecticides usually depend upon a consensus
evaluation of individual trials conducted over a broad
range of conditions. The process of consensus is a
practical phenomenon whereby experimental field trial
results are combined with the on-farm experiences of
consultants and growers to arrive at a general and
informal ranking of insecticides and how each
performs against particular pest and crop
combinations. While this system of evaluating
insecticide efficacies has provided pest managers with
rough guidelines on what works best in a given
situation, it has done little towards identifying the
activity profile of an insecticide in a crop over time.
Greater confidence in the fate of an application might
help reduce ‘insurance’ treatments that pest managers
are sometimes compelled to apply because of
uncertainty about persistence in the crop and level of
control being exerted on a target population. The
commercial availability of ELISA kits for quantifying
residues of imidacloprid and thiamethoxam has made it
possible to monitor titers of each of these systemic
insecticides in plant tissues and extracted fluids.
Quantification of residues provides a direct measure of
activity within a plant, in contrast to indirect measures,
such as monitoring insect densities post treatment. In
the case of spring and fall cantaloupes grown in
Arizona and California, titers of imidacloprid and
thiamethoxam were measured in leaf tissue collected
from plants in different stages of growth and under
various treatment regimes. One of the choices facing
growers each spring is whether to apply only one
application at planting, as a sidedress, or as split
applications. Results from two consecutive spring trials
showed that significantly higher and more persistent
titers of imidacloprid were attained when a single
application was made at planting, compared to a full
sidedress or split treatments. Much higher titers
(>10-fold) occurred in older, fully expanded leaves,
compared to younger, growing leaves. Mean titers
remained moderately high up to six weeks after
planting, but then declined to a level where Bemisia
tabaci nymphs began to establish. The relationship of
imidacloprid titers to mortality of B. tabaci was further
investigated by conducting in-field bioassays of adults
attached to leaves with clip cages. Mortality was
scored after 48 h, then leaf punches taken from the
location where clip cages had been attached. Although
adult mortality was generally low in each set of
bioassays conducted, even when imidacloprid titers
were high in leaves, nymphal densities remained low
until later in the season. Differential mortality between
adults and nymphs may be the reason why nymphs are
controlled in the field at the same time that adults do
not readily die. Sub-lethal effects, however, may
prevent adults from actively feeding and
ovipositioning on treated plants.
Journal of Insect Science: Vol. 8 | Article 4 13