Page 51 - Volume 2,Issue 2, July 2009

Sublethal effects of essential oils on

Acanthoscelides obtectus

97

damage of the beans. A similar rapid action

has been reported for citrus peel essential

oil vapours against larvae of

Callosobruchus

maculatus

F. (7).

Previous studies have shown that the

essential oils and their main components,

monoterpenoids, adversely affect the

growth and development of larvae of vari-

ous species via ingestion (8, 13, 15). Our re-

sults showed that the essential oils can also

affect larval and pupal growth and develop-

ment as well as fecundity of the survivors via

their vapour action.

Exposure of

A. obtectus

larvae and pu-

pae to sublethal doses of lavender and rose-

mary essential oil vapours did not affect the

developmental time to adult emergence ex-

cept for the doses causing about 70% mor-

tality, where a slight prolongation of the de-

velopmental time was observed mainly in

the male adults. On the contrary, in the case

of the eucalyptus essential oil, a prolonged

larval developmental time was observed at

all doses tested. Differences in the chemical

composition of the three essential oils test-

ed (levanter, rosemary and eucalyptus) (17)

may account for their different biological ac-

tion against

A. obtectus

.

Regarding the effect of essential oil va-

pours on the weight of adults derived from

treated larvae or pupae, a slight decrease

was observed which, in most cases, was

not statistically significant. In similar experi-

ments, Gunderson

et al.

(8) and Lee

et al.

(15)

recorded higher levels of decrease in adult

weight (in some cases up to 60%). However,

in those experiments the insects were not

exposed to oil vapours as the essential oils

had been incorporated into their diet.

Adult longevity of

A. obtectus

seems to

depend on the stage exposed to the essen-

tial oil vapours and the sex of the surviving

adult. In fact, the longevity of the adults of

both sexes derived from larvae exposed to

essential oil vapours was not affected at all

doses tested, whereas the exposure of pu-

pae to essential oil vapours resulted in an

average 14-34% reduction in the longev-

ity of the female adults at all doses tested.

The males were less affected than females,

as the longevity of male adults was reduced

only in the case of eucalyptus essential oil.

The most severe physiological effect of

exposure of larvae and pupae to sublethal

doses of the essential oil vapours was the re-

duction in the female fecundity although the

viability of the eggs laid was not decreased

significantly. In fact, females emerged from

treated larvae or pupae laid 25-40% fewer

eggs than control females. The effects of es-

sential oil vapours on the male reproductive

system were not investigated in the pres-

ent study, but similar studies (8) have shown

that males were less sensitive than females.

Overall, an adverse impact on the devel-

opment and especially fecundity of

A. obtec-

tus

individuals surviving the exposure to va-

pours of three different essential oils should

be added to their main rapid mortality ac-

tion. However, in the interpretation of sub-

lethal effects it is difficult to differentiate

whether these effects are due to insecticid-

al action or to inherent characteristics of the

insecticide-selected individuals (6). Subse-

quently, comparison between treated and

control individuals would, thus, be biased

(2). In fact, in preliminary experiments with

A. obtectus

adults resistant to lavender es-

sential oil vapours (DP Papachristos, un-

publ.), a reduced biological potential (lon-

ger developmental time and lower intrinsic

rate of increase compared to susceptible

population) was observed, but no data are

available at the moment for the other insect

stages. The practical implications of these

findings suggest that, when

A. obtectus

is

exposed to lavender, rosemary or eucalyp-

tus essential oil vapours, satisfactory control

may be achieved even if some insects sur-

vive the treatment.

Literature Cited

Abbott, W.W. 1925. A method of computing the

1.

effectiveness of an insecticide.

Journal of Eco-

nomic Entomology

, 18: 265-267.

Alford, A.R. and Holmes, J.A. 1986. Sublethal ef-

2.

fects of carbaryl, aminocarb, fenitrothion, and

Bacillus thuringiensis

on the development and

fecundity of the spruce budworm (Lepidoptera: