Table Of ContentPROC. ENTOMOL. SOC. WASH.
93(3), 1991, pp. 786-788
A PORTABLE PENETROMETER FOR MEASURING LEAF
TOUGHNESS IN INSECT HERBIVORY STUDIES
D. P. A. Sands and V. A. BRANCATE^^
CSIRO, Division of Entomology, Long Pocket Laboratories, Private Bag No. 3, PC,
Indooroopilly, Queensland 4068, Australia.
Abstract.—An inexpensive, portable penetrometer and tongs for measuring leaftough-
ness are described. These allow measurements to be made while leaves remain attached
to the plant.
Key Words: Penetrometer, tongs, leaftoughness, insect herbivores
Leaftoughnesshasbeenrelatedtofeeding can be used to monitor the progressive
preferences of insect herbivores. Several changes in toughness of leaves whilst they
penetrometers formeasuring leaftoughness attached to the growing plant.
have been described (Williams 1954, Cher- Theportablepenetrometerandtongswere
rett 1968, Feeny 1970, Beckwith and Hel- devised to relate toughness of leaves on
mers 1976, Wright and Fuller 1984), based growing plants of the vine, Stephania ja-
on measuring the weight or pressure re- ponica (Thunberg) Miers (Menisperma-
quired to force needles or probes through ceae), to feeding by larvae ofthe fruit pierc-
leaves. However,theycanonlybeusedwhen ing moths, Othreis fullonia (Clerck) and
held upright or horizontal and are difficult Eudocimasalaminia(Cramer) (Noctuidae).
or impossible to use without removing Thepenetrometer(Fig. 1)consistedofadial
leaves from plants. A simple leafpenetrom- gram gauge, with removable probes. The
eter was described by King (1988), but it probes were made from stainless steel pins
required more than one operator for use in or rods with the apex ground flat, each 12
mm
the field. Designs for portable penetrome- long, soldered at right angles onto a
mm
tersusedto measure fruit firmnessweredis- modified 3 electrical spade receptacle.
cussed by Abbott et al. (1976) while more Probes were slid over the flat feeler tip of
sophisticated, non-portable penetrometers the gauge and easily changed if different
gave greater accuracy for measuring fruit probe diameters were required for testing
firmness (Blanpied etal. 1978, Lidsteret al. leaves of varying toughness. The gauge
AG
1978, Abbott et al. 1984). These penetrom- (Chatillon 50*) used for experiments
etersalsomeasureddeformation ofthe fruit with S. japonica was calibrated 0-50 g but
by compression (Bourne 1980) and are not gauges with 150, 300 and 500 g capacities
suitableformeasuringchangesinthetough- are available for use on plants with greater
ness ofgrowing leaves in the field. leaftoughness. A probe measuring 400 ^ni
We describe an inexpensive, portable in thickness was used for S. japonica. The
penetrometer which can be easily calibrat- tongs (Fig. 2) were constructed from 2 rect-
ed, and tongs for holding leaves while the angular polyacrylamide plates, 160 X 33 X
penetrometer is operated. Our instrument
does not require destructive sampling and John Chatillon and Sons, Inc., New York.
VOLUME NUMBER
93, 3 787
removable probe 5 mm, with tapered apices, drilled (2 mm
mm
in upper, 2.5 in lower plate) to accom-
modate the penetrometer probe. A hinge
mm
mounted between the plates 55 from
the base allowed the tongs to be used in one
hand to holdeach leaftested, whilethe pen-
etrometer was operated in the other hand.
The penetrometer was calibrated by
clamping it in a boss head clamp, pivoted
on a retort stand so that the probe rested
under pressure on an open pan ofan elec-
tronic balance. Weights registered on the
tension gauge were corrected for error by
comparing readings with those on the elec-
tronic balance. Different pressures to cover
the range ofgauge readings were obtained
by adding weights to the clamp arm.
When used on leaves ofS.japonica, pen-
etrometer readings had coefficients ofvari-
ation of generally less than 5%. Readings
showed that veins were tougher than areas
betweenveinsandthattoughnessdecreased
as leaves lost turgor after removal from the
plants (Table 1). Hairs, glands, lesions and
foreign bodies (such as scale insects) were
avoided when taking measurements. Leaf
Dial tension gauge tearing and compression were minimized
by using a small diameter probe which also
increased the sensitivity ofgauge readings,
scale I— when compared with readings using larger
2cm diameter probes.
1
Acknowledgments
We are grateful to Mr. Tony Wright for
Fig. 1. Penetrometer. advice on selectingthe tension gauge and to
Mr. Ron Smith for making the calliper and
Table 1. Penetrometerreadings* for leaves ofStephaniajaponica.
788 PROCEEDINGS OFTHE ENTOMOLOGICALSOCIETY OFWASHINGTON
scale
I 1 21 cm
1
hinge
drilled hole
mm
2 rrj
2.5mm
Fig. 2. Tongs.
probes. Mrs. TiniSchotzandMs. RoseBroe Bourne, M. C. 1980. Textureevaluationofhorticul-
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