Detection of larval pea weevil (Bruchus pisorum) (Linnaeus) infestations in imported peas

3 Pages·0.091 MB·English

by  Somerfield K.G.

Detection of larval pea weevil (Bruchus pisorum) (Linnaeus)
infestations in imported peas
K. G. SOMERFIELD*
Plant Protection Centre, Ministry of Agriculture & Fisheries, P.O. Box 24,
Lincoln, New Zeland
ABSTRACT
Pea weevil (Bruchus pisorum) is an important potential pest of peas grown in New
Zealand and imports containing the pest must be intercepted. Detecting immature
stages inside dried peas is difficult. However soaking peas in water allows them to
swell and soften, and facilitates cutting and checking peas internally for immature
stages of the bruchid. This increases the efficiency of detection compared to visual
methods of external examination.
Keywords: Bruchidae, Bruchus pisorum, pea weevil, detection, Pisum satiuum.
INTRODUCTION
Pea weevil, Bruchus pisorum (Linnaeus) (Coleoptera: Bruchidae), has been detected
entering New Zealand in imported peas (Pisum sativum) for many years (Richardson 1979;
*
Present address: Plant Protection Centre, Ministry of Agriculture and Fisheries, Private Bag,
Levin, New Zealand.
82 New Zealand Entomologist, 1989,Vol. 12
Keall 1981; Townsend 1984). The potential importance of pea weevil to the New Zealand pea
industry became more apparent in 1973 when pea weevil infestation was found in New Zealand
"Greenfeast" peas grown in the same area as imported North American peas, at Irwell,
Canterbury (Somerfield 1977). Previously detection of immature stages of pea weevil in peas
relied heavily on visual inspection of the outside of peas for weevil entry points ("stings"). In
1983 a water soaking technique described here was tested for improving the detection of pea
weevil infestations inside peas when live B. pisorum occurred in imported peas at Christchurch
(Fig. 1).
Fig. 1: Pea weevil (Bruchus
pisorum) adult on imported peas.
Note exit holes.
MATERIALS AND METHODS
In September 1983 a consignment of 17.3 tonnes of peas in 692 multiwall bags imported from
Twin Falls, Idaho, U.S.A. were delivered to a Christchurch store. The peas were found to
contain live adult pea weevils (B. pisorum) and the Ministry of Agriculture & Fisheries (MAF)
was advised. A large composite sample of peas was taken for assessment of infestation levels at
Lincoln Plant Protection Centre. From this a random subsample of 2,256 peas was drawn and
adult pea weevil infestation inside the peas was assessed by external examination for entry
points (stings), emergence holes, peas with "windows" (small round translucent areas on the
surface of peas indicating areas where weevils would subsequently emerge) or the presence of
adult pea weevils. Only 1 "window" and 1 adult pea weevil were found during this examination
indicating an approximate rate of infestation in the pea bulk of < 0.1 %.
To test the soaking technique, 13 pea samples (100/sample) were drawn randomly from the
large composite sample. These were soaked in 35 DC water for 3 hours to allow the peas to
swell and soften. Assessments were then made of every pea in each of the 13 samples, first by
careful external examination of each pea, and then by cutting each open with a scalpel to check
for the presence of larvae. The size of larvae, and the amount of boring inside peas, varied
considerably. Each l00-pea sample required 15 to 20 minutes to examine, cut open and record
the number of larvae. Total time required over 13 samples was 1 day using 4 persons.
RESULTS AND DISCUSSION
Results of examination of each of the samples are given in Table 1.
Individual infestations recorded for 100-pea samples show infestation rates ranging from 1 %
to 6%, with an average value of 3.6%. This represented a 36 times increase in the rate of
detection of infestation compared with the results of examining unsoaked peas. Soaking peas in
water increases the efficiency of detection of pea weevil infestations in two ways: (i) the peas
swell up, giving a smooth round surface which is easier to inspect for entry points, and (ii) the
peas are then easily split open revealing any pea weevil larvae
New Zealand Entomologist, 1989, Vol. 12
Table 1: Infestation of pea weevil larvae in imported peas from
a Christchurch store
Sample No. of peas
Number with visible No. of peas
(100 peas/ entry points with weevil %
sample) ("stings") larvae inside Infestation
1300 45 47 3.6%
Average infestation
* Based on an actual sample of 158 peas. Count scaled to 100
inside the peas. The technique provides a worthwhile increase in detection efficiency,
although this could vary considerably depending on the extent of development of larvae
inside peas, and the ratio of emerged adults to unemerged larvae. By comparison, pea
weevil infestation rates of up to 75% have been reported from overseas.
There is a close correlation betweer, the number of peas considered to be infested on
the basis of observed entry points (45) and the number of peas subsequently confirmed
infested by dissection (47). Assessment of "stings" on soaked peas is therefore a useful
guide to the number of larvae present internally.
Detection of "stings" on unswollen wrinkled peas is difficult but possible. A MAF
Advisory Officer visiting the labocatory was requested to examine a random sample of
100 peas for "stings". He detected 1, which indicated an infestation rate at the lower end
of those recorded using soaked peas.
Based on other observations in the laboratory it is estimated that it should be possible
to detect infestations of pea weevil as low as 0.5 % to 1% within a few minutes of checking
samples of soaked peas.
ACKNOWLEDGEMENTS
Assistance with pea weevil infestation assessments was provided by Miss Lindsay Emms,
Dr Barney Stephenson and Mr Maurice O'Donnell, Plant Protection Centre, Lincoln
and is gratefully acknowledged.
REFERENCES
Keall, J. B., 1981: Interceptions of insects, mites, and other animals entering New Zealand during
1973-1978. Levin Plant Health Diagnostic Station, Ministry of Agriculture and Fisheries.
661 pp.
Richardson, C. A., 1979: Interceptions of insects, mites, and other animals entering New Zealand
during 1966-1972. Levin Plant Health Diagnostic Station, Ministry of Agriculture and Fisheries.
473 pp.
Somerfield, K. G., 1977: Insects of economic significance recently recorded in New Zealand.
New Zealand journal of agricultural research 20: 42 1-428.
Townsend, J. I., 1984: Interceptions of insects, mites, and other animals entering New Zealand
during 1979-1982. Levin Plant Health Diagnostic Station, Ministry of Agriculture and Fisheries.
647 pp.