Thrips
and Pansy Spot on Apple
At a recent orchard meeting, we discussed a symptom of
thrips infestation on apple called pansy spot.
This injury looks like the petals of a blossom, light in color against
the color of the apple skin (Fig. 1). Thrips
injury referred to as pitting was recognized in the 1950s (Swift and Madsen
1956); if the injury was no too severe, fruit outgrew the injury before
harvest. A paper a few years later
reported a discoloration of apple skin referred to as pansy spot (Madsen and
Jack 1966). Pansy spot was determined to
be caused by a reaction to the egg punctures made by thrips (the main culprit
was western flower thrips (WFT), Frankliniella
occidentalis). Certain varieties
were more sensitive than others, notably McIntosh and Spartan (the variety of
concern at our orchard meeting was Pink Lady).
These thrips move from host to host as each comes into bloom – their
host range is broad. Thrips are present
during bloom, when they lay eggs in flower parts. The resulting generation
matures, and the resulting adults perform additional oviposition into the
developing apples. The apple tissue
surrounding this cohort of punctures develop the pansy spot discoloration. Other apple varieties experienced the thrips
oviposition, but did not develop pansy spot.
Madsen and Jack (1966) determined that the best time to apply control
measures was petal fall, when nymphs are present but have not produced the
offending adults. Later work on WFT in
apple has shown that the highest egg injury is not necessarily associated with
the highest thrips numbers (Terry 1991).
Often eggs are laid in sepals or other tissues where the injury does not
become apparent. Eggs are often laid on
more mature clusters, and on king blooms rather than lateral buds.
A similar timing to Madsen and Jack (1966), but possibly a
little earlier, was determined more recently in British Columbia (Cockfield et
al. 2007), who found the greatest impact by a spray resulted from application from
full bloom to about 5 mm fruit diameter.
Few eggs were laid in ovary tissue during bloom – eggs in this tissue
(most likely to cause pansy spotting) increased starting about 8-13 days
following full bloom. Obviously bloom
sprays can’t be used in order to protect pollinators; therefore the optimal
spray timing would be from petal fall until 5 mm fruit diameter.
Sampling
Thrips can often be detected by examining the interior of
floors, or rapping blooms on a sheet of paper to dislodge the tiny
insects. However, additional sampling
methods have been developed. Different
color sticky panel traps were compared in British Columbia (Bradley and Mayer
1994). Blue and white traps caught 4-
and 3- times more thrips than yellow traps.
Several chemical attractants were added, but these did not provide
enough added attraction to be worthwhile.
Beating trays still compared favorably for versatility and speed of data
collection.
In Australia, sticky traps were found to be better than
beating samples, and blue traps were also considered the best (Broughton and
Harrison 2012). Chemical attractants
helped with some thrips species (including WFT) but not others.
Environmental effects and biological control
There is variable natural mortality of thrips. There is a non-feeding pupal stage that
occurs beneath the soil surface. If
there is significant rainfall at this time, many of these pupae drown, resulting
in lower adult populations.
Various types of ground cover were compared in British Columbia
apple orchards: Bare soil, grass and weedy ground covers were compared for
their impact on WFT populations. Weedy ground cover increased WFT in trees
during the first week of bloom, but this did not cause an increase in fruit
injury. The F1 generation of WFT (the generation causing pansy spot) in cluster
samples were lower in trees where green lacewing larvae were introduced at
bloom. Still, these population differences were not associated with differences
in fruit injury (Cossentine et al. 1999). Nicholls et al. (2000) also found thrips
populations were reduced by flowering ground covers, apparently by fostering
natural enemies. Anthocorids such as Orius
are effective predators of thrips.
Chemical management of thrips
Thrips are prone to develop resistance to insecticides; it
is therefore important to rotate among mode of action classes. However, options are limited. In our control recommendations (VCE 2018),
the following materials are listed for thrips on stone fruits (we have not
included thrips in the apple section until now; the peach materials are also
available in apple): Assail (G), Delegate (E), Entrust (E), Lannate (G). Another material, with an additional mode of
action is Surround (kaolin). This may be
an effective tool against WFT; Larentzaki et al. (2008), reported that kaolin
increased mortality, decreased oviposition, and decreased hatch of eggs in
onion thrips, Thrips tabaci.
At our orchard meeting, we discussed the potential impact
of the plant growth regulator Apogee (prohexadione-calcium). While there is little information on the
efficacy of this use, there has been some promising research on effect of
Apogee on other insects. Paulson et al.
(2005) examined the role against pear psylla (PP), spirea aphid (SA) and
obliquebanded leafroller (OBLR). PP
populations were reduced in pear, as well as SA and OBLR in apple. Not only was there direct mortality, but
there was a synergistic effect against PP and SA when imidacloprid was added. Tsagkarakis et al. (2012) reported that
Apogee reduced egg production and survival of Asiatic citrus psyllid. There should be research to further
investigate the role of Apogee since not all studies have been consistent – for
example, no effect was reported for potato psyllid (Prager et al. 2013).
References
Bradley, S. J., and D. F. Mayer. 1994. Evaluation of monitoring
methods for western flower thrips,
Frankliniella occidentalis (Thysanoptera: Thripidae), during the blossom
period of 'Granny Smith' apples. J. Entomol. Soc. Brit. Colum. 91.
Broughton, S., and J. Harrison. 2012. Evaluation of
monitoring methods for thrips and the effect of trap colour and semiochemicals
on sticky trap capture of thrips (Thysanoptera) and beneficial insects
(Syrphidae, Hemerobiidae) in deciduous fruit trees in Western Australia. Crop
Protect. 42: 156-163.
Cockfield, D., E. H. Beers, D. R. Horton, and E. Miliczky.
2007.
Timing of oviposition by western flower thrips (Thysanoptera: Thripidae) in
apple fruit. J. Entomol. Soc. Brit. Colum. 104: 45-53.
Cossentine, J. E., E. J. Hogue, and L. B. M. Jensen. 1999. The influence of
orchard ground cover and introduced green lacewings on spring populations of
western flower thrips in apple orchards. J. Entomol. Soc. Brit. Colum. 96.
Larentzaki, E., A. M. Shelton, and J. Plate. 2008. Effect of kaolin
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Prager, S. M., O. M. Lewis, K. Vaughn, and N. Nansen. 2013. Oviposition and
feeding by Bactericera cockerelli (Homoptera:
Psyllidae) in response to a solar protectant applied to potato plants. Crop
Protect. 45: 57-62.
Swift, J. E., and H. F. Madsen. 1956. Thrips damage to
apple. J. Econ. Entomol. 49: 398-399.
Terry, L. I. 1991. Frankliniella
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E. Rogers, and T. M. Spann. 2012. Applications of plant growth regulators to
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VCE. 2018. 2018 Spray Bulletin for Commercial
Tree Fruit Growers. Va. Coop. Ext. Serv. Publ. 456-419.
