Scolypopa Australis) Eggs

Horticultural Insects 120 Cold hardiness and effect of winter chilling on mortality of passionvine hopper (Scolypopa australis) eggs D.P. Logan1, C.A. Rowe2 and P.G. Connolly1 1The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand 2The New Zealand Institute for Plant & Food Research Ltd, 412 No. 1 Road, RD2, Te Puke 3182, New Zealand Corresponding author: [email protected] Abstract Passionvine hopper (PVH; Scolypopa australis) is a significant production pest of kiwifruit in New Zealand and an occasional pest of some other crops. It is also associated with toxic honey. Estimated losses of kiwifruit due to sooty mould associated with feeding by PVH varies from c. 0.5–3% of Class-1 fruit packed, depending on year. Mortality of overwintering eggs due to winter chill may contribute to this inter-annual variation. After cold-hardening under ambient winter conditions, eggs were exposed to six temperatures between -11°C and 0°C. The median minimum lethal temperatures for a 1-h exposure was -9.1°C. Longer exposures (up to 24 h) did not strongly influence mortality at different sub-zero temperatures. Mortality of eggs held at a range of North Island sites was most strongly correlated with the sum of chilling hour degrees (CHD) below a threshold of 10°C in August (r=0.89). Keywords Scolypopa australis, kiwifruit, winter, mortality. INTRODUCTION Passionvine hopper (PVH), Scolypopa australis wood, between January and June. (Walker) (Hemiptera: Ricaniidae), is a significant In kiwifruit, feeding by nymphs and particularly production pest of kiwifruit in New Zealand and adults is associated with the development of sooty is also associated with toxic honey (Robertson moulds on fruit, which makes them unsuitable et al. 2010). Its native range is the coastal areas for export markets. The estimated seasonal loss of south-east Australia and it has been present due to sooty mould is c. ≤0.5% of Class-1 fruit in New Zealand since the late 1800s (Riley & packed; however, in some seasons losses may be c. Howard 1891). Passionvine hopper has a one- 2–3% with minimum annual costs to the industry year life cycle with nymphs hatching in late spring of NZ$15–20M (Shane Max, pers. comm., 2010). and adults present from summer until early Annual variability of PVH numbers is one factor winter (Cumber 1966). Adults and nymphs tend that may influence inter-annual variation in fruit to aggregate on the actively growing shoots of a loss due to sooty mould. Populations of PVH tend wide variety of plant species (Spiller & Wise 1982, to be higher in areas adjacent to kiwifruit orchards, Logan et al. 2002) where they feed on phloem such as areas of weedy regrowth or home gardens, sap. Eggs are inserted shallowly into dying and than on kiwifruit crops (Steven 1990) and dead plant material, as well as structures built of effective control within orchard blocks is negated New Zealand Plant Protection 70: 120-130 (2017) www.nzpps.org © 2017 New Zealand Plant Protection Society (Inc.) www.nzpps.org Refer to http://www.nzpps.org/terms_of_use.php Horticultural Insects 121 by immigrating adults. Hence it is important to air temperatures recorded between 2001 and 2016 measure mortality factors in non-crop areas that at the Te Puke Research Orchard of Plant & Food may influence PVH populations. The best-studied Research (TPRO) (37.8205S, 176.3175E, WGS84 natural PVH mortality factor is egg parasitism, ellipsoid) and Nelson aerodrome weather station which can be substantial (up to 87%) but has not (41.3008S, 173.2160E) (hortplus.metwatch.co.nz prevented pest problems in kiwifruit production and cliflo.niwa.co.nz respectively, final access on areas (Cumber 1966; Gerard 1985; Charles & 4 April 2017). Allan 2004). Climate and weather events can be important drivers of insect population dynamics Egg collection and mortality assessment in general (Andrewartha & Birch 1984; Chown & Adult PVH were collected from various host Gaston 1999) and, according to Charles & Allan plants at TPRO during early March 2012–2015 (2004), large seasonal variation in PVH numbers and were placed in cages with sticks of balsa wood may also be induced by seasonal weather patterns. (100 mm x 5 mm) as oviposition sites secured to Passionvine hoppers overwinter as eggs and need potted mahoe plants (Melicytus ramiflorus). Eggs to survive frosts in most kiwifruit production were laid during March and early April, and at areas. The absence of PVH from much of the least 120 balsa-wood sticks with >100 eggs each North Island volcanic plateau and from the South were placed in fine muslin bags to exclude egg Island (except for the coastal margins of the parasitoids and predators. Bags were hung in a northern quarter) is consistent with a hypothesis shaded area outdoors at TPRO before and after that cool weather, either as severe chill events or experiments (July–August for lab experiments as an accumulation of cold, is associated with or May–September for field surveys). Eggs were egg mortality. thus acclimated to Te Puke autumn and winter Our objectives were to assess the mortality temperatures. Fruit stalks (approximately 50) of PVH eggs following exposure to relatively heavily infested with PVH eggs were collected brief periods of cold shock at temperatures from orchard blocks of kiwifruit (Actinidia ≤0°C in laboratory tests and to actual winter chinensis var. deliciosa ‘Hayward’) at TPRO for temperatures experienced at a selection of 16 one experiment in 2015. field sites in the North Island that differed in To determine mortality, eggs were dissected latitude and elevation. These two approaches from balsa-wood sticks or kiwifruit stalks during were used as there may be different mortality the autumn following the year of the experiment. responses to short-term chill associated with Assessment was delayed to allow hatching to be extreme events such as frosts and long-term completed naturally and for any parasitoids to exposure to winter temperatures (Bale 1996). emerge. Hatched eggs were readily distinguished The data will be important for assessing whether from dead eggs by having an empty hyaline or not cold weather in kiwifruit-growing regions chorion with a characteristic split (Cumber is associated with variation of losses to sooty 1967). Parasitised eggs were separated from dead mould through its effects on PVH eggs. or hatched eggs by the round hole created by the escaping parasitoid (Cumber 1967). There were MATERIALS AND METHODS some parasitised eggs in most samples but the rate Extreme minimum air temperatures was generally less than 5% (range 0.4–9.9%) and The major kiwifruit production area in New these eggs were excluded from counts of dead eggs. Zealand is around Te Puke in the Bay of Plenty region of the North Island. The southern-most Laboratory experiments area where kiwifruit are grown commercially is Cool-store: The effect of 0°C at durations of 1, 2, Nelson in the South Island. Therefore, the coldest 4, 7, 10, 14, 17, 21 and 24 days on the mortality air temperatures typical of these two areas were of PVH eggs in balsa wood sticks was determined determined using minimum monthly dry-bulb in a cool store unit during July and August © 2017 New Zealand Plant Protection Society (Inc.) www.nzpps.org Refer to http://www.nzpps.org/terms_of_use.php Horticultural Insects 122 2012. The temperature was controlled to 0°C of treatments was randomised for each group of by a commercial calibrated monitoring system replicates. Temperatures were scanned at 1-min accurate to 0.5°C. intervals with thermocouples (Type T, copper- Water bath: Four subsequent experiments were constantan) and recorded by a multi-channel carried out using three refrigerated water baths data logger (Campbell CR10). All thermocouples filled with a 50% antifreeze solution (PEG6000). were calibrated against a standard thermometer Eggs within balsa wood sticks or fruit stalks were immediately prior to the experiments. To measure inserted into an aluminium tube with base sealed temperatures experienced by eggs in each water (120-mm high, 20-mm wide, inner wall thickness bath experiment, a thermocouple was attached to a 0.5 mm). The tube was then plugged at the top and balsa wood stick or fruit stalk containing PVH eggs. held immersed in the antifreeze solution with the aid of a retort stand. In Water-bath Experiment 1, Statistical analyses for laboratory experiments ten replicate balsa wood sticks (average 172 eggs; A quasibinomial Generalised Linear Model range 77–227) were used to examine the effect (GLM) was used to establish the effect of of two 1-h exposures compared with a single storage time (1–24 h) at 0°C on egg mortality. 1-h exposure at each of six constant sub-zero The quasibinomial model adjusts for the temperatures (-1, -3, -5, -7, -9 and -11°C). The over-dispersion of the data. A comparison second 1-h exposure was given 14 d after the first. of deviance for binomial GLMs was used to During the interval eggs were kept at ambient determine the effect of 1-h and 2-h exposures winter temperatures (range of approximately -2 to sub-zero temperatures on egg mortality. To to 18°C). A 1-h period was selected to represent estimate an LT50, data were divided into four a natural period of extreme chilling. In Water- pseudo-replicates (Chambers & Hastie 1992). bath Experiment 2, the median lethal temperature Separate binomial GLMs were used to fit lines (LT 50) was estimated using 20 replicate balsa wood to mortality points at each of six temperatures. sticks (average 183 eggs, range 69–298) exposed at From each of these lines, the temperature each of six sub-zero temperatures (nominally -1, corresponding to 50% mortality was estimated, -3, -5, -7, -9 and -11°C).

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