Control of insect pests in persimmon with spray oils.
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CONTROL OF INSECT PESTS IN PERSIMMON
WITH SPRAY OILS
R.E. GASKIN1, B.H. ROHITHA2 and P.T. HOLLAND2
1NZ Forest Research Institute Ltd, Private Bag 3020, Rotorua
2HortResearch, Ruakura Research Centre, Private Bag 3123, Hamilton
The efficacy and safety to plants of mineral spray oils for use against
mealybugs were assessed in field trials on persimmon. Repeated
applications of oil and adjuvant-enhanced oil in the three months prior to
harvest had no deleterious effects on fruit size or quality. Fortnightly
applications of such oils in a commercial orchard maintained fruit insect-
free. Oil sprays, enhanced with alkylsilicone adjuvant, provided significant
control of mealybugs, and mites, in a persimmon orchard subjected to
intensive pest pressure close to harvest. Addition of oil to insecticide used
at half the recommended rate gave equivalent insect control to the full-rate
insecticide regime. Oil had no effect on insecticide residues, which were
halved in the half-rate treatment.Keywords:
pest control, mineral oil, alkylsilicones, pesticide residues,
The use of mineral oils as environmentally benign materials with useful insecticidal
properties is increasingly attractive, as consumer concerns over pesticide residues inproduce continue to grow and our export markets impose more stringent residuecontrols. Oils are exempt from Maximum Residue Limit (MRL) requirements and thewithholding periods to which synthetic insecticides are subject, thereby offering thepotential to produce pest-free fruit that can meet strict export quarantine and residuestandards.
It is well known that oils, both mineral and vegetable, can kill insects (Martin and
Woodcock 1983), but they depend upon physical contact and have no residual activity(Johnson 1980); therefore, complete spray coverage of the target is essential. A newclass of agrichemical adjuvants, the alkylsilicones (AS), can greatly enhance thespreading of oils (Policello et al.
1995). In theory, such enhanced spread shouldincrease the insecticidal activity of an oil.
A particularly troublesome quarantine pest on persimmon is the mealybug
), which resides under the calyx of fruit, thereby avoidingcontact with many pesticide sprays. AS-enhanced oil sprays could potentially accesssuch sites and improve contact with these cryptic pests. Additionally, if the effectivenessof existing chemical insecticides could be increased with enhanced oils it may allowfor a reduction in chemical rates and reduce residues proportionately.
The primary concern in the use of oils on fruit crops is their potential for
phytotoxicity. While pest control is likely to be improved by applying higher rates ofoil, this increases the risk of damage to fruit (McKenna and Steven 1993).
The present studies were directed at (i) increasing the insecticidal activity of
mineral oil, thereby providing a means of pest control on persimmons during thecritical pesticide-withholding period and into harvest, and (ii) determining if ASsurfactant-enhanced oil could increase the efficacy of an insecticide, enabling lowerrates to be used and minimising residues in fruit at harvest.
Proc. 49th N.Z. Plant Protection Conf. 1996: 27-31
Phytotoxicity of oil sprays
Mineral oil (Shellspray; Shell Chemicals) and AS-enhanced oil (containing 5%
v/v Y-12780 alkylsilicone; OSi Specialties Inc., USA) were applied at 1% and 2% v/
v, to fruit-bearing persimmon trees (Diospros kaki
L. cv. Matsumoto Wase Fuyu, 2-
3 m tall) on five different dates approximately two weeks apart (24 February-18 April).
Trees were subject to a standard insecticide regime (NZ Persimmon Export Council
1994a) until two weeks prior to the study. Sprays were applied (15 randomly selected
trees per treatment) at 1500 litres/ha equivalent (research hand sprayer, D4-45 nozzle,
250 KPa) between 8-11 am. Trees and fruit were visually examined fortnightly for
phytotoxicity, with a final assessment on fruit (5 per tree) at harvest (18 May).
Additionally, chlorophyll fluorescence (CF) measurements were made fortnightly on
treated and control trees (2 leaves per tree) with a Morgan CF-1000 meter to monitor
any treatment-induced stress (Lichtenthaler and Rinderle 1988).Effect of late-season airblast applications on fruit quality and pest control
Three blocks (4 rows x 25 trees; cv. Japanese Fuyu) in a commercial orchard were
randomly allocated to treatments. These blocks were subject to a standard programme
of chemical insecticides (NZ Persimmon Export Council 1994a) until 4 March. The
control continued to receive this programme through to harvest. Three applications of
oil and AS-enhanced oil, both at 1% v/v (Table 1), were made to separate blocks by
airblast sprayer (1500 litres/ha) at fortnightly intervals (17 March-26 April). All fruit
from the middle two rows of each block (50 trees) were harvested on 23 May. All fruit
from each tree were individually weighed and graded for export as “fancy”, “select”,
or “reject” (NZ Persimmon Export Council 1994b), and data subjected to analysis of
variance to determine any differences between treatments. Insect infestation was
determined on 100 randomly selected fruit from the middle rows of each treatment.
The calyx was removed and insects present counted and identified under a microscope.
The incidence of cavity beneath the fruit calyx, which may provide a better refuge for
cryptic pests, was also recorded.Effect of reduced-rate pesticide sprays on pest control and residues in fruit
Forty five fruit-bearing persimmon trees (cv. Japanese Fuyu, 2-3 m tall) were
allocated to a completely randomised block design of 5 treatments (including control) x9 replicates. All trees were treated with fortnightly airblast applications at 70 ml/100 litresof Attack (permethrin 25 g/litre + pirimiphos-methyl 475 g/litre; Crop Care Holdings Ltd)until 20 March. Two weeks later laboratory-bred mealybugs (P. longispinus
) wereintroduced onto fruit on all trees. Pieces of sprouted potato containing mealybugs weretied onto the calyx of 7 randomly selected, tagged fruit on every tree. In total,approximately 8000 mealybugs were released on 315 fruit. Full- and reduced-ratediazinon (Averte WP; 450 g/kg diazinon + 22.5 g/kg permethrin; Ciba-Geigy) treatments(Table 2) were applied to experimental trees on 18 April and repeated on 2 May, twoweeks after mealybug infestation, by which time all mealybugs had moved out frompotato sprouts. A further application, of the enhanced oil treatment only, was made withinthe pesticide withholding period, on 16 May. Sprays were applied with a research handsprayer as described previously. Fruit were harvested on 30 May. Six randomly selectedfruit were sampled, in addition to all tagged fruit, on each tree and insect infestation onall fruit was determined as described previously. Insect data underwent log(x+1)transformations prior to analysis of variance to compare differences among treatments onthe basis of numbers of live insects present. Pesticide residue decay curves wereestablished for diazinon, permethrin and pirimiphos-methyl. Randomly selected fruit(two replicates of 5 fruit each) were harvested at regular intervals up to 28 days after thelast pesticide spray and stored deep frozen prior to residue analysis using methanolextraction, toluene partition, carbon column cleanup (Holland and McGhie 1988),followed by GC determination (Holland et al.
RESULTS AND DISCUSSION
No damage due to application of any oil treatment was observed on fruit (75 per
treatment) or foliage throughout this study. Additionally, CF measurements provided no
evidence for any damaging effects of spray treatments on the photochemical efficiency
of foliage, as no treatment means differed significantly (P<0.05) from the unsprayed
control (data not presented). This indicates that oil and AS-enhanced oil sprays can safely
be applied at regular intervals to persimmon, from February through to harvest.Airblast applications
Oil treatments had no deleterious effects on fruit size, with all means falling within
the “fancy”/“select” grade (Table 1). There were no differences (P<0.05) between theinsecticide control and oil treatments in percentage of fruit graded “fancy”. Nostatistical analysis of insect data was possible due to very low levels of infestation. Noinsect damage was detected and neither mealybugs nor leafroller larvae were foundon any fruit. While the pest pressure in this well-managed commercial orchard was toolow to distinguish any differences between oil treatments, both were effective inmaintaining the insect-free status during the 11 weeks pre-harvest.
Effect of oil treatments on size and quality of persimmon fruit.
Reduced rate sprays
The numbers of mealybugs present on infested fruit (Table 2) confirmed that all
treatments gave highly significant control of mealybugs compared with the untreated(since late-March) control. The half-rate insecticide+AS-enhanced oil providedcontrol equal to full-rate insecticide and better than half-rate insecticide or theenhanced oil treatment alone. Mealybug numbers on the randomly sampled fruit weretoo low to be analysed, but showed similar trends (Table 2). Leafroller numbers weregenerally too low overall for analysis (Table 2).
Insects per fruit (transformed means) present on deliberately
infested and randomly selected fruit treated with full and half rate
insecticides and oils.
1 Averte applied on 18 April and 2 May; full rate 112 g/100 l, half rate 56 g/100 l.
2 1% (v/v) Shellspray containing 5% Y-12780. Additional spray application on 16 May.
3 assessed over all fruit sampled** significantly different from the untreated control at P
Mite numbers (tydeid mites and others) on the randomly sampled fruits were also
extremely low and no analysis was possible as a result. Mites were not introduced withthe mealybug potato cultures, but mealybugs, through their production of honeydew
and sooty moulds, are known to encourage the proliferation of mites (D. Steven, pers.
comm.), and they were present on mealybug infested fruit in sufficient numbers toevaluate the effect of treatments. All spray treatments reduced mite numberssignificantly (Table 2). Half-rate insecticide+enhanced oil again reduced mite numbersmost, although differences between treatments were not significant (P<0.05).
Pesticide residues (mg/kg whole fruit) in persimmons treated with
full and half rates of insecticide.
1 sampled on 9/05, ie 50 days after airblast treatment with Attack2 nd; not detected (less than 0.01 mg/kg).
There was no external insect damage on any fruit, nor was there any association
between any of the treatments and the occurrence of calyx cavity (data not presented).
The efficacy of oil spray without the benefit of an AS “enhancer” was not tested in thisstudy. There is a need to determine whether such a treatment could provide equivalentpest control to the AS-enhanced oil spray, as it would offer substantial cost benefits.
AS-enhanced oil had no effect on residues (Table 3), as has been reported
previously for organosilicone surfactants (Walker et al.
1992; Stevens et al.
1994),and diazinon levels were more than halved in the half-rate treatment. These resultsindicate that the withholding period of at least 28 days could be reduced to 14 days forhalf-rate treatments. At this time, diazinon residues from half-rate sprays were 0.1 mg/kg which is less than half the MRL for export to Japan. The most stringent MRL forpermethrin is demanded by Singapore (0.1 mg/kg pending), and this was also achievedapproximately 14 days after the last spray of half-rate treatment (Table 3). Pirimiphos-methyl residues required approximately 50 days after treatment to decline to a “nil”level (0.02 mg/kg), as required by the Australian and SE Asian markets, after intensiveapplications (six treatments during Jan-March) of the pesticide.
The use of an enhanced oil+half-rate insecticide regime up to 14 days prior to harvest
could provide insect control on persimmons for longer than is currently possible, whilestill achieving acceptable MRLs. In situations where fruit are subject to intensive pestpressure in the immediate pre-harvest period, oil formulations alone could providesignificant pest control in the absence of pesticides. Alternatively, in orchards with lowpest infestation, oil sprays could safely substitute for insecticides in the late season.
Thanks to Bruce Cameron and Brent Wilcox for use of their orchards and to Arthur
Vanner, Annette Ah-Chee and Joe Taylor for their valuable technical assistance. ColinMalcolm undertook residue analysis. Funds for this research were provided by theAgricultural and Marketing Research and Development Trust (AGMARDT), theFoundation for Research, Science and Technology, the Persimmon Export Counciland OSi Specialties Inc. (USA).
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