International Journal of Enviornmental Science and Technology, Vol. 3, No. 1, 2006, pp. 1-7
Evironmental monitoring of economically important invertebrate pests in Bramley apple orchards in Northern Ireland
1*A. G. S. Cuthbertson and 2A. K. Murchie
1Central Science Laboratory, Sand Hutton, York YO41 1LZ, UK
*Corresponding Author, E-mail: email@example.com
Received 14 January 2006;
Code Number: st06001
Apple orchards are host to many species of both pest and beneficial invertebrates. Many of the pest species can cause severe economic damage if populations are left unchecked. Within Northern Irish Bramley apple orchards only four species are deemed to be of economic importance; Panonychus ulmi, Aculus schlechtendali, Rhopalosiphum insertum and Archips podana. Little information exists on the current population status and economic damage potential of these four pests. The population levels of these species and their potential to cause economic damage on the Bramley fruit is therefore discussed in regard to current economic spray thresholds.
Key words: Bramley apple, chemical pesticides, economic spray threshold, Panonychus ulmi,Aculus schlechtendali, Rhopalosiphum insertum, Archips podana, biological control
Fruit cropshavea relatively high valueand therefore cosmetic damage and insect contamination must be kept to a minimum. The highest prices are paid for quality undamaged fruit; insect-damaged fruit will be accepted only for pulp (fruit juice, apple piefiller) production. For this reason, the economic threshold of those pests which attack fruit is low. A vast array of insect species has been recorded as feeding on apple and other fruit trees (Massee, 1946). Themajority of these causelittle damage or occur only sporadically, although each crop has several species of major importance. These species often attack the fruit directly, affecting its appearance or otherwise lowering its quality. Out of this considerable complex only four species show signs of reaching pest status in Northern Ireland (Mowat and Clawson, 1996): (1) fruit tree red spider mite (Panonychus ulmi (Koch), Acarina: Tetranychidae); (2) apple rust mite (Aculus schlechtendali (Nalepa),Acarina:Eriophyidae); (3) applegrass aphid (Rhopalosiphum insertum (Walker), Homoptera:Aphididae);(4)fruit tree tortrixmoth (Archips podana (Scopoli), Lepidoptera: Tortricidae). Within the United Kingdom (UK), the Agricultural Development and Advisory Service (ADAS), have set out spray thresholds for integrated production of pome fruits (Cross and Berrie, 1994). For the apple pests found in Northern Irish orchards the treatment thresholds used on unspecified cultivars in English and Welsh orchards are shown in Table 1. The spray practice in northern Ireland’s orchards is probably derived from ADAS recommendations based on the greater occurrence of a wider range of pests in England and Wales. The spray thresholds assume that populations will increase at a certain rate, but this is likely to differ in the cooler climate of Northern Ireland compared to, for example, southern England. Mowat and Clawson (1996) found winter populations of P. ulmi to exceedADAS actionthresholds in four orchards out of 15 sampled. In the most extreme case, the mean number of eggs per node was 83.8 (n = 10) which suggested that the ADAS threshold of >30 nodes with >5 eggs was well exceeded; yet summer populations of adults never exceeded two per leaf. Similarly, A. schlechtendali exceeded the winter ADAS action threshold of ten mites per bud in seven orchards out of the 15, and in one case, mean number of mites per bud was 204 (n=10). However, despite high winter populations, mite numbers in orchards exceeded the summer ADAS threshold of five mites per leaf only in late August. The reasons for these discrepancies are unclear. Mowat and Clawson (1996) speculated that biocontrol may be operating in the case of A. schlechtendali, as significant populations failed to develop in orchards untreated with acaricides. Similarly, Mowat and Clawson (1996) found populations of R. insertum to exceed ADAS action thresholds (>30 trusses infested per 25 trees) in all three orchards sampled (two managed and one unmanaged) although a high proportion of the aphids sampled were parasitised by Diaeretiella rapae (MacIntosh). In the untreated orchard, trees became heavily infested during mid-May with a peak of 131 aphids per leaf. Spraying trees in this orchard against R. insertum tended to increase yields (Mowat and Clawson, 1996). Theaim of this studywas to investigatethe current population status ofeconomically important invertebrate pests in the Bramley’s Seedling apple orchards and to determine if the current ADAS spray thresholds are relevant in this area of the UK horticultural industry. As the population status of A. podana has recently been well documented (Cuthbertson and Murchie, 2005a) only the population structures of the first three listed species above are presented in this study. The study was carried out in the Bramley’s Seedling apple orchards in County Armagh,Northern Ireland,UK (N 054o24.3' W 006o 35.8') during 1997.MATERIALS AND METHODS
Sampling for Panonychus ulmi and Aculus schlechtendali
Five managed and two unmanaged commercial orchards were monitored for P. ulmi and A. schlechtendali throughout 1997. Managed orchards were defined asthose whichwere treatedwith fungicides, acaricides, aphicides, calcium and fertiliser sprays. The acaricide and aphicide sprays are applied when the grower deems them necessary, but they usually follow a calendar spraying regime. Unmanaged orchards were those that received routine fungicide sprays but no acaricides. However, one grower (owner of an unmanaged orchard), applied two sprays of demeton-Smethyl (an acaricide) on the 26 May, and the 6 June 1997 for control of apple-grass aphid (R. insertum). The orchards were all situated within a five mile radius of the village of Loughgall, CountyArmagh, Northern Ireland, UK (N 054° 24.3’ W 006° 35.8’). Mites were monitored throughout the year but due to the different life stages (adults and eggs) different techniques were used. During the summer months(May – September), four leaves were removed fortnightly from the lower branches of each of two trees. These were washed in 30% alcohol, the solution filtered using filter paper (Whatmans no. 2, 70 mm), and the number of adult mites counted using a binocular microscope (Zacharda et al., 1988). Winter populations of the mites were assessed by counting the number of P. ulmi eggs per node (ridged area at junction of two twigs) of at least 1.5 year–old wood, and the number of over-wintering A. schlechtendali under buds. Ten nodes and ten buds per orchard were examined every two weeks during the months of January to April and October to December 1997.
Sampling for Rhopalosiphum insertum populations
Seasonal occurrence of apple-grass aphid was monitored in a managed orchard and an unmanaged orchard belonging to a private apple grower. Winter R. insertum populations were assessed as number of eggs per node found on 1.5 year-old wood. Ten nodes (two per five trees) were sampled at fortnightly intervals from 10 October until 16 April 1997. The spring populations of nymphs were monitored by sampling trusses (5 per 25 trees, n=125) washing them in 30% alcohol and counting the number of aphids present. Only two observations were taken (7 and 21 May 1997), as winged nymphs were beginning to migrate off the trees.
Panonychus ulmi and Aculus schlechtendali populations
In the unmanaged orchards, P. ulmi was not detected, apart from a few eggs in mid-November. In the managed orchards, P. ulmi was found mostly in the winter period (Fig. 1). Eggs were obtained on nodes from November to April, with peak numbers in early-December reaching 30 eggs per node. The managed orchards’ P. ulmi populations were all above the ADAS spray threshold (>5 eggs per node) during winter time, but not during the summer when only small populations were detected. Aculus schlechtendali populations in managed orchards were above the ADAS spray thresholds during both winter (ten mites per bud) and summer (five mites per bud) periods (Fig. 2). Numbers fell at the end of April, then increased again to peak (80 mites per eight leaves) at the end of July. The two unmanaged orchards showed small numbers of A. schlechtendali all year round until one received aphicide spray (Fig. 2). In this orchard from mid-July until the end of August, A. schlechtendali numbers rose to peak higher than the managed orchards’ populations with a total of 157 mites per eight leaves. The resultant over-wintering population during November until December was also higher than the managed orchards.
Rhopalosiphum insertum populations
The managed orchard had a larger aphid population than the unmanaged orchard (Fig. 3). Spring populations (when aphids can cause damage) in both orchards exceeded the ADAS spray threshold (>30 trusses infested per 25 trees), with a peak number of 120 trusses infested per 25 trees in the managed orchard and a peak of 70 trusses infested per 25 trees in the unmanaged orchard.
DISCUSSION AND CONCLUSION
For both P. ulmi and A. schlechtendali, a distinct difference was obtained between managed and unmanaged orchards. In the managed orchards, mites were detected almost all year round and had larger overwintering populations than the unmanaged orchards. Much literature suggests that spider mites, and other pest species, remain at low levels in situations where their natural enemies are not inhibited by sprays or other factors (for example: Porter, 1947; Lord et al., 1956; Chant, 1966; Huffaker and Flaherty, 1966; Putman and Herne, 1966). Therefore, it is likely that the unmanaged orchards had greater populations of natural enemies than the managed orchards because they received fewer pesticide applications. This hypothesis is supported by the increase in A. schlechtendali numbers after the unmanaged orchard was sprayed for R. insertum. However, the situation is complicated as trees in unmanaged orchards may be deficient in nutrients or injured by disease, so they are probably unfavourable for spider mites. Others have found marked differences between P. ulmi reproduction on leaves from trees in abandoned orchards and on those from trees in well-kept fertilised orchards (Kuenen, 1946, 1949; Kuenen and Post, 1958). In Northern Ireland, apple scab (Venturia inaequalis) is a major problem (Watters and Sturgeon, 1990). If this fungus is not kept under control the effects on the trees are clearly visible by mid-season. The leaves become dry and brittle which could make them unfavourable for mite feeding. Defoliation begins on the infected trees as early as September (Cuthbertson, A. G. S. personal observation). Quality of the leaves can play a major role in the pest mite populations in these orchards (Cuthbertson and Murchie, 2003), however, distinguishing between the effects of pesticide treatment and host quality on mite populations is difficult and it is likely that a combination of both are important. The phenomenon of large over-wintering populations of pest mite species followed by little damage in summer, as found by Mowat and Clawson (1996), was repeated again in 1997. There may be two reasons for this. First, from the monitoring it would appear that both P. ulmi and especially A. schlechtendali populations do not reach significant levels until mid-August. This may be too late in the season to cause any significant damage. The fruit is well formed and therefore the risk of damage from the mites is low. The second reason is possibly the presence of natural enemies during early season offering biological control (Collyer, 1953). Predatory mites such as Anystis baccarum (Linnaeus), the most abundant predatory species in Northern Irish orchards (Cuthbertson and Murchie, 2005b), are active around the same time as both P. ulmi and A. schlechtendali summer populations are beginning to develop from mid- May onwards (Cuthbertson, 2004, 2005; Cuthbertson and Murchie, 2004a,b). Therefore, predatory activity may hinder the pest mite population growth at the start of the season (Cuthbertson et al., 2003a; Cuthbertson and Murchie, 2005c,d). As the season progresses, predation on both P. ulmi and A. schlechtendali may decrease due to a greater range of available prey for the predators, allowing their numbers to increase. Larger populations of R. insertum occurred in the managed orchard compared to the unmanaged orchard.
Although this study was not replicated, the results support other observations (Department of Agriculture and Rural Development, Northern Ireland (DARDNI), unpublished data) that Bramley orchards that receive acaricide treatments tend to have greater pest populations. The reasons for this may again be related to higher natural enemy numbers on untreated trees. Anystis baccarum is known to readily feed upon both juvenile and over-wintering eggs of R. insertum in the field (Cuthbertson et al., 2003b). This over-wintering and early spring predation by A. baccarum along with other predatory species may act to slow the population growth of R. insertum, compared to treated trees, where the natural enemy population could have been depleted. With numbers of R. insertum exceeding the ADAS action threshold, damage would be expected; however, no significant loss occurred to fruit of the apple trees in 1997 (DARDNI, unpublished data). The spring populations of aphids are not present on the trees for long and soon migrate to grasses depending on the advancement of summer temperatures. For this reason Mowat and Clawson (1996) concluded that a pesticide of short persistence would suffice for R. insertum control. Fromthisstudy it would appear that the current ADAS economic spray thresholds, based on a UK wide basis, are of little relevance to the Bramley’s Seedling apple orchards in Northern Ireland. The invertebrate pest species (P. ulmi, A. schlechtendali and R. insertum) monitored in this study, though regularly occurring above recommended spray threshold levels, rarely produce significant economic damage to either fruit or foliage. The same phenomena was also recorded for A. podana (Cuthbertson and Murchie, 2005a). Further research and investigation is therefore required in order to determine more cultivar specific action spray thresholds in relation to the Bramley crop (Cuthbertson and Murchie, 2005e). This will help move the Northern Irish apple industry into a more environmentally sustainable production system by helping to reduce unnecessary pesticide applications against what would appear to be a minimal pest problem.ACKNOWLEDGEMENT
Dr. Andrew G. S. Cuthbertson was funded by the Department of Agriculture and Rural Development (Northern Ireland) Studentship.REFERENCES
© 2006 Center for Environment and Energy Research and Studies (CEERS)