March 26, 2013
Do you really know your moths?
Confident identification of the major moth species of economic importance to tree fruit growers is a vital skill for fruit growers. Luckily the list is not particularly long but some are easily confused with non-pest species. The definitive reference work for fruit growers, The Colour Atlas of Fruit Pests by David Alford, which is sometimes found gathering dust on a shelf in the farm office, devotes nearly one hundred pages to moths but many species are only of academic interest or not commercially significant. Undeniably complacency can creep in as more growers rely on consultants and crop walkers to tell them what is present. To avoid this, a spot of revision at the start of the season (even more important if you are training new entrants) to recognise the main features of the five major problem moth pests will be time well spent. Loss of actives has also meant that some moth species are growing in importance like the light brown apple moth (LBAM).
The lures in pheromone traps attract males of the target species but sometimes identification is difficult as other non-pest species that look similar can end up on the sticky trap card and be counted, thus throwing the value of the threshold number. Magnifying photographs of moths helps to show the details of wing markings etc but these can be confusing when the observer is looking at the actual size individuals which are pretty small. Neil Obbard Agrii technical adviser and company weather station manager considers that for good identification photographs need to show both a blown up image and an actual size version of the moth in the trap. “Otherwise you have no idea of what size you are looking for”. Neil and Agrii fruit team members have put together a handy new moth ID Chart for “the Big Five” which they believe is the clearest yet and will help to avoid inaccurate recording of numbers of pests flying in the orchard.
Once the threshold number is reached (these are included on the reverse of the new chart) the timing of a ‘flight event’ is known. This must be combined with weather station data from nearby to the orchard using the various predictive models to pin point the amount of day degrees necessary to “cook the eggs” to give a date of egg hatch for that particular species. Decisions can then be made on spray timing according to whether an ovicide to kill the eggs or an insecticide to deal with the newly hatched caterpillars will be used.
The Light Brown Apple Moth
Originating in Australia and a serious fruit tree pest in many countries, the LBAM was first found in the UK on spindle bushes in Cornwall in 1936 and has subsequently spread around the country. It causes damage via its larval stage as a leaf-rolling tortrix caterpillar. Until recently it was not causing economic damage to commercial fruit trees (apple, pear, plum and cherry) although it had started to become a significant problem in hardy nursery stock throughout England. The moth has a very wide host range and the larvae are known to feed on over 120 plant species. In the last few years LBAM has become a bigger issue for growers of blueberries and has also been found to be attacking grapes so vine growers need to be able to identify it too. According to Kevin Workman at Agrii, “the larvae get inside the blueberry fruit and web the end making detection difficult; control is difficult as there are few effective insecticides approved for blueberries making identification even more important.”
For top fruit LBAM is generally controlled by sprays targeted against codling moth and other tortricid pests but damage to cherry and plum is more likely because of the limited choice of insecticides available for and used on these crops. Following a severe and extensive attack of LBAM in a Kent cherry orchard in 2005, HDC commissioned a survey, TF 169, using sex pheromone traps, to determine how widespread the species was in commercial orchards. Moth numbers were monitored during 2006 from pheromone traps in apple, pear, plum and cherry orchards distributed throughout the fruit growing areas of England. The LBAM was more abundant on cherry than other crops but no one factor alone (including the age or size of the orchard) seemed to influence the abundance of LBAM on fruit crops. The most likely explanation for the abundance of LBAM in cherry orchards was that they receive the fewest insecticide sprays. Climate change means that LBAM could become more of a pest in cherry crops which receive no specific caterpillar protection. It is an increasing problem in cherry crops under protection where caterpillars continue to feed.
Growers are advised to monitor the moth populations in all orchards, particularly cherry which are at higher risk, using sex pheromone traps. LBAM adults vary considerably in appearance (there are two distinct forms on the new Agrii ID chart) and the caterpillars are very similar to other tortrix species so are difficult to tell apart. The pest should be monitored with pheromone traps weekly from petal fall of apple to the end of August. There is no industry standard as yet so growers should consult their agronomist for specific advice about their situation.
More HDC funded research on LBAM control using biocontrol methods has been unsuccessful because moth numbers have fallen due to very low levels following two severe winters so researchers have turned their attention to the control of summer fruit tortrix moth on stone fruit at present.