FLESH-EATING MAGGOTS, SORT OF
(Harvey Reissig whr1@nysaes.cornell.edu,
Entomology, Geneva)
The apple maggot (AM), Rhagoletis pomonella (Walsh), is a
native insect that originally infested hawthorn trees throughout the northeastern
United States and Canada. The AM has been a major pest of apples since
they were introduced into North America. In unsprayed habitats, it is
not uncommon for nearly 100% of apple and hawthorn fruit to be infested
by AM, because natural enemies do not reduce population levels of this
pest in natural settings. Therefore, some type of control program will
continue to be necessary to control this pest at acceptable levels in
commercial apple plantings in the foreseeable future.
Biology
The AM overwinters as pupae in soil beneath apple trees. Adults emerge
from the ground in late June or early July and begin to lay eggs in the
fruit after a 7–10 day pre-oviposition period.
Apple Maggot Adult
Adults remain active during July and August, and a few adults remain
active throughout September and even in October in seasons when the weather
is mild. AM females lay eggs underneath the skin of apples.
[INSERT:
Close-up of apple maggot oviposition puncture
Apple Maggot larva showing black mouth hooks
These eggs hatch in about a week and larvae begin to tunnel throughout
the fruit. Usually, particularly in cultivars with very hard fruit, larvae
grow very slowly while the apple remains on the tree. Larvae usually complete
their development after apples have dropped from the tree in the fall.
Then they leave the fruit and tunnel into the soil to pupate, where they
spend the winter.
General Management Principles
Organophosphate insecticides are very effective in controlling AM
adults, and it is very rare to find detectable levels of AM injury in
fruit sampled in commercial apple orchards in New York State. Therefore,
management programs for AM are based on the assumption that there are
no indigenous populations of this pest inside orchards and are designed
to prevent flies from immigrating into orchards from outside habitats.
Unfortunately, in NY there are usually numerous hosts (abandoned or uncultivated
apple and hawthorn trees) that are chronically heavily infested with AM
and relatively close to commercial orchards. Apple maggot flies are capable
of moving at least several hundred yards to infest other hosts and at
least a few flies will always move longer distances of up to one mile.
Extensive research has been done to compare the biology and host
preferences of AM reared from apple fruit and various species of hawthorn
fruit. Populations living in these two different hosts are considered
to be some distinct and are called "host races". There is considerable
disagreement among various authorities about whether or not flies infesting
hawthorns will immigrate into commercial apple orchards and oviposit in
apples. For all practical purposes, heavily infested hawthorn trees near
apple orchards should be considered just as much of a potential threat
as heavily infested wild apple trees.
Elimination of Wild Hosts and Cultivar Differences
Since wild hosts (apples and hawthorns) in close proximity to commercial
orchards are considered to be the only sources of potential infestations
of AM flies, it is a sensible strategy to eliminate as many of these pest
sources as possible. Obviously, it is desirable to create as large a "host-free"
area around orchards as possible, but most authorities recommend removing
alternate hosts for a distance of at least 100 m from the borders of commercial
orchards. It is best to survey wooded areas surrounding apple orchards
in the early spring when apples are in bloom because they are easier to
detect at this time.
AM prefer to oviposit in certain cultivars of apples, and larvae
survive better in some varieties of fruit than others. Early ripening,
soft cultivars such as ‘Wealthy’, ‘Cortland’ and ‘Early
McIntosh’ are generally more favored for AM oviposition and larval
survival than harder, later ripening, cultivars such as ‘Rome’,
‘Delicious’, and ‘Idared’. ‘Northern Spy’,
which is a cultivar with hard, late ripening fruit, appears to be one
exception to this general rule because it is reported to be a favorite
cultivar for AM infestation. Although no commercially produced cultivars
are immune to AM infestation, management strategies can be relaxed somewhat
in less preferred, harder varieties.
Conventional Protective Control of Apple Maggot Flies
This program does not require monitoring of specific orchard blocks.
Whenever it is determined that AM flies have first emerged in an unsprayed
habitat (preferably in close proximity to the targeted orchard) the entire
orchard should be sprayed initially with an organophosphate insecticide
7–10 days (their pre-oviposition period) later. Additional sprays
should be applied at 10–14-day intervals until about the middle of
August. Since flies emerge in late June-early July in New York State,
this protective program will usually require about 4 sprays annually.
Usually, this type of program is only necessary in blocks in which detectable
levels of AM-infested fruit have been found, or in orchards located adjacent
to extensive numbers of heavily infested wild hosts.
Reduced Protective Control Schedule for AM Control
This program also does not require monitoring of specific orchard
blocks and is very similar to the previously described program, except
that the first spray is applied on a calendar basis on July 15. Then,
two more sprays will be applied, on August 1 and August 15. The delay
of the first spray for AM control is based on the principle that extensive
monitoring studies conducted in NY have shown that flies usually do not
begin to immigrate into commercial apple orchards from wild habitats until
about the middle of July. This type of program usually is quite effective
unless environmental conditions result in a shortage of fruit on wild
hosts outside of orchards. Then, AM flies may alter their usual behavior
of initially ovipositing in fruit on wild host trees close to their emergence
site and may immediately begin to disperse to find suitable oviposition
hosts in commercial apple orchards.
Conventional AM Monitoring Program
This program is described in detail in the Apple
IPM Scouting Manual (IPM Pub. No. 207, Apple IPM: A guide for sampling
and managing major apple pests in New York State)
and is based on the idea that it is not necessary to spray an orchard
unless a certain population level of flies (monitored by red sticky spheres)
is detected immigrating into a monitored block. This technique has been
used quite successfully by many growers in NY in "typical" orchards, and
the average orchard monitored by this strategy will usually require 1–2
sprays annually for control of AM. Although many growers in NY use apple
maggot traps hung along the edges of commercial orchards as a general
indication of when to start spraying for AM, most do not adhere strictly
to the formal recommendations described for the monitoring program. Some
of the most common deviations from the protocol are: (1) Many growers
use apple maggot traps only to determine when the first AM spray should
be applied and then spray at 14-day intervals thereafter, regardless of
subsequent trap catches. (2) Growers often monitor for apple maggots in
one or two blocks and then spray the remainder of their orchards based
on trap catches in the monitored blocks. (3) Many growers simply apply
sprays whenever any flies are captured and ignore the recommended threshold
level of 5 flies/trap.
Growers and consultants using an AM monitoring program often are
concerned about late season catches of flies on traps during September
and October in commercial apple orchards. Studies conducted in NY have
not shown that there is any need to apply control sprays after the middle
of August, even though flies can still be captured on traps after the
estimated period of residual effectiveness of the last spray. Apparently,
female AM active late in the season in apple orchards do not oviposit
in fruit, even though most of them have completely developed eggs in their
ovaries.
This monitoring program should not be used in "high risk" blocks
that are adjacent to extensive sources of AM infestations from wild hosts.
Using this program in such blocks will not only result in a potential
risk of low levels of AM injury, but will also not result in any reduction
of pesticide use because experience has shown that in such blocks the
traps will simply indicate that a spray is needed every 10–14 days
throughout the season after the traps are deployed.
Although there have never been any formal recommendations presented
on exactly how many AM traps should be deployed to completely monitor
a grower’s entire acreage of apples, it should be noted that AM traps,
in contrast to pheromone traps for moths, have a very short range of attraction
(10–25 yards). Therefore, it is clearly unreasonable to expect that
trap catches in any one particular block can be used to monitor fly immigration
into another orchard 1–2 miles away! Also, there is some margin of
safety built into the monitoring recommendations. The monitoring directions
assume that the protective residue from an organophosphate spray will
last 10–14 days before another spray is needed. Obviously, residues
from organophosphate sprays gradually degrade and become less effective,
so that the residual effectiveness in killing flies does not decline abruptly
on the 14th day after a spray to become completely ineffective.
AM Monitoring, Border Spray Program
This strategy is similar to the standard recommended monitoring program,
except that whenever trap catches indicate a need for an AM control spray,
only the 3–4 border rows of the monitored block and the ends of rows
are sprayed (as previously described in the PC border row program). This
program is based on the principles that there are no indigenous populations
of AM flies inside monitored orchards, and that AM flies immigrating into
orchards from outside sources will be killed by residues on treated border
rows trees before they can move into the interior of the orchard.
Although some growers and consultants have reported excellent success
using border sprays for AM control, very little research has been done
in NY to formally test the effectiveness of this type of program. Therefore,
growers should be cautious in using this strategy. This program should
probably be used only in "low risk" blocks that are not near sources of
potential outside AM infestations and are planted to cultivars which are
not favored for AM oviposition or larval survival.
New Insecticides for AM Control
Organophosphate insecticides offer many advantages to growers for
AM control. They are very effective, relatively inexpensive, generally
not toxic to predaceous mites, provide good residual control, and there
is no evidence to suggest that flies are becoming resistant to these compounds.
However, as previously discussed, changing pesticide regulations may result
in either the loss of registration of some of these compounds or changes
in the pre-harvest interval, which may adversely affect using these materials,
particularly for late season control of AM.
Recent laboratory and field tests have shown that the new "reduced
risk" compounds, SpinTor and Provado, have activity against the AM. When
these materials were tested recently in NY, they provided comparable control
to a standard treatment of Guthion, but weekly sprays were necessary for
SpinTor because of its short residual effectiveness. These materials,
particularly Provado, have very little contact activity and must be ingested
by the flies to be effective. Laboratory trials have shown that the effectiveness
of Provado against AM can be increased by adding sugar as a feeding stimulant,
but these same effects have not been demonstrated in the field. Currently,
cooperative work is being conducted with Dr. Dan Moreno, a USDA fruit
fly specialist in Weslaco, TX, to develop an improved feeding stimulant
bait that can be mixed with these types of new insecticides to increase
their effectiveness. Additional trials of other new materials are being
tested in the laboratory and field against AM, including: photoactive
dyes, YRC 2894, and Proclaim.
End of this issue: Scaffolds 1999 Index
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