Long-time almond expert Walt Bentley offers tips. |
Editor’s note: This season, we welcome again our guest
blogger UC IPM entomologist Walt Bentley, whose specialty includes managing
pests in almonds.
Ant
damage to almonds continues to be a difficult problem for farmers to manage. In
all areas of the state populations of the three species of ants which cause damage
appear to be increasing. From Fresno County going south to Kern County the
California fire ant (also called the southern fire ant), Solenopsis xyloni, is the damaging species.
Another,
less common species, is the thief ant, Solenopsis
molesta, which is slightly smaller than Xyloni. In Merced, Stanislaus, and
San Joaquin Counties both the California fire ant and the pavement ant Tetramorium caespitum can be found
causing damage. In the Sacramento Valley the pavement ant is the predominant
species.
Pavement ants are predominant in the Sacramento Valley. |
Although
almond farmers will know, from past history of damage, whether or not they have
a problem species in the orchard, there are other species that are considered
more beneficial than harmful. These include the bicolored pyramid ant, Doriomyrma bicolor and the native gray
ant, Formica aerata. This latter
species has been found to actively feed on peach twig borer in unsprayed peach
orchards.
The
best way to separate the damaging species from those not causing damage is by
examining the thin waist between the last pair of legs and the abdomen
(stomach). Both the pavement and the California fire ant possess two bumps or
nodes while the beneficial species have only one node. Also, the damaging species will often be
found with weeds growing within the nest and are easily excited by disturbing the
soil near their nests. Finally, the
pavement, California fire ant, and thief ant will aggressively bite and
sting. The California fire ant sting, in
particular, is quite painful.
Look out for the pavement ant. |
While
control information has applied to all three damaging species, monitoring
methods and thresholds were only developed for the California fire ant. This
species is considered the most damaging. We do know that feeding on almonds
occurs when the nuts are on the ground, after being shaken from the tree. In only a few instances will ants climb into
trees to feed on nuts. This usually
occurs when limbs touch the ground giving ants easy access to hullsplit
nuts.
Learn to identify the California fire ant. |
The
primary factors that influence damage include the population of ants in the
orchard and the length of time the nuts are left on the ground to dry. The
longer the nuts are on the ground, the more damage you can expect. And, as you
might guess, the greater the population in the orchard the more potential for
damage.
Years
ago work was done on treatment thresholds for the California fire ant and the
damage potential for 3 different population densities is shown in Figure 1. This
information is also available in the Almond Pest Management Manual and the Pest
Management Guidelines for Almond.
In
general colony counts of 3 or more per 1,000 square feet can lead to damage at
harvest, if conditions are right. This is a big if, especially for the
California fire ant, because high temperatures (95 degrees Fahrenheit or
higher) will restrict their feeding. If
such high temperatures exist while the nuts are drying on the ground, damage
from the California fire ant will be minimal.
This is true even for very high populations. If daily highs are less than 90 degrees,
damage can be extensive. The same information is not known for the pavement ant
although it appears to behave similarly.
Limit the time nuts are on the ground to avoid ant damage. |
There
are some cultural management techniques that can be used to keep ant damage to
a minimum but be cautious, particularly if navel orangeworm is also a problem. Where
ants are the primary pest, leave the almonds on the tree for as much drying as
possible. This will allow you to pick
the nuts from the ground without a delay to dry. You can try to schedule the
shaking of heavily infested blocks late in the season, again to keep the nuts
on the tree for as long as possible. Beware, however, if you have a navel
orangeworm problem. The delayed shaking
will increase worm damage.
Ants damage soft shell varieties. |
Another
point to remember is that only the soft shell varieties, such as Nonpareil, are
damaged by ants. Hard shells such as Mission, Butte, Padre and, to some extent
Carmel, are not fed upon. Don’t bother
to treat for ants if you have a hard shell orchard.
Ant
baits have been very effective in reducing damage, when applied at the correct
time. These include such products as
Esteem©, Clinch© and Extinguish©. Each
requires a specific time interval, prior to shaking, to be applied. In applying baits, be sure to mow any cover
prior to the application. For further information go to the IPM Guidelines for Almonds.
Figure
1. Projected ant damage to soft shell
almonds based on colony number and days left on the orchard floor.
% Damage by Ants to Almonds on Orchard Ground
Days nuts on ground
# colony entrances
5,000
sq ft in Apr-May
|
4
|
7
|
10
|
14
|
21
|
15
|
0.9%
|
1.6%
|
2.1%
|
3.1%
|
4.9%
|
45
|
1.4
|
2.3
|
3.2
|
4.7
|
7.0
|
185
|
2.0
|
3.6
|
5.0
|
7.0
|
11.1
|
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