By
Christine Souza
Ag Alert
(Reprinted
with permission from the May 14, 2003 edition of Ag Alert, a publication of the
California Farm Bureau Federation. This article is based on a presentation made
at the Jan. 29, 2002 field day hosted by the Stanislaus County UCCE and the
Almond PMA.)
California
almond growers in the last several years have seen a re-emergence in the number
of tenlined June beetles that appear in their orchards. This pest typically
appears in almond trees in sandy regions and can result in the partial loss of
almond orchards.
"One
of the reasons this beetle was a problem about 15 years ago and now is
re-emerging as a problem could be that the parasite that ordinarily controls the
beetle has broken away from the biological control, which means it will take a
little bit of time for the parasite to bring the beetle back under
control," said Marshall Johnson, University of California Kearney
Agricultural Center entomologist.
The
tenlined June beetle, Polyphylla decemlineata, a species that evolved in
California, is about 1 inch long and brown in color with longitudinal while
lines on the back, according to information from the UC Integrated Pest
Management Program. Larvae are cream-colored grubs with a brown head capsule.
They live in the soil and are about 1 inch at full maturity. There is one
generation every two years and adults emerge from the soil from late June
through August. The tenlined June beetle is included in a genus of about 22
species in California that are common to sandy soils. It is not considered an
invasive or exotic species.
About
20 years ago, the tenlined June beetle was discovered in the Manteca area and
has been found in Fresno and Madera counties. The pest can affect crops in the
San Joaquin Valley such as almonds, apples, cherries and others.
Johnson
is trying to locate grant funding that will aid researchers in their study and
control of this potentially economically dangerous pest. In his research of the
tenlined June beetle, Johnson plans to look more closely at its distribution and
life history, as well as injury caused to almond trees, challenges to control
and management options for growers.
"The
injury almond growers have to worry about most is damage to the roots, which can
be serious to almonds. It is not the adult stage that growers have to worry
about, at least from the damage side. It is the immature stages that cause the
problem," Johnson said. "You can see these things often flying around
at nighttime."
Larvae
feed on the roots of the trees and can cause serious damage to mature almond
trees. Johnson noted that the tenlined June beetle can be as far as 5 feet below
the surface of the ground. Above ground, leaves on damaged trees have what looks
like salt damage and the trees can actually lose leaves. The tree can also
experience limb dieback and eventually death. In heavily infested orchards, the
orchard floor will show that it has been protruded with numerous holes.
Females
and males emerge as adults from the pupae in mid-June, Johnson said, and live
through the summer in that stage. Females lay eggs in August and these are the
first instars that hatch. These feed in September and then overwinter. In about
March and April, the first instars will actually molt to a second instar. The
second instar feeds underground. In June, that instar will molt into a third
instar and it feeds through the summer and overwinters. In June of the next
year, it emerges as an adult. It is after these two years that the tenlined June
beetle completes its life cycle. If a grower finds it in the field, Johnson
said, he can confirm that he has had it for at least two years, maybe more,
because it takes some time for the population to build.
When
it is time for the adults to mate, they make holes in the soil and then the
males fly around looking for the females. Males are often found in the sticky
traps but females are not found in the traps. How females move around in the
orchards is one area where researchers would like more study, Johnson said.
"The
female will come out, stay by the side of the hole and wait for a male to find
her. From what we understand, the female gives off a sex pheromone that attracts
the male to her," Johnson said. "Before the female will even come out
of the hole, some of the males will be standing by the hole waiting for her, so
she must be giving off a pretty good scent."
Once
the females have mated with the males, they go underground and lay their eggs.
There
are currently no registered materials for control of the tenlined June beetle
after trees are planted. The control of this pest requires the removal of
infested trees and soil fumigation before they are replanted. Although
pesticides exist that can kill the beetle grubs, something that makes
eradication of this beetle a difficult task is not having an effective method
for delivering the material to infested sites under the soil.
"Currently
we have no silver bullet tactic other than the removal of infested trees
followed by a long fallow period and perhaps soil fumigation to eliminate the
beetle grubs," Johnson said. "Given the life history of the beetle,
early detection and treatment of the first and second instar grubs near the soil
surface may be the most effective and feasible approach. Right now more work is
needed to identify effective controls and pathogens."
Once
an almond grower has removed infected trees and fumigated the soil, it is still
unclear whether the tenlined June beetle will return. That is something else
Johnson and other researchers would like to study.
A
biological control agent exists in California to control the tenlined June
beetle. This parasite is a large Camsomeris wasp enemy of the tenlined June
beetle. It is believed that the wasp enters the holes made by the tenlined
beetle and can locate the grubs. As a grub is found, the wasp parasitizes it and
will eventually kill the grub. It takes four to eight weeks for the immature
parasite to transition from egg to adult.
"Although
we find this wasp quite commonly in almond orchards and other places in the
valley, we are not sure how long it controls the tenlined June beetle,"
Johnson said.
To
control the tenlined June beetle, Johnson intends to complete research in a
number of areas including possible use of an insect pathogen commonly used to
control the Japanese beetle, introduction of a nematode effective on grubs, and
use of a male attractant pheromone.