I know that recent weather events have changed our focus and this may no longer be a top priority. However, it is important to note that this thrips species may be established in our area and is one we need to keep a close eye on as it is known to infest a number of different plants.
Recent reports of foliage bronzing and leaf drop in cotton
has sparked interest in areas of the Rio Grande Valley. Upon closer investigation the cause of this
damage was identified as a type of thrips, commonly called chilli thrips, Scirtothrips
dorsalis.
Chilli thrips are not new to the Rio Grande Valley, but they
have the potential to become a pest in multiple crops. These thrips are thought to originally be
from Southeast Asia and were first detected in Florida in 1991 and in Southeast
Texas in 2005. Dr. Mamoudou Sétamou, Professor of Entomology & IPM
Specialist Texas A&M University-Kingsville Citrus Center, first discovered
and confirmed chilli thrips in the Rio Grande Valley in citrus in 2018 and has
detected them every year since in citrus groves located in Hidalgo and Cameron
counties. They are known to infest an impressively wide range of host plants,
more than 100 species from at least 40 different plant families, and the list
may continue to grow as they expand their range. Among known plant hosts are bean, citrus,
corn, cotton, eggplant, melon, peanut, pepper, rose, and tomato. So, this pest is one we are going to keep a
close eye on.
Damage Identification and Life Cycle
In cotton, the first clue that a field is infested with chilli
thrips is the bronzing appearance of leaves in the upper canopy. Like other
thrips species, they have piercing-sucking mouthparts used extract contents
from individual plant cells. This
feeding causes characteristic bronzing and may also lead to cupping or other
distortion of leaves. Severe feeding
causes leaves to become brittle and drop from the plant.
Detection of chilli thrips in the field is difficult, and identification
is nearly impossible without magnification.
The tiny (»2mm
long) cigar-shaped adults are pale in color with black, feathery wings. Immatures, called larvae, look similar to
adults but are even smaller and lack wings.
Females insert anywhere from 60 – 200 microscopic eggs into plant
tissue. The length of time it takes to
complete their life cycle varies depending on temperature and host plant but
ranges from 14 – 20 days.
Unlike similar looking species such as the Western flower
thrips, which may be found in flowers feeding on pollen, chilli thrips only
feed on foliage and are typically found on the undersides of leaves near the
mid-vein or borders of leaves. However, when population densities are high,
some individuals may be found feeding on the upper surface of leaves. When scouting for this pest one must observe
the underside of a leaf with a hand lens for several moments. Initially thrips may not be visible as they
are near the mid-vein but will are fast moving and will begin to move around on
the leaf surface, making them more visible.
Damage Potential
Chilli thrips have been present in citrus (and possibly
other crops) in the Valley since at least 2018.
However, this is the first time we have seen populations outside of
citrus large enough to cause noticeable damage on cotton. It is difficult to determine why populations
have “exploded” in cotton this season or to predict what will happen in other
crops this year and in seasons to come.
Because we know that this species is capable of feeding and reproducing
on many crops grown year-around here in the Valley, we plan to monitor them
closely by initiating Valley-wide surveys and management strategies.
Chilli thrips are known to vector at least seven viruses to
various crops including chilli leaf curl virus, peanut necrosis virus, tobacco
streak virus, melon yellow spot virus, watermelon silver mottle virus, and
capsicum chlorosis virus, although there are no reports that they have been
vectors of any of these viruses in Texas.
Management
In most cases, there is no longer a need to control chilli
thrips in cotton this growing season.
Looking down the road, management may be needed if they move into fall
crops. Although chilli thrips are an invasive
species, research in Florida has shown that natural enemies, including minute
pirate bugs, lacewings, and lady beetles may aid in reducing chilli thrips
populations. Thus, the use of broad-spectrum insecticides may promote thrips
outbreaks by removing key beneficial insects.
Studies conducted by Seal et al. (2006, 2008, 2009) have indicated that
pyrethroids are not effective in controlling chilli thrips while imidacloprid
and spinetoram provide good control and allow for the continued growth of
natural enemy populations. Rotating
between different classes of insecticides will be important in reducing the
development of resistance. In addition,
controlling weeds, which may serve as alternative hosts is important.
While this is a relatively new potential pest for South
Texas, Florida has had a few years to learn about chilli thrips and have
produced some very useful information that can be found at the following links:
Good general information:
Photos of damage on various host plants and more detailed
control information:
Addition resources cited:
·
Seal
DR, Ciomperlik M, Richards ML, Klassen W. 2006. Comparative effectiveness of
chemical insecticides against the chilli thrips, Scirtothrips dorsalis Hood
(Thysanoptera: Thripidae), on pepper and their compatibility with natural
enemies. Crop Protection 25: 949-955.
·
Seal DR,
Kumar V, Klassen W, Sabine K. 2008. Response of chilli thrips, Scirtothrips
dorsalis, and melon thrips, Thrips palmi, to some selected
insecticides. Proceedings of the Caribbean Food Crops Society 44: 578.
·
Seal
DR, Kumar V. (2009, in review). Biological response of chilli thrips, Scirtothrips
dorsalis Hood (Thysanoptera: Thripidae), to various regimes of
chemical and biorational insecticides. Crop Protection.
