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Introduction
The native New Zealand grass grubs (Costelytra giveni (white), formerly Costelytra
zealandica, family Scarabaeidae) are the most costly pest in the country, causing $215mil585mil annually just on beef and sheep & Dairy alone ($75mil-$205mil sheep and beef,
$140mil-380mil dairy) (Ferguson, et al., 2019). The most destructive and costly phase is the
larval stage, particularly the two-year life cycle larvae in spring, where they feeds on the
roots of pasture and crops, but also cause damage to shrubs and trees in their adult form by
eating the young foliage (AgResearch, 2022). This happens within a one-year life cycle,
unless it is too cold or there is a lack of available food, where the grub has the possibility to
enter the 2nd year life cycle where they stay in the larval stage (2nd/3rd instar) for another 12
months. Due to being univoltine (one generation per year), it can take 2-4 year for the
populations within a pasture to reach a damaging threshold. (Mansfiled, Townsend, &
Jackson, 2022)
Detection & identification is key for controlling the population of grass grub. The life cycle to
the grub starts with the emergence of the adults, typically around Guy Fawkes Day in
Canterbury (emergence happens earlier in more northern areas, later in more southern
areas) (Mansfield, 2021) and the females lay their eggs typically right within the area of
emergence, after attracting the males with their sex pheromone, Phenol. The eggs hatch,
and around December-January, where the first instar larval stage is entered. They last in this
stage for around 3 weeks, transitioning to the second instar for around 6 weeks
(approximately February-March) and then onto the third instar in April-September/October.
In July, depending on conditions (temperature and food source), the grub will go through
diapause (a period of suspended development) to over-winter to then pupate in October for
a couple of weeks before an adult emerges (although this can occur as late as February).
During the second and third larval instar stage, the two-year life cycle can occur if
temperature, food, and soil conditions are not optimal. These two-year life cycle larvae are
the most damaging to crops and will pupate and emerge at the same time as the next years
larvae. This occurs frequently in Canterbury. The life span of an adult is around 2-4 weeks.
The Female will then fly away to a new site to infest after mating and laying eggs.
(Fenemore, 1984)
Currently, the best way to detect grass grub is to dig a hole in the questioned area with a
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spade, and make a ‘spade square’ (a square hole with each face being the width of the
spade head). Break up the soil and count the grass grubs within, and if the count exceeds
more than two per spade, you have a damaging number (to make a m2 you multiply the
count by 30) (Mansfield, 2021). This identification is to late for preventive action however,
and the actions taken during this season end up being preventative for the following season.
One potential detection method is to apply small amounts of insecticides in strip in a
paddock. This reference strip allows early detection as if the strip looks green compared to
the rest of the pasture, then farmers will be alerted to possible grass grub infection in the
unsprayed patches, allowing them to have earlier knowledge to go dig a spade square
(Barratt et al., 1990). Sadly, this is not an accurate method, as grass grub infection is patchy,
and not uniform within the paddock, although sometimes for cereal crops it can be easy to
follow as they just follow the sown crop line. (Mansfield, 2021)
While there are many control methods out there to apply, these can be costly and
contaminating. Biological control agents such as birds and other invertebrates can be
effective, but can also cause high amounts of damage to the crop/pasture they are to be
protecting, or have other unseen consequences. Pasture disturbance, such as drought,
cultivation or (re)sowing can push back the population growth of the grass grub, allowing a
two - four year break till the population gets back up to proper damaging numbers (Jackson
T. , 1990). The down side to this is that recently disturbed pastures have very few natural
defences against the pest, as disease cycle, pesticide application and plant growth need to
start all over again. Luckily, when pest population numbers are high enough, they grubs will
start having combat mortalities, reducing the numbers in younger pastures. (Jackson T. ,
1990)
Microbial control for insect pests has been labelled a promising but limited control strategy,
as the microbes have the inability to dwell in UV rich environments, or more simply stated,
above ground. This has led to the use of microbial control agents being used in pastures and
gardens, to control the soil-dwelling pests (Jackson, Alves, & Pereira, 2000). One that has
been developed is the use of the naturally occurring NZ soil bacterium, the highly host
specific Serratia entomophila. It is a soil borne bacterium that ingested by the grass grub as
it feeds. It then multiplies in the grub, causing amber disease. This disease enters upon
ingestion and adheres to the foregut to breach the tissue barriers to the foregut, causing
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cessation of feeding and gut clearance within the grub, so that the bacteria can be released
back into the soil where it can be re-ingested (BioStart, 2022). This process takes
approximately 1-3 months, as the bacterium has to enter the hemocoel of the grub to
effectively cause symptoms, while the pathogen itself take 5-7 years to establish control
over a new population or new disturbance (naturally), although biopesticides can speed up
this cycle through deliberate introduction. S.entomophila does not affect adults.
First isolated in 1981 in Canterbury by Trought, it became a commercially produced biocontrol agent soon after under the name Invade™. This was a liquid formulation, needing
cool storage with a short shelf life, while also requiring specialist equipment to drill into the
soil, making it an effective but extremely limited product to market. It was then re-marketed
and sold for under two years under the name Bioshield™, where it is a granule formulation
that is drilled into the soil, or can be broadcasted onto the surface for irrigation or rainfall to
breakdown and drain into the soil. It was pulled for not being profitable enough Finally, it
was sold to BioStart where it is sold under the name BioShield Grass Grub™ in 2017, where
it is a liquid formulation. (Koppenhöfer & Wu, 2017) (Khachatourians, 2009) (Mansfiled,
Townsend, & Jackson, 2022)
BioStart does not recommend application of BioShield Grass Grub™ if soil temperatures are
expected to drop below 10°C and/or soil moisture is below 15%, or if the larvae have
surpassed the middle of the third instar stage, as the larvae still need to be feeding to
induce effectiveness. It has no withholding period compared with insecticides, but has no
manufacturer within NZ, instead being developed in Germany and shipped back.
Since then, new progress has been made with the use of another bacterium, Serratia
proteamaculans, which has an increased host range (only includes the Manuka beetle
Pyronota festiva) and faster disease development, which produces faster mortality (within
five-twelve days). This control agent is currently being field tested underway in arable crops,
in a granulation form that is drilled into the ground.
Serratia entomophila is a promising biocontrol agent for the grass grub pest. Unfortunately,
it has some serious faults that make it hard to incorporate in place of insecticides. One of
the biggest faults is that it is only affecting one pest. While the grass grub pest is
devastating, and the idea of not having to use insecticides for it is enticing economically and
environmentally, there are still other pest that will be have to be treated with insecticides in
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most cases, which will end up killing the population of S.entomophila in the soil, requiring
more usage of the Bioshield product. This cycle will defeat to purpose of the product, which
is to make build up a population of the bacterium within the soil to combat the grass grub
population. Many insecticides are being removed from the market, with popular insecticides
such as Diazinon leaving in 2028, and will leave a niche in the market to fill, but the lack of
investment within the BCA production industry means that there is a huge cost in getting
these products to shelves, and into the pastures. This is increased by the lack of early
detection methods, as this product requires early enough detection so as to effectively
combat the pest population growth of the grub, since it does not affect the adult.
Development of detection methods are more crucial in all aspects, as while it nice to have
the answer, we still need to be able to ask the question. Techniques/Methods such as
thermal or multispectral imaging need to be invested in and advanced first so that it is
affordable and available for all farmers, and then we can answer the problem of how to deal
with it, which needs to be multiple products to target multiple pests, and which don’t cancel
each other out, like using S.entomophila and insecticides.
I personally would love to see an increase in biocontrol agents like BioShield (or the
developing Serratia proteamaculans) but multiple hurdles need to be conquered before we
cross the finish line.
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References
AgResearch. (2022). Grass-grub, brown beetle adult. Retrieved September 21, 2022, from AgPest:
https://agpest.co.nz/?pesttypes=grass-grub-brown-beetle-adult
Barratt, B., van Toor, R., Ferguson, C., & Stewart, K. (1990). Grass grub and porina in Otago and
Southland: a guide to management and control. Ministry of Agriculture and Fisheries, 1-104.
BioStart. (2022). BioShield Grass Grub Liquid: Grass Grub Control in Vineyards. Retrieved September
21, 2022, from https://biostart.co.nz/wp-content/uploads/2020/12/BIOS-037-Bioshield-A4Flyer_no-print-marks.pdf
Fenemore, P. (1984). Grass grub, Costelytra zealandica (White), life-cycle. Department of Scientific
and Industrial Research. doi:https://doi.org/10.7931/DL1-DIS-105-4R
Ferguson, C. M., Barratt, B. I., Bell, N., Goldson, S. L., Hardwick, S., Jackson, M., . . . Wilson, M. (2019).
Quantifying the economic cost of invertebrate pests to New Zealand’s pastoral industry.
New Zealand Journal of Agricultural Research, 62(3), 255-315.
doi:https://doi.org/10.1080/00288233.2018.1478860
Jackson, T. (1990). Biological control of grass grub in Canterbury. New Zealand Grassland Association
(pp. 217-220). Lincoln: MAF Technology.
Jackson, T., Alves, S., & Pereira, R. (2000). Success in Biological control of soil-dwelling insects by
pathogens and nematodes. In Biological Control: Measures of Success (pp. 271-296). Lincoln,
New Zealand: Kluwer Academic. Retrieved September 21, 22
Khachatourians, G. G. (2009). Insecticides, Microbial. In Encyclopedia of Microbiology (pp. 95-100).
doi:https://doi.org/10.1016/B978-0-12-809633-8.13066-3
Koppenhöfer, A., & Wu, S. (2017). Microbial Control of Insect Pests of Turfgrass. In L. A. Lacey (Ed.),
Microbial Control of Insect and Mite Pests: From Theory to Practice (pp. 331-341).
doi:https://doi.org/10.1016/B978-0-12-803527-6.00022-6
Mansfield, S. (2021). Keep an eye out for grass grub. (A. Stewart, Interviewer) Retrieved from
https://vimeo.com/513214411
Mansfiled, S., Townsend, R., & Jackson, T. (2022). Grass grub control with Serratia entomophila.
AgPest. Retrieved from https://learn.lincoln.ac.nz