Grass Grub - may not be the only culprit in stem and root damage!

28 February, 2007

Executive Summary

The larval and adult stages of black beetle and white-fringed weevil can cause major damage to the roots and lower stems of young vines. The problem may be worse in warm spring and summer conditions when populations of both insects may increase rapidly.

A sustainable viticulture approach, using monitoring of adults and juvenile stages, an understanding of the life cycles of these insects, cultural practices and biological control, can be used to control populations effectively.

Research Results

Yellowing stunted leaves and damage to the roots or stems of your young vines (Figures 1 and 2 below) are caused by insect damage but they may not necessarily have been caused by grass grub.

Research conducted recently by Corbans Viticulture showed there are several other root and stem chewers in the soil and vineyard and that may be present throughout the whole growing season.

Figures 1 and 2: Damage to the stem caused by insect damage

Most of the published information and research in New Zealand has pointed the finger solely at grass grub (Costelystra zealandica) shown in Figure 3, the adult stage of which is the bronze beetle (Figure 4).

Figure 3 Grass grub (Larvae of Bronze beetle) Figure 4 Bronze beetle (Costelystra zealandica)

Soil surveys for grubs and other insects and use of pitfall traps to capture adult beetles (as shown in Figure 5) in all Corbans Viticulture blocks revealed a variety of insects. Samples of captured adults, larvae and pupae were sent to entomologists at AgResearch, and Landcare Research for identification.

It was found not only grass grub larvae (Figure 5) but also the similar looking but larger African black beetle (Heteronychus arator) larvae (Figure 6) and significantly more black beetles (Figure 7) were captured in the pitfall traps than bronze beetles.

Fig 5 Pitfall trap for capturing adult beetles

Figure 6 Black beetle larvae or grub      Figure 7 Black beetle (Heteronychus arator)

In spring and early summer there were also large numbers of white-fringed weevil (Graphognathus leucoloma) larvae and pupae in the soil (Figures 8 and 9) and adults (Figure 10) which were found at the base of the stems.

Figure 8 White-fringed weevil larvae      Figure 9 White-fringed weevil pupae

Figure 10 White-fringed weevil (Graphonathus leucoloma)

 

The research found a few greasy cutworm larvae (Figure 11) and porina moth caterpillars (Figure12). Porina moth caterpillars and greasy cutworms were not present in large enough numbers to warrant concern. Also found were some adult garden weevils but no Argentine stem weevil or Tasmanian grass grub were discovered during the survey.

Figure 11 Greasy cutworm (Agrotis ipsilon)      Figure 12 Porina Moth caterpillar (Wiseana cervinata)

The bronze grass grub beetle and white-fringed weevil were known to cause damage to grapevines but little was published about black beetle damage to grapevines.

After the initial survey, damage was discovered to vines soon after planting in spring. This appeared to be caused by more than one species. The research team also felt that there were large enough populations of black beetle and white- fringed weevil, along with bronze beetle, to cause significantly more damage to young vines given another warm spring and dry summer.

Trials in which black beetle larvae or weevil larvae were placed in enclosed bins with young vines showed that both did considerable damage.

The white-fringed weevil larvae was found to cause damage in early spring and summer while the black beetle larvae began to chew the vines after Christmas.

Early advice suggested the use of Suscon green and other environmentally harmful sprays to achieve control of these pests. However, the research team chose to use research, expert advice and a sustainable viticulture approach to this problem.

Trials conducted to assess causes of damage

Trials were set up in enclosed bins to assess damage done by both larval and adult stages of beetles and white-fringed weevil and garden weevil. The adults and larvae of each species were separated into separate bins containing young two-month old vines in soil. It was found after several weeks that all could do damage to the stems and roots of the young vines. Even the adults preferred to chew on roots and stems instead of the leaves. The damage caused by each was slightly different as shown in Figures 12, 13 and 14.

It was also discovered that root and stem damage in the bin with adult white- fringed weevil adults (see Fig 15) was similar to the uniform chewing of the bark by the weevil grubs found earlier (2006-11a), but further up the stem. There was also root and stem damage in another bin with black beetle grubs (see Fig 16).

Fig 15 White-fringed weevil adult damage     Fig 16 Beetle grub damage

Surprisingly the white-fringed weevil adults did not cause noticeable damage to the leaves and most of the vines had good shoot growth. The damage done to the stems and roots by the beetle grubs was different in appearance with some shallow damage to the bark of the stems surrounding deeper damage. All of the vines with stem damage made by black beetle grub or white-fringed weevil adults had very few roots remaining compared to undamaged control vines. It appears that white-fringed weevil grubs and adults can cause similar stem and root damage. To date it has been established also that the black beetle larvae cause stem and root damage. It is yet to be discovered if adult black beetles cause similar damage to the larval stage.

Fig 17(a) Damage done by White- fringed weevil larvae with an undamaged root and stem shown in (b).

All of the damage caused by white-fringed weevil adults, grubs and beetle larvae are likely to either cause death or stunting in young vines or vines will be rejected for scarring on stems.

All life stages of both insects should be monitored and controlled throughout the growing season until vines are uplifted.

Biology and life cycles of beetles and weevils

Dr Anna Graham, Corbans Viticulture Research & Development Manager, researched the biology, life cycles and hosts of these insects in order to find the most effective strategies for control. HortResearch Hortfact sheets available on their website were excellent sources of information, as well as articles written by East et al. (1981, 1982) and Henzell & Lauren (1982) from Raukura Agreasearch station, and MAF publications on white-fringed weevil.

Below is a summary of this work.

The life cycles of these beetles and weevils includes an adult stage of one to two months followed by eggs at two to three weeks. Eggs hatch into larvae or grubs which go through several growth stages (instars). After several instar stages, they pupate further down in the soil from September – March . The pupa doesn’t feed. The time of each stage in the life cycle of these different pests varies so several measures need to be taken to control them.

Grass Grub, Costelytra zealandica (White)

Adults occur mostly in October/November and can cause damage to the leaves of grapevines. The larval stage begins from December throughout summer, autumn and winter. It is thought that the later stages (instars) of the larvae cause the most damage. Young larvae feed more off new roots, later instars found in late summer/ early autumn feed off stems one to three centimetres below the soil surface.

The larvae cause damage in horticultural environments and in forest nurseries (e.g. to pine seedlings). The beetles eat leaves. They have little impact on pasture but are known to strip the foliage from grapevines and roses, and from apple, cherry, plum, peach and apricot trees.

Most grass grubs hatch in December and January and pupate nine to 10 months later. They are found up to 150 mm below the soil surface. The first larval stage lasts about three weeks and the second about six weeks. The third instar is present until the following September or October, but completes its growth and stops feeding about July, depending on the conditions. The pupal stage lasts three to four weeks.

Black beetle (Heteronychus arator)

Black beetle outbreaks are associated with higher than average spring and summer temperatures. Adults chew at the base of plant stems.

Black beetle has only one generation a year, but it is quite common to find life stages out-of-phase with the main generation. In some areas up to 20 per cent of the population over-winters as third stage larvae or as pupae, and the adults which develop from these stages probably lay their eggs in January and February, so that virtually all life stages may be found in the soil at any time of the year.

The usual cycle is for adult females to lay seven to 10 eggs in the soil from late September, most egg laying occurring from late October to late December. Larvae develop through three instars over the summer and pupate in February-March. New adults begin to emerge from late February and feed actively for a few months. When numbers are high, usually following summer drought, massed dispersal flights may occur in autumn. Adults develop over winter in the soil, emerging in spring to start feeding again and to lay eggs. Populations can spread quite rapidly as the adults are mobile and will infest neighbouring blocks.

White-fringed weevil (Graphognathus leucoloma)

Adults cause damage to leaves. Larvae (especially mature ones) do most damage to the roots rather than stems. Most larvae become mature during the autumn and winter so most damage is likely to occur in winter months.

According to a MAF report written by R. East, the white-fringed weevil larvae are distributed in all New Zealand viticulture regions. They exist all year round as grubs can take nine to 15 months to become adults (i.e. a two-year life cycle).

All stages may occur throughout the year. The adults do not cause much damage, and only to leaves, but lay eggs at the base of plants or under plastic on the soil surface from December to April, with peak numbers in January. They do not fly. With good food sources, the adults will lay many eggs which hatch after rain.

Most of the damage is caused by the weevil grubs and it is not uniform. The white- fringed weevil grubs are resistant to many insecticides.

Control of beetles and weevils causing damage to young grapevines

These insects are difficult to control as different methods are needed to control larvae and adults, and their life cycles do not always coincide. The Agrichemicals recommended for their control - Suscon green and Diazanon, tend to be harsh on the environment and not conducive to sustainable practices.

Biocontrol agents such as Bioshield are available are recommended for keeping grass grub numbers down during a normal season but their effectiveness against black beetle and white-fringed weevil is unknown. Unlike bronze beetle grass grub, in which populations gradually increase if not checked, these two species can have outbreaks (rapid increases in number) that last for two years after warm spring/dry summer weather. Monitoring for adults and larvae is recommended. It is useful to understand the life cycles of these insects to determine the most effective critical control points.

Control measures

 

1. Adults

 

Monitoring numbers of weevil and beetle adults is important to ensure that any control measures are done at key times in the life cycle. Beetles are most active during dust so pitfall traps can be easily made using ice cream containers (see Figure 5). Drill drainage holes in each side of the container, about half way up. Place the ice cream container in a hole so that the edge of the container is level with the soil surface. Fill the ice cream container to a depth of two-three cm with antifreeze mixed with water (50:50 mixture). Check traps fortnightly during spring and summer and weekly during autumn. Remove any beetles. Adult bronze and black beetle numbers can be monitored using pitfall traps.

Bronze beetle adults stay in a fairly localized area e.g. adults lay eggs in, or adjacent to, soils where larvae and pupal stages were. However, black beetles and white- fringed weevils are more mobile and will come from a neighbouring block into the vineyard.

It is important to limit adult beetle numbers once young vines are planted as the adults may crawl down the trunk of vines after feeding on the leaves and lay eggs just below the soil surface. An increase in adult population found in the pitfall traps during this time signals the time to use a deterrent such as Deltaphar, a synthetic pyrethroid. There are other products on the market. We recommend that you consult with your regular agrichemical supplier or handbook.

 

1. Grubs and pupae

 

The larvae and pupae are present in the soil in early spring and that is the time to begin monitoring those populations. Spring is a critical time for reducing young larvae and pupae populations and preventing worse damage in autumn caused by mature larvae. Weevils begin chewing roots and stems before Christmas and beetle grubs after Christmas.

The first instar (growth stage) of the grub is the best time to control these pests in severe cases of soil infestation as they cause the least damage during feeding, are more susceptible to disruption of their environment by tilling.

Tilling is the most effective and sustainable preventive measure against larvae and pupae of beetles and weevils in the soil. Most grubs occur in the soil to a depth of up to 20 to 30 cm. The larvae or grubs move closer to the soil surface to feed as they mature. The third or fourth instar is usually when the larger grubs cause the most damage, and generally only one to three cm below the soil surface, so control measures should be used before they reach this stage. Weekly cultivation for three consecutive weeks by regular fine tilling and ploughing in late winter/early spring is recommended to control white-fringed weevil. However, if you also have beetles, this will not be a completely effective strategy as their life cycles do not coincide.

Removal of other host plants such as clover, lucerne or ryegrass from the vineyard can help to keep populations down. Mustard and Phelarus grass, which are not host grasses for these pests, can be sowed on new blocks of land. Bioshield is a biological control product available for control of bronze beetle grass grub, but it is not known whether this is effective against black beetle and white-fringed weevil larvae. Biological control agents and management of soils to encourage growth of beneficial soil microbes are good long-term solutions to prevent stem and root damage.

Chemical treatments such as Diazanon, Counter and Suscon green can be used to kill larvae in the soils. Many of the granular forms need good rain or irrigation to release the effective chemicals into the soil. With the exception of Suscon green, which has a residual effect for up to three years, a single application of any of other treatments is unlikely to be effective against all of the beetle and weevil larvae because they do not all share simultaneous first instar stages.

Acknowlegdements

Colin Ferguson of AgResearch, Mosgiel for helping with the identification of the various grubs and beetles and advice on monitoring black beetles using pitfall traps.

References

HortResearch website www.hortnet.co.nz/publications/hortfacts

East R, King PD, Watson RN (1981) Population studies of grass grub (Costelytra zealandica) and black beetle (Heteronychus arator) (Coleoptera: Scarabaeidae). NZ journal of Ecology 4: 56-64.

Henzel RF & Lauren DR (1977) Control of grass grub adults with synthetic pyrethroids. Proceedings of the 30th NZ weed and Pest Control Conference: 211-216.

Henzel RF, Lauren DR, Yortt ML (1979) Alternative formulations and chemicals for the control of adult grass grub with foliar sprays of synthetic pyrethroids. Proceedings of the 32nd NZ weed and Pest Control Conference: 101-106.

Henzel RF & Lauren DR (1982) Development of methods for prediction of the starting date and duration of insecticide spray programmes against adult grass grub, Costelytra zealandica (White) (Coleoptera: Scarabaeidae). NZ Entomologist 7: 266-271

King PD & Watson RN (1982) Prediction and monitoring of black beetle (Heteronychus arator) (Coleoptera: Scarabaeidae), outbreaks in New Zealand. NZ Entomologist 7: 227-231.

Pioneer maize insect pest series Greasy cutworm (2006) Pioneer Technical insights No. 317.

Pioneer maize insect pest series Black Beetle (2006) Pioneer Technical insights No. 328.

For further information, please contact Dr. Anna Graham anna@corbansviticulture.co.nz