This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

The pests apple weevil, Fuller’s rose weevil and garden weevil were important in the production of pome fruit crops in Australia. They were most important in Western Australia, but also caused problems in other apple producing states. Adults of all three species were pests because they defoliated crops, disfigured fruit and layed eggs blocked mini-sprinklers.

Control of these weevils was based on using broad-spectrum insecticides which were disruptive to integrated pest management in orchards and lacked reliable effectiveness.

An earlier project showed that both insect parasitic nematodes and ground cover plants affected the survival of garden weevil larvae. This opened up the possibilities for completely new approaches to control of these species of weevil.

The insect parasitic nematode the researcher worked with was a new strain discovered in NSW and became commercially available before this report was published. The researcher undertook a number of field trials to assess their effectiveness against weevil larvae in orchards and vineyards, the latter location was selected because higher populations of garden weevil could be found there more readily. The nematode trials included a range of variables such as method of incorporation and application, different rates and different timing.

After extensive field trials, the researcher concluded that using parasitic nematodes against the soil borne larval stage was not feasible at this time. This was in contrast to situations where the nematode was used commercially in amenity turf. While this was a negative result, the researcher could not rule out the possibility that as other strains of nematodes became available, or methods of application improved, parasitic nematodes may have proven to be of value in the future.

In relation to ground cover plants and suppression of survival of weevil larvae, earlier work showed that survival of garden weevil larvae was very low when fed on clover. This result was considered useful for two reasons – clover had a confirmed effect of reducing survival of weevil larvae and it was already grown as a cover crop in many orchards.

The researcher clarified this effect in a series of pot trials, which showed that clover was not actually killing weevil larvae, but more that it was simply an unsuitable food plant by itself. When clover was grown with apple trees, larvae survived satisfactorily. The same situation was seen for grubs of Fuller’s rose weevil. Pot trials also indicated that ryegrass and kikuyu may have had a negative impact on larval survival.

Poor survival of weevil larvae on crop hosts such as apple and plum reduced the reliability of pot trial results and tentative conclusions could only be drawn for the best cover crop composition to help reduce weevil abundance.

The researcher was unable to overcome rearing problems to test the effect of different plants on larvae of apple weevil. Laboratory experiments were undertaken with the aim of enhancing their ability to produce apple weevil larvae. They showed that greater egg production could be achieved by rearing adult weevils at 180C compared to 250C and that eggs were more likely to hatch when exposed to moisture and temperature of 180C.

In addition to examining the role of selected cover crop plant species on larval survival, they tested their suitability as food sources for the adult weevils. Grasses such as oats and ryegrass were the least suitable as food plants for all three species of weevil adults and were worthwhile for orchardists to consider. They would have been especially effective if they could be combined with a canopy exclusion treatment, such as sticky bands.

Further work on the role of ground covers in suppression of these weevil pests was recommended.