Phenology and management of hemipteran pests in white clover (Trifolium repens L.) seed crops, Canterbury (New Zealand)

Abstract

New Zealand currently supplies approximately 60% of the world white clover (Trifolium repens L.) seed market, of which 99% is produced in the Canterbury growing region. However, this important cash crop may be infested by a range of insect pests which contribute to economic losses. The main aims of the research in this thesis were first, to identify the key economic pests in white clover seed crops by studying the interactions between plant and insect pest phenologies; second, to quantify the effect of selected pests on white clover seed production; third, to determine the most effective timing and placement of insecticide applications for controlling the pests; and fourth, to determine the synchrony of potential natural enemies with key insect pests.

Twenty four and fifteen white clover seed crops in the main Canterbury production areas were monitored for insect pests and arthropod predators in two successive growing seasons (1994-95 and 1995-96, respectively). Suction samples were taken at varying distances from the crop edge into the crop from mid-November until harvest to determine the abundance, spatial and temporal distribution of arthropods. The most abundant pests found in crops were potato mirid (Calocoris norvegicus [Gmelin]) and bluegreen lucerne aphid (Acyrthosiphon kondoi Shinji) which occurred in high numbers during the critical inflorescence formation and developmental stages (up to seed set). Potato mirid were found in higher densities around the crop margin (edge to 4 m into the crop) compared with bluegreen lucerne aphid that were more widely distributed throughout the crop.

A field cage experiment was conducted to determine the impact of different potato mirid densities (0,10,20,40 and 80/cage) and high numbers (200/plant) of blue green lucerne aphid released at different plant growth stages (first and second inflorescence development, and full flowering) on inflorescence expression, seed production and yield. Loss of inflorescences and increased proportions of damaged inflorescences increased with increasing potato mirid densities and a significant linear relationship between increasing potato mirid density and declining seed yield was established. The highest level of inflorescence damage from bluegreen lucerne aphid feeding occurred at full flower and in a treatment where low infestation numbers (50/cage) were allowed to build up to 2100/cage. Significantly lower seed quality (thousand seed weight), but not overall seed yields were recorded from these field cages.

The timing and placement (crop edge to 6 m into the crop) of three insecticides (lambdacy! talothrin, fluvalinate and pirimicarb) for the control of potato mirid and blue green lucerne aphid were evaluated in a field experiment. Lambda-cyhalothrin gave the most effective control of potato mirid, while pirimicarb was ineffective against potato mirid, but gave effective bluegreen lucerne aphid control. The earlier (late November) rather than traditional (early December) application of insecticides resulted in an extended period of control of potato mirid and bluegreen lucerne aphid during inflorescence development and produced higher seed yields compared with later application timings.

In a single white clover seed crop an intensive sampling experiment was conducted to study the spatial and temporal distribution of insect pests for the development of a crop sampling programme. An insecticide spray action threshold of 10 potato mirid nymphs in the crop margin (crop edge to 4 m into the crop) was established, while an action threshold (2 or 4 bluegreen lucerne aphid/inflorescence) for bluegreen lucerne aphid control was developed from other studies in white clover seed crops. Application of insecticides to control potato mirid and bluegreen lucerne aphid numbers exceeding thresholds were economically viable.

The synchrony of common natural enemy populations with potato mirid and bluegreen lucerne aphid populations was evaluated using correlation analysis of data collected during the 1994-95 Canterbury survey season. Although some natural enemy populations showed synchrony with bluegreen lucerne aphid (e.g., ladybird, Coccinella undecimpunctata L. and lacewing, Micromus tasmaniae [Walker]) and potato mirid nymphs (harvestman, Phalangium opilio L.), they were unable to prevent bluegreen lucerne aphid or potato mirid densities increasing to economic injury levels.

The development of a pest management programme for white clover seed production taking into account crop agronomy, growing practices and changes in the industry is discussed.