(a) Fig. 1.1 shows a crop field where aphids have been feeding on bean plants. Fig. 1.1 (i) State two ways in which aphid feeding damages crop plants. [2] (...

Labelled answer diagram:

(a)(i) Two ways aphid feeding damages crop plants [2]:

1. Aphids suck sap from the phloem, removing sugars and nutrients that the plant needs for growth. Heavy infestations weaken the plant, causing stunting, leaf curling and reduced yield. [1]
2. Aphids transmit viral diseases from plant to plant as they feed. Their piercing-sucking mouthparts inject virus particles directly into the phloem when they move between hosts. [1]

Also acceptable: aphids excrete honeydew, which promotes sooty mould growth on leaves and reduces photosynthesis.

(a)(ii) Why aphid populations increase very rapidly in warm weather [2]:

1. Aphids can reproduce asexually by parthenogenesis, giving birth to live young without mating. Each female can produce many offspring per day without needing to find a mate. [1]
2. Warm temperatures accelerate their development and shorten the time between generations. In favourable conditions a generation can be completed in as little as one week, so numbers grow exponentially. [1]

(b)(i) One natural predator of aphids [1]:

Ladybirds (ladybugs) - both adults and larvae are voracious aphid feeders. [1]

Also acceptable: lacewings; hoverfly larvae; parasitic wasps (e.g. Aphidius).

(b)(ii) One advantage and one disadvantage of biological control compared with insecticide spraying [2]:

Advantage: Biological control does not leave chemical residues on the crop and does not harm beneficial insects such as pollinators. It is a sustainable, long-term approach that does not cause pesticide resistance. [1]

Disadvantage: Biological control is slower to reduce pest numbers than chemical spraying. The predator population takes time to build up, and during that lag period crop damage continues. The predator may also leave the area if conditions are unfavourable. [1]