Isoprenyl diphosphate synthases (IDSs) produce the ubiquitous branched-chain diphosphates of different lengths that are precursors of all major classes of terpenes. Typically, individual short-chain IDSs (scIDSs) make the C ₁₀ , C ₁₅ , and C ₂₀ isoprenyl diphosphates separately. Here, we report that the product length synthesized by a single scIDS shifts depending on the divalent metal cofactor present. This previously undescribed mechanism of carbon chain-length determination was discovered for a scIDS from juvenile horseradish leaf beetles, Phaedon cochleariae . The recombinant enzyme P. cochleariae isoprenyl diphosphate synthase 1 ( Pc IDS1) yields 96% C ₁₀ -geranyl diphosphate (GDP) and only 4% C ₁₅ -farnesyl diphosphate (FDP) in the presence of Co ²⁺ or Mn ²⁺ as a cofactor, whereas it yields only 18% C ₁₀ GDP but 82% C ₁₅ FDP in the presence of Mg ²⁺ . In reaction with Co ²⁺ , Pc IDS1 has a K _m of 11.6 μM for dimethylallyl diphosphate as a cosubstrate and 24.3 μM for GDP. However, with Mg ²⁺ , Pc IDS1 has a K _m of 1.18 μM for GDP, suggesting that this substrate is favored by the enzyme under such conditions. RNAi targeting Pc IDS1 revealed the participation of this enzyme in the de novo synthesis of defensive monoterpenoids in the beetle larvae. As an FDP synthase, Pc IDS1 could be associated with the formation of sesquiterpenes, such as juvenile hormones. Detection of Co ²⁺ , Mn ²⁺ , or Mg ²⁺ in the beetle larvae suggests flux control into C ₁₀ vs. C ₁₅ isoprenoids could be accomplished by these ions in vivo. The dependence of product chain length of scIDSs on metal cofactor identity introduces an additional regulation for these branch point enzymes of terpene metabolism.